3. Puberty, Oestrus and Pregnancy

3.1 Puberty
3.2 The oestrous cycle
3.3 Gestation length, parturition and uterine involution
3.4 Summary
3.5 References

3.1 Puberty

3.1.1 Importance and estimates of age at puberty
3.1.2 Anomalies in age at puberty

Puberty is a gradual quantitative phenomenon rather than an acute and qualitative endocrinological event. It occurs when the gonads begin to secrete sufficient steroids to accelerate the growth of the genital organs and the development of secondary sexual characteristics. Post and Reich (1980) defined puberty in Bos indicus heifers in Australia as the age at which plasma progesterone levels reach 1.0 ng/ml.

3.1.1 Importance and estimates of age at puberty

Age at puberty is an important determinant of reproductive efficiency. Many heifers, especially taurine, can reach puberty and breed fairly satisfactorily at one year old. However, the cost of achieving this varies among breeds and among heifers within the same breed. Heifers with the inborn ability to reach puberty early thus attain puberty and breed at less cost than those with later inherent age at puberty (Brinks, n.d.).

Estimates of age at puberty in Bos indicus cattle in the tropics and subtropics range between 16 and 40 months (Table 1). Bos indicus cattle reach puberty later than Bos taurus x Bos indicus crossbreeds or purebred taurine cattle (Table 2). This is due to genetic and environmental factors, including nutrition, disease, temperature and season of birth. These factors affect heifer growth rates.

Table 1. Some estimates of age at puberty among Bos indicus cattle in the tropics and subtropics

Breed	Location	Estimate (months)	Source
Boran	Kenya	15.6	Ronningen et al (1972)
Africander	Louisiana (USA)	18.1	Reynolds et al (1963)
Mashona	Central Africa	19	Rakha et al (1970)
Sokoto Gudali	Nigeria	19.0-23.5	Oyedipe et al(1982)
Brahman	Florida (USA)	19.4	Plasse et al (1968a)
Angoni	Central Africa	20	Rakha et al (1970)
Africander	Central Africa	20	Rakha et al (1970)
Kankrej	India	22.5	Fulsounder et al (1984)
Zebu	Ethiopia	22.6	Alberro (1983)
Brahman	Venezuela	23.3	Ordonez et al (1974)
Boran x Sahiwal	Tanzania	26	Macfarlane and Worrall (1970)
Brahman	Louisiana (USA)	27.2	Reynolds et al (1963)
Haryana	India	30	Ahuja et al (1961)
Zebu	Somalia	31.5	Aria and Cristofori (1980)
Gir	India	36.5	Malik and Ghei (1977)
Red Sindhi	India	36.7	McDowell et al (1976)
Ankole	Rwanda	37	Compere (1963)
White Fulani	Nigeria	40.2	Knudsen and Sohael (1970)

Table 2. Some estimates of age at puberty among Bos taurus cattle and their crosses with Bos indicus types in the tropics and subtropics

Breed	Location	Estimate (months)	Source
Brown Swiss	Rwanda	8-15	Compere (1963)
Jersey	India	14.1 ±0.14	McDowell et al (1976)
Angus	USA	14.4	Reynolds et al (1963)
3/4 Bos taurus cross	India	14.5 ±0.23	McDowell et al (1976)
Brahman x Angus	USA	15.3	Reynolds et al (1963)
1/2 Bos taurus cross	India	15.3 ±0.23	McDowell et al (1976)
1/4 Bos taurus cross	India	16.8 ±0.46	McDowell et al (1976)
Brahman x Shorthorn	Florida (USA)	17.0	Plasse et al (1968a)
F1 crosses	Ethiopia	17.0 ±1.5	Alberro (1983)
Africander x Angus	USA	18.1	Reynolds et al (l963)
3/4 Friesian x 1/4 Zebu	Trinidad	19	Duckworth (1949)
Brahman	Florida (USA)	19.4	Plasse et al (1968a)
Pure Friesian	Nigeria	19.5	Knudsen and Sohael (1970)
3/4 Friesian x 1/4 White Fulani	Nigeria	20.0	Knudsen and Sohael (1970)
1/2 Friesian 1/2 White Fulani	Nigeria	21.2	Knudsen and Sohael (1970)
Crossbreeds	South America	22.5	Linares et al (1974)
Brahman x Criollo	South America	23.3	Ordonez et al (1974)
Crossbreeds	Somalia	24.5	Aria and Cristofori (1980)
Ankole x Jersey	Rwanda	26.5	Compere (1963)

3.1.1.1 Genetic factors affecting puberty

Age at puberty varies among species, breeds and even strains and families. On average, the zebu reaches puberty 6 to 12 months later than Bos taurus cattle (Warnick, 1965; Wiltbank et al, 1969). Temperate taurine breeds of dairy cattle reach puberty at 30-40% of their adult body weight, compared with 45-55% for beef cattle (Hafez, 1980). In contrast, ranched Boran zebu heifers in Ethiopia do not attain puberty until they reach 60% of their adult bodyweight. Zebu heifers raised traditionally attained puberty at an even higher percentage of adult weight.

Estimates of the heritability of age at puberty range from 0.20 to 0.67 (Arije and Wiltbank, 1969; Smith et al, 1976; Laster et al, 1979; Rathi, 1979; Werre, 1980; Lunstra, 1982; King et al, 1983). Heritability of weight at puberty ranged from 0.30 to 0.44.

Several studies, particularly among taurine cattle, have attempted to relate age at puberty with other production traits. Werre (1980) found strong, negative genetic correlations between age at puberty and measures of growth: faster growing heifers reached puberty earlier. Genetic relationships with average daily gain to weaning, weight at puberty and yearling weight (0.31 ±0.82, -0.44 ±0.41 and -0.25 ±0.70, respectively) were stronger than the phenotypic correlations (-0.13,0.07 and -0.03, respectively). Thus, heifers growing faster for genetic reasons are likely to be younger and heavier at puberty. Arije and Wiltbank (1971) and Steffan et al (1983) reached similar conclusions.

Wiltbank et al (1966) observed that the breed of sire and dam and their interaction affected age and weight at puberty. Laster et al (1972) thought that both heterosis and maternal effects (lactation and mothering ability) affected age, but not live mass, at puberty.

Reynolds et al (1963) estimated the average age at puberty in Brahman heifers in Louisiana, USA, to be 27.2 months, compared with 14.4 months for Angus heifers. The estimate for Angus x Brahman crosses was 15.3 months (26.4% heterosis). McDowell et al (1976) calculated heterosis in age at puberty to be 18.4% among Red Sindhi x Jersey crosses in India. Ordonez et al (1974), working with Brahman x Criollo crosses in Latin America, ascribed the heterotic effect on age at puberty to better growth rates in the crossbreds.

