Effects of sexual condition and dietary protein level on feedlot performance of lambs in Zimbabwe - Effets du status sexuel et du niveau protéique d'alimentation sur les performances d'agneaux élevés en feedlots au Zimbabwe

S. Sibanda, V.B. Kiwanuka and T. Smith

Grasslands Research Station
Department of Research and Specialist Services
Private Bag 3701
Marondera
Zimbabwe

Summary
Resume
Introduction
Materials and methods
Results
Discussion
Acknowledgements
References

Summary

Effects of 3 sexual conditions (entire, cryptorchid and castrate) and 4 dietary protein levels (120 g/kg, 140 g/kg, 160 g/kg and 180 g/kg) on the feedlot performance of lambs were studied in a 3 x 4 factorial experiment. Merino and Dorper x Merino x Suffolk lambs (average weight 30 kg) were fed a high energy diet ad libitum for 70 d, slaughtered, and physical and chemical carcass measurements made. Level of protein in the diet had no effect (P > 0.05) on any variable. There was, however, a significant (P < 0.05) interaction between protein level and sexual condition. Weight gain and eye-muscle area, water content and protein content in the carcass increased with protein level while carcass fat and feed conversion ratio decreased in both entire and cryptorchid lambs: the opposite was the case in castrates. Neither protein level nor sexual condition affected carcass mass. The results suggest that carcass composition can be manipulated through higher dietary protein levels for entire and cryptorchid lambs. There seems to be no justification for increasing the protein level above 120 g/kg for castrates.

Resume

Les effets de 3 status sexuels (entier, cryptorchide et castré) et 4 niveaux protéiques (120 g/kg, 140 g/kg, 160 g/kg et 180 g/kg) sur les performances d'agneaux élevés en lots ont été étudiés en utilisant un plan factoriel 3 x 4. Merinos et Dorper x Merino x Suffolk (poids moyen 30 kg) ont été alimentes pendant 70 j avec un haut niveau d'énergie distribué ad libitum. Des analyses physiques et chimiques ont été réalisées sur les carcasses. Le niveau de protéine dans la ration n'a pas d'effet (P > 0.05) sur aucune des variables. Par contre il y a un effet significatif (P < 0.05) entre les niveaux protéiques et le status sexuel. Les gains de poids et la surface du muscle oculaire ainsi que la concentration en eau et en protéine dans les carcasses augmente avec le niveau de protéine et le niveau d'engraissement de la carcasse. Ces 2 indices de consommation diminuent chez les animaux entiers et cryptorchides alors que l'inverse se produit chez les castrés. Ni le niveau protéique ni le status sexuel n'affectent le poids de carcasse. Les résultats suggèrent que la composition de la carcasse peut être modifiée au niveau de protéine sur des animaux entiers ou cryptorchides. Il semble qu'il n'y a pas de justification d'augmenter le niveau de protéine au dessus de 120 g/kg pour les animaux castrés.

Introduction

Provided an adequate plane of nutrition is available, entire and cryptorchid lambs generally grow faster, utilize feed more efficiently and produce heavier carcasses with less fat and more protein than castrates (Turton, 1962; Hudson et al, 1968; Wilson et al, 1970; Glimp, 1971; Wilson et al, 1972; Corbett et al, 1973). In spite of this, no attempt has been made to develop specific diets for entire and cryptorchid animals. In most cases diets designed for finishing castrates have been used to compare animals of the 3 sex conditions.

With their higher rate of protein deposition it is probable that entire and cryptorchid animals have higher protein requirements than castrates. It has also been shown that lamb carcass composition can be manipulated to provide more protein at the expense of fat by using diets with high levels of rumen undegradable protein (Fattet et al, 1984).

This study investigated the response of entire, cryptorchid and castrate lambs on diets containing different levels of protein, in order to determine whether there is a need to develop rations containing more protein for the better-performing entire and cryptorchid lambs.

Materials and methods

Experimental animals were 36 each Merino and Dorper x Merino x Suffolk lambs. Average weight when the sexual treatments were imposed was 16.9 kg. Although the sex treatments were imposed at 2 mo, the lambs remained with their dams until weaning at 3 mot The experiment was a 3 x 4 factorial with 3 sexual conditions (entire, cryptorchid and castrate) and 4 levels of dietary protein (120 g/kg, 140 g/kg, 160 g/kg and 180 g/kg). Lambs were stratified by breed and weight and randomly allocated to 12 treatment groups of 6 animals each.

Animals were castrated by Burdizzo's forceps. Cryptorchidism was induced by pushing the testes into the abdomen and placing two elastrator rings as high as possible on the scrotum, distal to the testes. Scrotal ablation occurred in 2-3 wk. resulting in the testes remaining close to the abdominal wall. Lambs were weaned and dosed against internal parasites when the oldest ones had reached 3 mot The 6 lambs in each treatment were penned and group-fed ad libitum on one of the 4 diets. Water was available ad libitum. Diets were formulated so as to provide equal amounts of metabolizable energy (ME) and rumen degradable protein (RDP), with increasing levels of rumen undegradable protein (UDP) (Table 1). The coefficients used in computing RDP and UDP contents were from an earlier study on rumen-fistulated steers using similar diets and protein sources while ME was calculated using figures from Topps and Oliver (1978).

Lambs were weighed after withdrawing water for 15 hr and feed for 24 hr at the start of the feeding period. Full-gut live weights were taken every 14 d thereafter until slaughter 70 d later. At the end of the feeding period, water and feed were withdrawn for 15 hr and 24 hr before the lambs were weighed and slaughtered. Carcasses were chilled for about 24 hr. after which physical measurements were made on the left side, sectioned between the tenth and eleventh ribs, of each carcass. The outline of the longissimus dorsi was traced on paper and the cross-sectional area determined by planimetry. Backfat thickness over this muscle was taken as the average of measurements made at 3 points. Carcasses were stored in sealed plastic bags at -20°C pending chemical analysis. The half carcasses were later cut into smaller portions by band saw and ground in a commercial cutter-grinder with a 9 mm screen (Jeffco Cutter-grinder, Model 262B). The ground carcass was mixed by hand and a representative sample of 100 g was taken and analysed for water, ether extract (fat) and protein (O'Donovan and Elliot, 1971).

Table 1. Composition of diets fed to feedlot lambs in Zimbabwe.

Diet composition1)		Protein level (g/kg of diet)
		120	140	160	180
Physical
Crushed maize		824	704	660	554
	Star grass hay	150	150	140	150
	Cottonseed meal	-	120	183	285
	Urea	15.2	13.5	10.0	5.0
	Limestone flour	4.9	5.75	3.0	2.0
	Salt (NaCl)	2.45	2.87	1.5	1.5
	Monocalcium phosphate	2.45	2.88	1.5	1.5
	Vitamin A & D powder	1	1	1	1
Chemical
	Dry matter	903	899	902	898
	Rumen degradable protein	92	92	92	92
	Undegradable protein	28	48	68	88
	Metabolizable energy	12.2	12.2	12.2	12.2

Note: 1. Figures are expressed as g/kg except metabolizable energy which is MJ/kg.

Analysis of variance, adjusted for initial starved weight as a covariate, was carried out on all post-weaning variables to establish the effects of sexual condition and dietary protein level and their interactions. Analysis of variance was also performed on gain to weaning to determine the effects of sex condition in that period.

Results

Pre-weaning performance

Sexual condition did not affect (P > 0.05) growth rate before weaning when the dietary treatments were imposed. Average weight increases were 13.6 kg over an