3.1.1.2 Environmental factors affecting puberty

Nutrition and bodyweight

The major factors controlling the onset of puberty are body weight and growth rather than age (Sorensen et al, 1959; Boyd, 1977; McDonald, 1980): until heifers reach a particular (target or critical) weight, oestrus is unlikely to occur. Short and Bellows (1971) observed high pregnancy losses and low milk production in heifers that were fed poorly prior to puberty.

Poor nutrition significantly delays puberty in both zebu (Morales et al, 1977; Mancio et al, 1982; Oyedipe et al, 1982) (Table 3) and taurine cattle (Joubert, 1954; Sorensen et al, 1959; Bedrak et al, 1969; Short and Bellows, 1971).

Table 3. Age and weight at puberty and conception among Nigerian zebu heifers on high, medium and low levels of dietary protein intake

	High 1	Medium 1	Low 1
Age at puberty (days)	570.4	640.8	704.2 *
Weight at puberty (kg)	207.1	187.0	161.7 *
90-day conception rate (%)	58.8	27.8	16.7 *
Weight at conception (kg)	240.0	240.2	248.0
Age at conception (days)	624.3	759.0	930.8 *

* Differences between treatments significant at P<0.05.
¹ The high, medium and low rations contained 150,100 and 41% of estimated requirements, respectively

Source: Oyedipe et al (1982).

Heifers on a high-protein diet are often younger and heavier at puberty than those on a low-protein diet (Sorensen et al, 1959; Wiltbank et al, 1966; 1969; Garcia and Calderon, 1978) and are more fertile after puberty. However, although poor nutrition delays puberty, very high levels of feeding do not necessarily result in earlier puberty than adequate levels. No data are currently available that indicate how much energy or protein zebu heifers need daily to achieve a given weight and puberty at a given age. The role of nutrition on pubertal development is detailed in Chapters 4 and 6.

Disease and parasite burdens

Post and Reich (1980) studied the effects of parasite burden and reproduction in 212 heifers from 10 breed groups in Australia. The animals were grazed in 4 groups: (1) control with no treatment for gastro-intestinal or ecto-parasites; (2) treated monthly with an acaricide against ecto-parasites; (3) treated monthly with an anthelminthic for endo-parasites; and (4) treated monthly with both acaricide and anthelminthic. By 25 months, 197 of the 212 heifers had reached puberty (Table 4). Weight at puberty differed significantly (P<0.05) among treatment groups but age did not. No significant interactions were recorded between breed and treatment for either age or weight at puberty. Similar data for traditionally raised zebu cattle, in which both problems are often more prevalent, are not available.

Table 4. Effect of acaricide and anthelminthic treatment on age and weight at puberty

Treatment	n	Number attaining puberty	Age (days)		Weight (kg)
			Mean	Range	Mean	Range
No treatment	50	45	557	413-735	253 a	158-338
Acaricide	56	47	533	328-760	253 a	169-330
Anthelminthic	50	50	511	390-670	262 ab	169-398
Anthelminthic + acaricide	56	55	522	340-735	274 b	163-360
Overall	212	197	530	328-760	261 b	158-360

In the mean weight column, means without at least one common superscript are different (P<305).

Source Modified from Post and Reich (1980).

3.1.2 Anomalies in age at puberty

In some heifers, first oestrus, conventionally the first sign of puberty, is not followed by ovulation and a number of oestrous periods may occur before the start of persistent cyclic ovulation. This phenomenon of "adolescent sterility" was referred to by Rutter and Randel (1986) as non-pubertal oestrus (NPO) - behavioural oestrus without subsequent development of functional luteal tissue.

NPO was demonstrated by Macfarlane and Worrall (1970) in Boran and Boran x Sahiwal heifers in Tanzania. The NPO period was significantly longer (P<0.001) during the dry season (Table 5) which suggested that nutritional status might be involved.

Of 43 Simmental x Hereford-Brahman heifers studied by Rutter and Randel (1986) in Texas, USA, 27 (62.8%) exhibited NPO after their first behavioural oestrus. NPO tended to be more common in light-weight heifers than in heavier heifers (P = 0.12).

Table 5. Interval between first oestrus and ovulation in Boran and Boran x Sahiwal heifers in Tanzania

	n	Interval (days)	C.V. (%)
Rainy season	56	67.0	16.9
Dry season	24	103.4	24.5

Source: Macfarlane and Worrall (1970).

After first oestrus (puberty), the ability of heifers to conceive improves with age, reaching optimum levels at sexual maturity. Thus, although oestral activity was exhibited at 15.6 months in Boran cattle at Muguga in Kenya, first service was delayed until 21.7 months (Ronningen et al, 1972).

3.2 The oestrous cycle

3.2.1 Oestrous cycle length
3.2.2 Oestrus duration
3.2.3 Oestrus detection

3.2.1 Oestrous cycle length

The oestrous cycle comprises all events related to reproduction occurring between two periods of sexual activity. But this definition can be erroneous in the cow as it overlooks silent heats, which Plasse et al (1970) found in 26% of Brahman cows studied in the Florida Gulf Coast area of the USA, and fails to accommodate very early embryonic mortality, both of which can increase oestrous cycle length.

Several efforts have been made to define the length of the oestrous cycle of zebu cattle. Anderson (1936,1944) reported the normal range to be 17 to 24 days in zebu cattle in Kenya. Cycle lengths within this range have since been reported by several other investigators (Table 6).

Cycle length has generally been determined by monitoring cow sexual behaviour but similar information can be obtained by palpating ovarian structures or by measuring progesterone levels (Martinez et al, 1984; Llewelyn et al, 1987). Cycle length varies among breeds and animals within a breed, and with season, nutrition, disease, age, management and cow production status. For example, Zakari et al (1981) analysed 379 oestrous cycles of 4- to 5-year-old Bunaji (White Fulani) and Bukoloji (Sokoto Gudali) cows in Nigeria. Average cycle length was 22.87 ±0.70 and 23.76 ±0.65 days, respectively. There was no significant (P > 0.05) difference in cycle length between the breeds within season but cycles were significantly longer during the dry (pre-rainy) season than at other times of the year (P<0.01). A similar trend was observed by Rakha and Igboeli (1971) in Central Africa. Rectal palpation of animals failing to return to oestrus showed that 55% had developed quiescent ovaries. Ovaries were functional in the remaining 45% but it was not clear what proportion might have been exhibiting silent heat. The associated bodyweight changes that would have given more information on the physical condition of animals failing to show oestrus were not reported. However, Baker (1967,1968) observed that Sahiwal x Shorthorn heifers showing oestrus were heavier (P<0.01) in any month than anoestrous heifers (Table 7) and Bartha (1971) reported more irregular cycles in Azaouak cows that had high milk yields or that had lost a lot of weight postpartum.

Zakari et al (1981) found that about 43% of the oestrous cycles occurred during the rainy season, 25% during the pre-dry, 19% in the dry and only 12% in the pre-rainy period. In India, Purbey and Sane (1978a) recorded that about 40% of the oestrous cycles in Dangi cows occurred in the summer, 34% in the winter and 26% in the monsoon season. These differences might have been due to fluctuations in nutrition level but the periods when oestrus was irregular and less frequent were also characterised by low relative humidity, high ambient temperature and increased sunshine.

Table 6. Some estimates of oestrous cycle length in Bos indicus cattle

Breed	Location	Estimate (days)	Source
Zebu	Cuba	17-31	Martinez et al (1981)
Africander	South Africa	19.3-21.7	Coetzer et al (1978)
Zebu	Zambia	19.6 ±1.2 to 22.0 ±1.5	Rakha and Igboeli (1971)
InduBrazil	Mexico	20±1.9	Vaca et al (1985)
Azaouak	Sahel	20-22	Bartha (1971)
Zebu	Kenya	20.1	Anderson (1936)
Dangi	India	20.4 ±0.2	Purbey and Sane (1978a)
Nganda	Uganda	20.9 ±1.4	Rollinson (1963)
Garre	Somalia	21.5	Aria and Cristofori (1980)
Bunaji cross	Nigeria	21.5 ±3.4	Johnson and Oni (1986)
Angoni	Zambia	21.9	NCSR (1970)
Small East African Zebu	Ethiopia	22.2 ±6.0	Mattoni et al (1988)
White Fulani	Nigeria	22.4 ±0.7	Johnson and Gambo (1979)
Grade	Kenya	22.42	Anderson (1944)
Sahiwal	Somalia	22.5	Aria and Cristofori (1980)
White Fulani	Nigeria	22.9 ±0.7	Zakari et al (1981)
Boran	Kenya	23 ±0.4	Llewelyn et al (1987)
Zebu	Kenya	23.03	Anderson (1944)
Bunaji	Nigeria	23.4 ±2.7	Johnson and Oni (1986)
Red Sokoto	Nigeria	23.8 ±0.6	Zakari et al (1981)
Zebu	Cuba	24.2 ±10.7	Martinez et al (1984)
Zebu	Ethiopia	25.1 ±6.0	Alberro (1983)

Table 7. Average bodyweight of oestrous and anoestrous Sahiwal x Shorthorn heifers, Queensland Australia, November 1964 to June 1965

Month	Average bodyweight (kg)
	Oestrous heifers	Anoestrous heifers
November 1964	313	284
December	335	273
January 1965	315	291
February	320	258
March	321	301
April	331	289
May	358	306
June	337	314

Source: Baker(1967).

Kaikini and Fasihuddin (1984), using Sahiwal and Gir cows in India, and Hutchison and Macfarlane (1958), working with Boran and other unspecified zebu cattle in Tanzania, found that 3 to 7.5% of pregnant cattle exhibit a gestational oestrus. These are aberrant cycles with no ovulation or corpus luteum formation ¹.

¹ R D Randel, Texas A & M University, College Station, Texas, USA, personal communication.

3.2.2 Oestrus duration

The mean duration of oestrus in zebu cattle is around 10 hours, but with a range of 1.3 to 20.0 hours (Table 8). Estimates of the duration of oestrus in Bos taurus cattle in the tropics and subtropics range from about 11 hours to about 15 hours (Hall et al, 1 959) with intermediate values being reported by Esslemont and Bryant (1976) and Blanton et al (1957). The wide variation in the duration of oestrus in zebu cattle partly reflects variation in the observation methods and actual breed peculiarities of oestrus in zebu cattle, some of which are highlighted below.

Table 8. Some estimates of the duration of oestrus in zebu cows

Breed	Location	Estimate (hours)	Source
Zebu	Kenya	1.3 to 4.78	Anderson (1936)
Nganda	Uganda	2.2	Rollinson (1963)
Small East African Zebu	Ethiopia	2.53 to 7.36	Mattoni et al (1988)
White Fulani	Nigeria	3.1 ±0.2	Johnson and Gambo (1979)
Bunaji	Nigeria	3.6 ±2.2	Johnson and Oni (1986)
Bukoloji	Nigeria	4.03 ±0.96 to 10.70 ±0.92	Zakari et al (1981)
Zebu	Ethiopia	4.5 ±2.2	Alberro (1983)
Brahman	USA	4-6	Warnick (1965)
Bunaji	Nigeria	5.15 ±0.92 to 11.44	Zakari et al (1981)
Brahman	USA	6.7 ±0.8	Plasse et al (1970)
Bunaji cross	Nigeria	8.2 ±2.8	Johnson and Oni (1986)
N'Dama	Ivory Coast	8-9	Ralambofilinga (1978)
Zebu	Central Africa	13.3 ±0.22 to 16.5 ±1.0	Rakha and Igboeli (1971)
Shorthorn	Australia	13.4 ±0.7	Baker(1967)
Zebu	Cuba	14.3 ±3.1	Solano et al (1982)
Zebu	Cuba	15.3 ±4.3	Martinez et al (1984)
Zebu crosses	Latin America	15.3 ±4.37	Morales et al (1983)
Zebu	Cuba	15.3 ±4.4	Martinez et al (1981)
Angoni	Zambia	16.3	NCSR (1970)
Barotse	Zambia	17.4	NCSR (1970)
Zebu	Mexico	20 ±1.9	Vaca et al (1985)

Heat stress causes low intensity oestrus in zebu and, sometimes, Bos taurus cattle raised in the tropics (de Alba et al, 1961). NCSR (1970) and Cuq (1973) also noted that oestrus in zebu cattle tends to be subdued, with few external signs. In a general review of oestrus in tropical zebu cattle, Cuq (1973) estimated that 30-40% of oestrous activity occurred at night, which agrees with the 34.8% reported by Rollinson (1963) among Nganda cattle in Uganda.

Rollinson (1963) calculated the average duration of pro-oestrus and meta-oestrus to be 14 and 11 hours, respectively, compared with 3.5 ±1.5 and 2.8 ±0.7 hours obtained by Baker (1967) for Sahiwal x Shorthorn crosses and 3 to 3.5 hours by Mattoni et al (1988) for the Small East African Zebu in Ethiopia.

Rollinson (1963) found that although cows may remain attractive to bulls for an average of 27 hours, they stood for service for only 2.2 hours. Nearly a third (31.2%) of the heats started between 0600 and 1000 hours. Cows were most active soon after day-break. Similar observations were reported by Zakari et al (1981) in Nigeria and Solano et al (1982) in Cuba. Orihuela et al (1983) observed that up to 63% of mounts occurred at night (1800 to 0600 hrs) in zebu cows in Mexico. All these studies show that many of the heats start when animals are confined for the night and would be undetected by daytime observation only. Moreover, in the cooler Ethiopian highlands, Mattoni et al (1988) observed that, although 63% of heats started during the day, mounting activity was greatest when cows were not grazing. There thus appears to be a breed x environment interaction on oestrus manifestation in zebu cows. High daytime temperatures and the need for feed appear to have a strong influence on oestrus onset.

Oestrus can be prevented by psychological influences (Boyd, 1977). An animal used to being bullied may be unwilling to stand for riding, as will animals with painful lesions of the limbs or back. Hunger or extreme weather conditions may prevent an animal from expressing heat or non-oestrous animals from mounting those in oestrus. These factors act on both the male and the female.

3.2.3 Oestrus detection

Accurate detection of oestrus is essential for effective reproductive management, particularly where artificial insemination or hand mating is practiced. Poor heat detection can cause increased levels of apparent or managemental infertility.

In a controlled breeding experiment in Sri Lanka, Abeyratne et al (1983) found that most farmers were unaware that their animals were displaying oestrous cycles. About 10% of the non-pregnant animals were in heat when examined prior to treatment. In many cases animals had been seen in heat, served, and assumed pregnant, and subsequent oestrous cycles were overlooked. Many of the farmers kept only one or two breeding females, either tethered or confined. They used mainly bellowing, restlessness and mucous discharge or mounting by the bull to identify oestrus. Zemjanis et al (1969), working in Venezuela, estimated that 90% of animals thought to be anoestrous were actually cycling.

The heat detection method used should identify oestrus simply, accurately and positively. Successful heat detection is possible only when cows can be correctly identified, proper records are maintained, stockmen are properly trained and sufficient time is spent watching for oestrus.

Esslemont et al (1985) recommended that in order to improve heat detection among Bos taurus cattle, it is useful to watch for heat three or four times a day, leaving not more than 8 hours between visits. Such a system improves heat detection rates to 80% (Table 9). In a study of 1460 oestrous periods in 270 Holstein-Friesian dairy cows and heifers in Louisiana (USA), Hall et al (1959) found that 10% more oestrous periods were detected if the cows were checked three times daily (at 0600,1200 and 1800 hours) than when checked twice a day (at 0600 and 1800 hours). A fourth check (at 2400 hours) resulted in an increase of nearly 20% compared to twice-daily checks.

Table 9. Effect of number and times of observation on heat detection among temperate Bos taurus cows

Number of observations	Observation time					Detection rate(%)
2	0600		1800			69
3	0600	1400	2200			84
4	0600	1200	1600	2000		86
5	0600	1000	1400	1800	2200	91
2	0800		1600			54
2	0800		1800			58
2	0800		2000			65
3	0800	1400	2000			73
4	0800	1200	1600	2200		80
4	0800	1200	1600	2000		75

Source: Esslemont et al (1985).

Oestrus can be detected by visual, non-visual, hormonal and laparascopic means. Visual methods are covered extensively here because they constitute some of the first steps for improving reproductive performance.

3.2.3.1 Visual methods and aids to heat detection

Esslemont et al (1985) identified the following pattern in the oestral behaviour of taurine cows:

· Licking and rubbing each other.
· Sniffing the vagina of another cow.
· Mutual chin resting.
· Lining up to mount another cow.
· Mounting another cow.
· Standing to be mounted by another cow.

This is a general pattern only and not a sequence of events to expect in individual cows. In taurine or taurine x zebu cows the most characteristic signs to look for are standing to be mounted and mounting head to head.

Other signs of heat are less reliable. Cows in oestrus tend to be restless and nervous, and to feed less, and their milk production may also fall. They sniff the vulva or urine of other cows. They bellow, lick and bunt and may urinate more frequently (dribbling). In large herds a few, usually restive, cows tend to congregate and mount each other. Animals standing for mounting often develop a ruffling and abrasion of the hip, rump or pin bone hair (muddy, rubbed back and sides). Some cows discharge copious quantities of mucus from the vagina. The vulva of an oestrous cow may become swollen, soft and reddened, particularly if the cow is repeatedly mounted by a bull.

Changes in the amount of oestrogen secreted by the cycling cow are, probably, the main cause of variations in the visible signs of oestrus. From 2 to 3 days before oestrus until oestrus, oestrogen increases blood supply to the uterus and increases uterine tone and turgidity, relaxes the cervix, increases cervical mucus production and the number of endometrial glandular cells and initiates oestrous behaviour. Therefore, shortage of oestrogen may lead to failure or reduced intensity of oestrus symptoms, or short oestrus. Alternatively, low levels may fail to trigger the luteinising-hormone surge necessary for ovulation and the mature follicle would become atretic or cystic.

Rollinson (1963) found that zebu cows remain attractive to bulls for 12.1 to 38.3 hours, with variations both within and among animals. Over this period cows were successfully mounted an average of 4.4 times (range 1-11). The total number of mounts, including those without intromission, averaged 9.5 (range 1-28). The author suggested that the limited number of actual, rather than attempted, mounts was due to the short duration of oestrus relative to pro- and meta-oestrus in zebu cattle. This is supported by Esslemont and Bryant (1976) who found that taurine cattle, which tend to have a longer oestrus, were mounted 56.3 ±34.8 times.

Oestrus is difficult to detect in zebu cattle, probably because their highly organised social hierarchy is disturbed by changes in management, such as the introduction of a teaser bull not known to the herd or removing animals from their natural environment. Galina et al (1982) reported that zebu cattle do not allow themselves to be ridden repeatedly. Animals studied averaged only 1 mount per hour compared with 2.8 mounts per hour for Charolais. Mattoni et al (1988) estimated an average of 8 mounts per hour for the Small East African Zebu but with a range of 1 to 58. Up to 5% of heats were characterised by a single mount.

The number of cows in heat at any time, and the type of bull or teaser, can also affect oestrous behaviour. For example, Esslemont et al (1985) observed that among Bos taurus cattle, the number of mounts per cow tends to be greatest when three animals are in heat at the same time, and then declines. Johnson and Oni (1986) observed oestrous behaviour in Bunaji and Bunaji x Friesian heifers over 96 days using either vasectomised bulls or herd-mates as teasers. The animals were watched from 0600 to 1200 and 1300 to 1800 hours daily. The number of mounts per heifer over the study period ranged from 68 to 170. Crossbred bulls mounted crossbred heifers more than did Bunaji bulls or herd-mates: they averaged 28.3 ±5.3 mounts with crossbred heifers, compared with 24.5 ±4.9 and 22.0 ±4.6 mounts by Bunaji bulls and herd-mates, respectively. The pattern was the same with Bunaji heifers, crossbred bulls averaging 21.4 ±4.6 mounts compared with 18.6 ±4.3 and 16.4 ±4.1 mounts by the other two teaser types.

De Alba et al (1961) noted that some zebu cows refused to be mounted more than once during oestrus. Orihuela et al (1983) also observed that 85% of riding behaviour was by cows in heat. Some zebu cows change their oestrous behaviour when confined. In a study of 10 primiparous 3-year-old Boran cows, Llewelyn et al (1987) found that behavioural signs were only 27% accurate in indicating oestrus, although this appears to be a very low estimate.

In a study of ovarian function and oestrous behaviour involving 15 oestrous cycles of 5 zebu heifers in Zimbabwe, Symington and Hale (1967) observed clear sticky mucus in the vaginas of 5 heifers 2 days before oestrus and of 3 heifers 2 days after oestrus. At other times the vagina contained varying amounts of cream-coloured mucus. The volume of mucus was less and its consistency looser during the luteal phase of the cycle. In a study of Ethiopian Highland Zebu type cows, Mattoni et al (1988) found mucus discharge in only 64% of the oestrous periods. The appearance of the mucus may at times be haemorrhagic. Larger amounts tend to be discharged when the oestrous cow mounts others, lies down or urinates. It can thus be easily missed, especially at night. At other times, the only evidence of vaginal mucous discharge may be a pasting of the tail and/or legs. Therefore, although vaginal mucus discharge can be a useful indicator of oestrus, it is not precise.

The intensity of signs of oestrus appears to be related to conception rates. Hall et al (1959) used a scale of 1 to 3 to score intensity of oestrus:

1. Thin, glary vaginal discharge, nervousness, and unusual interest in herd-mates

2. More intense periods with increased vaginal discharge, considerable excitement and mounting of other females

3. The animal stands to be mounted, with or without the symptoms given for (1) or (2)

In a study 3/4 Holstein-Friesian x 1/4 zebu cows, Morales et al (1983) observed conception rates of 66, 50 and 93% (P<0.05) in animals showing weak, moderate and strong signs of oestrus.

The fact that few herders are properly trained to detect heat, the difficulty in identifying the signs of heat in zebu cattle, the large variability in the duration of oestrus, plus the reluctance of some bulls to mate, all make heat detection a major problem in peasant herds. These herds comprise few animals, sometimes a single cow, an ox and a yearling bull or a heifer. In such cases the best chance of detecting cows in heat would be at the communal grazing pastures when animals interact with others from elsewhere. However, daytime observations alone will miss those animals that exhibit oestrus at night, i.e. roughly one-third of the herd. In addition, animals are often not taken to pasture until late morning. As a result, several heats would be missed, some cows fit for insemination may not be presented, a few would be inseminated when they are actually pregnant and some would be inseminated during the luteal phase of the oestrous cycle. All of these factors contribute to poor results where artificial insemination or hand mating are used as the breeding methods. Several devices have, therefore, been developed that aid visual recognition of oestrus. Some are discussed below.

Tail paint marks

In the tail-paint-mark system, a strip of paint is applied to the rump of the cow: when the cow is mounted the paint becomes scuffed or cracked. For large taurine breed cows the paint is applied in a strip 20 cm long and 5 cm wide running from a point 12 to 15 cm behind the level of the tuber coxae towards the tail along the mid-line. Anatomical variations between breeds may lead to slight differences in the best position to place the paint mark.

The paint is stripped into the hair and should reach the skin. The animals should be examined at least daily to determine whether they have been mounted. With experience, the cracking pattern of the paint can be used to estimate the time at which the animal was in heat.

Although several special paints or pastes are commercially available, ordinary high gloss enamel, paste or water-based paint can be used successfully. Where ordinary household paint is used it should be applied every 3 or 4 days.

Kerr and McCaughey (1984) found the tail painting method to be 88% accurate in a study in which oestrus was also determined by regular progesterone assay. Smith and MacMillan (1980) reported 99% accuracy, 33% higher than the rate of detection by farmers using behavioural observation alone. Given the ease with which these paints can be acquired, they should be used more often.

Chinballs and harnesses

Chinball markers or harnesses (Elmore et al, 1986) consist of a halter, worn by the bull or teaser, with a built-in "roller ball" or crayon that leaves a strip of paint on the back of the cow after mounting. The crayons must be replaced regularly. If commercial crayons are not easily available, crayons can be made from beeswax (20%), stearic acid (15%), oleic acid (10%), mineral filler such as asbestos powder or tale (50%) end pigment (5%). The pigment must be insoluble. A softer product can be made by reducing the proportion of stearic acid relative to oleic acid.

Chinballs or harnesses are normally used on "prepared" teaser animals, e.g. epididectomised or vasectomised bulls, bulls with an amputated penis, a penile deviation, a preputial stenosis (pen-o-block), or a penis which is anchored posteriorly or ventrally to the scrotum, and steers injected with oestrogen or testosterone.

Androgenised cows (treated with male hormones) have been used as teaser animals. Kiser et al (1977) administered testosterone for 20 days (inducing regime) followed by a maintenance dose every 10-14 days. Alternatively, a single intra-muscular injection of 200 mg testosterone anathate can be used, followed by a booster after 3 to 4 weeks (personal experience). Such cows, which should not be milking, marked others in heat as effectively as surgically prepared bulls. Nymphomaniac cows or heifers that are cystic can also be used as teasers. Hackett and Lin (1985) used oestrogen, in addition to testosterone, to prepare teaser cows.

Because they actually serve the cow, vasectomised bulls can transmit venereal diseases. Their libido may also decline when they are used over several seasons. Bulls with a deviated penis offer a safer alternative since they mark cows without intromission, but they may also lose libido and a few have been observed to serve cows by mounting at an angle.

Three or four teasers should be used per 100 cows. They should be worked in groups with frequent rest. Bulls within a group should be moved frequently to avoid the formation of favourite units.

Marking web device

The marking web device is a simple and reliable oestrus detection aid recently reported by Broadbent et al (1989). It is constructed from a piece of calico (8.5 x 8.0 cm) and an 8.5 x 2.5 cm strip of household tape, covered by a layer of black mastic, stuck across the calico. The device is fixed to the sacral region of the cow or heifer using a suitable adhesive. Pressure from the chest of the animal mounting peels the mastic from the underlying tape. When tested on two groups of heifers kept at pasture (n = 22) or indoors (n = 134), the device was found to be 86 and 59% as accurate, respectively, as four times per day visual observation; there were 14 and 41%, respectively, of false positives, but no false negatives.

3.2.3.2 Non-visual methods

Rectal palpation

An experienced practitioner can predict when a cow is likely to come into heat by rectal palpation of the ovaries and uterus. Care has to be exercised in examining zebu cows because their ovaries tend to be smaller than those of Bos taurus cows. Rectal palpation is more effective in heifers because their ovaries tend to be smaller and smoother than those of older cows and the corpus luteum (CL) and follicles can thus be identified more easily. Rectal palpation is particularly valuable because bulls may fail to detect heat in some cows with apparently normally functioning ovaries (Symington and Hale, 1967). This could arise from silent heats, bull exhaustion, too many cows per bull or the physiological inability of oestral cows to attract bulls.

Five stages can be differentiated by rectal palpation during a 21-day oestrous cycle: (1) pro-oestrus (days 17-20), (2) oestrus (day 1), (3) early meta-oestrus (day 2), (4) late meta-oestrus (days 3-6), and (5) di-oestrus (days 7-16).

During pro-oestrus, a growing follicle and a regressing CL can both be felt. They may be on the same ovary or on different ovaries. The uterus feels swollen and its turgidity is increasing. Such a cow should be expected in heat in 1 to 4 days. For increased accuracy the cow should be palpated every 6 to 12 hours.

During oestrus the ovary has a mature follicle and a CL that is almost completely regressed. On palpation, the uterus is very turgid, or responds by toning up. Oxytocin released after gentle palpation causes the smooth musculature of the oestrogen-sensitised uterus to contract. Rough palpation can, however, cause the release of adrenalin, which would inhibit this response. The uterus must, therefore, be massaged gently to elicit response (Studer, 1975). When this condition is observed, the cow should be inseminated.

In early meta-oestrus, an ovulation fossa or depression can be felt on the ovary. However, this can be difficult to detect unless the ovaries have been repeatedly palpated near to oestrus to establish the presence, size and side of the follicle. There are often no other palpable structures, but it may be possible to detect a regressing CL and small follicles. The uterus is swollen and the tone decreasing. Such an animal can be expected in heat after 19 days. During late meta-oestrus the ovary has a new developing CL from the previous ovulation. Uterine tone is greatly decreased. Such cows can be expected in heat after 15 to 18 days.

In di-oestrus, ovaries have a mature CL and small follicles may or may not be palpable. The uterus is flaccid. Such cows can be expected in heat after 5 to 14 days.

A common problem in the zebu is failure to estimate the age of the CL. Symington and Hale (1967) suggested that the aberrant behaviour of the zebu CL may indicate that ovarian hormonal activity is less intense or that the morphological changes of the ovary are less pronounced than in taurine cattle.

The follicle is smooth and fluctuant and blends evenly into the ovary. In contrast, the CL is liver-like (friable) in consistency and often has a clear line of demarcation between itself and the ovary and, often, a crown. The CL also tends to change the size and shape of the ovary while the follicle usually does not.

Cervical or vaginal mucus crystallisation characteristics

Goel and Rao (1971) studied the characteristics of 88 samples of mucus, collected at different stages of the oestrous cycle of zebu cattle in India, and concluded that their crystallization pattern (arborisation) could be used to detect oestrus.

In samples of cervical and vaginal mucus collected daily from day 16 of the oestrous cycle until ovulation, Alliston et al (1958) found that the crystallization patterns before and after oestrus were "fern-like", and were more visible near oestrus than during the luteal phase. This pattern appeared 3.5 days (84 hours) before oestrus and started to decline before ovulation. Crystallisation patterns of vaginal mucus were less reliable indicators of oestrus than those of cervical mucus in a study in which the pattern was also related to rectal palpation findings (Bane and Rajakoski,

Although oestrus and ovulation were associated with "fern-like" patterns of air-dried cervical mucus from cows in Zimbabwe, Donkin (1980) observed that the same pasterns may occur et other times in the cycle. Ghannam and Sorensen (1967) found the mucous pattern method useful only for pregnancy diagnosis.

Cervical mucus glucose content was studied by Symington and Hale (1967) in zebu heifers in Rhodesia using Clinistix strips (Ames). Pooled information from all animals indicated a tendency for the glucose test to be more positive on day 0 than at other times. However, there was a large variation in individual results, leading to the conclusion that cervical mucus glucose content is of little predictive value.

Cervical or vaginal maces resistance and conductivity

Changes in the electrical resistance of mucus in the anterior vagina can be associated with oestrus in cattle. In a study of three cows, Metzger et al (1972) found that electrical resistance was lower at oestrus than during di-oestrus (P<0.001). Leidl and Stolla (1976) made a similar observation. Electrical resistance was well correlated (r = 0.92-0.99) with milk progesterone levels in Holstein cattle during the 4 days before and including oestrus (Heckmann et al, 1979). This agreed with Gartland et al (1976) and McCaughey and Patterson (1981). Foote et al (1979) inseminated 58 dairy cows on the basis of low probe readings and obtained a pregnancy rate of 52% versus 49% for 86 controls inseminated following the normal artificial insemination routine.

Vaginal smears

Symington and Hale (1967) stained mucus from the fornix area of the vagina of 5 zebu heifers with Leishman's stain. The preparations were examined microscopically ( x 100). There was little variation in the cytological characteristics of the mucus throughout the oestrous cycle. In contrast, Cuq and Pessinabe (1979) reported that oestrus could be detected in zebu cattle using the differential staining properties of vaginal smears and the demonstration of cytoplasmic lipids (histochemistry) in vaginal smears or urinary sediments. They found the histochemical method to be more accurate and was therefore recommended since urine samples are easier to obtain.

Vaginal and body thermal conductance and temperature

Using a self-retaining heat-flow probe, Abrams et al (1973) measured the thermal conductance of the vagina of four Jersey and one Angus x Holstein heifers 1 to 2 hours before and 4 to 5 hours after injecting 3 mg oestradiol 17 b intravenously. There was an oestradiol-17 b induced rise in vaginal thermal conductance that appeared to result from an increase in the rate of vaginal blood flow. This technique is not widely reported elsewhere.

Bane and Rajakoski (1961) reported observations by others that the average vaginal temperature of the cow increases gradually from the start of oestrus to a maximum after about 12 hours and then falls to the di-oestrus level.

Hurnik et al (1985), using infrared image display equipment, observed that the area enclosed by the 37°C isotherm on the gluteal region of 27 Holstein/Friesian cows tended to increase at the beginning of oestrus. The technique was, however, hampered by frequent false positives (33%) and inability to detect heat in 7% of the animals.

Ball et al (1978) measured rectal temperature of 10 cows each morning and milk temperature at morning and evening milking over 40 days, in order to determine the best time to inseminate, with or without standing oestrus. The best determination was based on the observation that milk temperature was at least 0.1°C higher during oestrus than before oestrus.

Vaginal pH

Zust (1966) and Schilling and Zust (1968) reported that cow intravaginal pH tends to decrease during oestrus. The pH was nearly constant during di-oestrus, ranging from 6.86 to 6.98 at the cervix, from 7.26 to 7.38 at the middle part and from 7.54 to 7.71 at the caudal part. The pH at the cervix decreased from 7.0 to 6.72 one day before oestrus, with a further decrease to 6.54 at the beginning of oestrus. The lowest pH (6.45) was recorded at the end of heat, immediately prior to ovulation.

3.2.3.3 Hormonal changes

Radio-immunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) enable prompt study of endocrine events. For example, the characteristic changes in blood or milk progesterone and other hormone levels can give an accurate indication of the time of oestrus (Hill et al, 1970; Dobson et al, 1975). Some ELISA techniques take as little as 5 minutes to yield results. However, samples must be taken frequently and this is impracticable under farm conditions. Moreover, the results may also require careful interpretation and, thus, adequate training of operators.

3.2.3.4 Laparascopic techniques

Many internal organs can be examined using the laparascope without causing extensive trauma (Wilson and Ferguson, 1984). The laparascope has been used to study the reproductive organs of many species, including cattle (Megale et al, 1956; Lamond and Holmes, 1965; Wishart and Snowball, 1973). Although the technique can be used to detect oestrus accurately (Ireland et al, 1980), it requires sophisticated equipment and training and is thus of little use to farmers and most field practitioners.

3.3 Gestation length, parturition and uterine involution

3.3.1 Gestation length
3.3.2 Parturition
3.3.3 Uterine involution

3.3.1 Gestation length

Gestation is the period from conception to parturition. Estimates of the gestation length among Bos indicus cattle average 285 days (Table 10), which is within the range of 270-292 days reported for Bos taurus cattle (Andersen and Plum, 1965; Hunter, 1980; Bazer and First, 1983; Rodriguez et al, 1983). McDowell et al (1959) suggested that gestation tends to be longer in animals with a high proportion of zebu breeding.

Table 10. Some estimates of the gestation period in Bos indicus cattle

Breed	Location	Estimate (days)	Source
Zebu	Ethiopia	275	Glenn et al (1963)
Arsi	Ethiopia	276.2 ±6.2	Swensson et al (1981)
Sahiwal	India	278.1-289.5	Sinha et al (1982)
Garre	Somalia	279	Aria and Cristofori (1980)
Zebu	Ethiopia	279.8 ±1.7 to 281±2.2	Azage Tegegn et al (1981)
Zebu	Cuba	281.5-287.7	Rodriguez et al (1983)
Zebu	Various	283-291	McDowell(1972)
Highland zebu	Ethiopia	283 ±4.8	Mukasa-Mugerwa and Azage Tegegne (1989)
Kankrej	India	284 ±5	Fulsounder et al (1984)
Dangi	India	284.7	Purbey and Sane (1978b)
Sahiwal	Somalia	285	Aria and Cristofori (1980)
West African Shorthorn	West Africa	285.3	Sada (1968)
Angoni	Zambia	285.9	NCSR (1970)
Angoni	Zambia	285.9 ±13.2	Rakha et al (1971)
Kenana	Sudan	286 ±0.2	Saeed et al (1987)
Red Sindhi	India	286.3	Ghose et al (1979)
Mashona	Zambia	286.5	NCSR (1970)
Zebu	Sudan	287	El-Amin (1976)
N'Dama	Ivory Coast	288.2	Ralambofilinga (1978)
N'Dama	West Africa	288.5	Sada (1968)
Gir	Brazil	289.7	Carregal (1975)
Sokoto Gudali	West Africa	290.5	Sada (1968)
Highland zebu	Ethiopia	291.0 ±37.2	Alberro (1983)
Brahman	USA	292.8	Plasse et al (1968b)
Africander	Zambia	297.5	NCSR (1970)

Gestation length is largely determined by foetal factors, with some modification due to parity and maternal environment. Ordonez et al (1976) gave the heritability of gestation length in zebu cattle in Latin America as 0.10, with a repeatability of 20%. Other estimates range from zero to 0.71 in taurine cattle, with most estimates between 0.25 and 0.50 (Andersen and Plum, 1965).

Egbunike and Togun (1980) noted a breed effect and Rodriguez et al (1983) observed that gestation averaged 281.5 ±7.25 days in dairy cows, compared with 287.7 ±9.23 days for beef cattle. Ordonez et al (1976) reported a significant effect of sire and dam of calf, sex of calf and year of calving and a significant sex of calf x parity interaction among zebu cattle in Venezuela. Bazer and First (1983) found that when embryos of short gestation breeds were transferred into dams of long gestation breeds, gestation was identical to the gestation average of the breed of the embryo, indicating the influence of the foetus on gestation length. This agrees with the findings of Sinha et al (1982), who calculated a gestation period average of 289 ±3.2 days in Sahiwal cows bred with Sahiwal semen, compared with 278.08 ±1.46 days in those bred with Jersey semen. Gestation was longer in cows carrying a crossbred male foetus (280.5 ±1.85 days) than in those carrying a crossbred female foetus (277.0 ±1.89 days), while the reverse was true for purebred foetuses (291.6 ±4.6 days for females vs 286.0 ±0.71 days for males). The observation that male calves are carried for 1 to 5 days longer than females has also been made elsewhere (Plasse et al, 1968b; Brito, 1973; Carregal, 1975; Chhabra and Goswami, 1980; Jainudeen and Hafez, 1980; Azage Tegegn et al, 1981; Chandramohan and Bhat, 1981; Khalafalla and Khalifa, 1983). These differences, however, tend not to be significant.

Gestation for twins tends to be 3 to 6 days shorter than for a single birth (Bazer and First, 1983) and heifers conceiving younger tend to have slightly shorter gestation periods (Hafez, 1980).

Lobo et al (1981) observed that, in Brazil, gestation length was significantly affected by year of calving but not genetic group (proportion of zebu blood), month of calving, parity or lactation length. El-Amin et al (1981), however, found a significant effect of month of calving in Red Butana cows in Sudan. Bartha (1971), working with Sahelian zebu cattle, and Carregal (1975), using Gir cows, failed to demonstrate significance due to season and Taylor et al (1984) found that season, calf sex, year of calving and dam parity were not important sources of variation in gestation length of Malvi cattle.

3.3.2 Parturition

Parturition is initiated by the foetus. This conclusion is based on the observation that defects in the foetal brain or adrenal gland prolong gestation in sheep, goats and cattle (Bazer and First, 1983).

The foetal pituitary releases adrenal-cortico-trophic hormone (ACTH) which stimulates the foetal adrenal gland to increase release of cortisol. In the cow, increases in foetal cortisol concentrations precede but parallel increases in oestrogen before delivery (Hunter et al, 1977). Foetal cortisol is therefore partly responsible for initiating parturition because it induces oestrogen synthesis in the placenta and elevated levels of prostaglandin F₂ a (PGF₂ a), which results in the regression of the corpus luteum of pregnancy

PGF₂ a (or its analogues) causes abortion when administered during the first 120 days of pregnancy and induces delivery in 1 or 2 days when administered after 250 days. Between 120 and 250 days it appears to have very little effect (Schultz and Copeland, 1981), or the incidence of abortion is lower, probably due to extra-ovarian sources of progesterone. Similar effects can be achieved by administering glucocorticoids, which tend to mimic foetal cortisol by altering placental steroid synthesis (Wagner, 1980). Long-acting glucocorticoids can be used between day 120 and day 250, while short-acting glucocorticoids are more effective from day 250 to day 280. Administering both glucocorticoids and prostaglandins, either simultaneously or 10 days apart, results in termination of pregnancy irrespective of the stage of gestation.

Parturition in cows often takes several hours (Bazer and First, 1983; Mukasa-Mugerwa and Mattoni, 1989). The first stage lasts 2 to 6 hours and the delivery stage about 30 to 40 minutes. The placenta is usually expelled 2 to 6 hours after delivery. Rao and Rao (1980a) calculated the average time for the expulsion of foetal membranes in Ongole cows and their crosses to be 5.16 ±0.20 hours.

Delivery involves the coordinated rhythmic contraction of uterine smooth musculature, involuntary contraction of abdominal muscles and softening and dilation of the birth canal. Smooth muscle contractions are initiated by an increase in intracellular calcium under the influence of oxytocin and prostaglandins. Although relaxin is known to control the relaxation of the birth canal in swine, its exact role in cattle is not known.

Rao and Rao (1980a) calculated the mean weight of the membranes in Ongole cows to be 2.62 ±0.08 kg, which is comparable to the 2.4 kg obtained by Mukasa-Mugerwa and Mattoni (1989) for Ethiopian Highland zebu. Foetal membranes were approximately 10-13% of calf weight and 0.66-1.0% of dam weight at parturition. On average, there were 91.56 ±2.16 cotyledons per placenta; this is within the range of 70-142 noted for other cattle genotypes.

3.3.3 Uterine involution

Uterine involution in cattle takes 23 to 35 days (Dennis and Gachon, 1974) in two phases (Tennant et al, 1967; Donkin, 1980).

Bastidas et al (1984) found that uterine involution took an average of 33.0 ±1.0 days in Brahman cows in Venezuela, and was influenced by month of calving (P<0.01) and cow age (P<0.05). Rao and Rao (1980b) estimated that, in Ongole cows, complete vulva involution took 20.2 ±1.64 days, cervical involution 34.1 ±2.61 days and involution of the gravid and non-gravid horns 28.20 ±1.45 and 21.50 ±1.23 days, respectively. Involution was more rapid in purebred Ongole cows than in those crossed with Jersey, Brown Swiss and Holsteins. Involution tends to be accelerated by walking.

Kadu and Kaikini (1976) calculated a significant correlation coefficient of 0.39 between the time taken for complete uterine involution and the interval from calving to first postpartum oestrus. This is important for reproductive management as it implies a link between postpartum ovarian activity and uterine contents and environment. In Bos indicus cattle, the duration of postpartum anoestrus is also affected by lactation and suckling. Maree et al (1974) observed, however, that calf presence did not significantly influence uterine involution time, as did El-Fadaly (1981) working on milked and suckled buffaloes in Egypt.

Involution rate is significantly affected by parity and month of calving (Bastidas et al, 1984). Involution among Brahman cows studied was slower during the dry season, but was not affected by suckling regime or calf birthweight. It was positively correlated (r = 0.55, P<0.01) with interval to postpartum follicle formation but negatively correlated with cow weight at calving (r = -0.31, P<0.05) end cervical involution (r = -0.63, P<0.01).

The regression rate (Y) was defined by the equation:

Y = 13.3 - 0.67X + 0.015X²

where X is the diameter of the gravid horn. The equation accounted for 83% of the variation. Martinez et al (1982) found no important seasonal effect on uterine involution rate in zebu cows suckled twice daily. The overall average was 24.5 ±8.74 days while complete absorption of the gestational corpus luteum was achieved after 27.83 ±11.41 days.

3.4 Summary

Puberty can be defined as a qualitative or quantitative trait. Zebu heifers attain puberty later than taurine or taurine x zebu crosses. Puberty precedes sexual maturity and is influenced by the output of pituitary hormones and the size and activity of the gonads. Age and weight are the most important factors affecting the onset of puberty in heifers. Many heifers, particularly younger and lighter animals, exhibit non-pubertal oestrus.

Oestrous cycles last about 21 days in zebu cattle and the oestrous period lasts about 10 hours (range 2-18 hours). Oestrus is often subdued and affected by physiological and psychological factors. Almost a third of heats start at night which, with their short duration, implies that many may be missed.

Heat detection is primarily a management problem. Considerable skill is needed to detect oestrus from visible signs. Farmers who maintain good records and spend more time watching for heat will obtain better results. Many farmers in the tropics raise few cows and the main signs to look for are standing for mounting and/or mucus discharge. When animals can be aggregated, the use of teaser animals, with tail painting, chinball markers or marking web devices, offers the best aid to visual detection of oestrus. Hormonal assays, laparascopic examination and vaginal probes are very expensive and require extensive training, and offer little added advantage under field conditions.

The length of gestation in the zebu is about 285 days. It is primarily determined by foetal genetic factors, with some modification due to parity and sex of calf. Uterine involution is independent of factors such as season, suckling or calf birthweight following normal delivery.

3.5 References

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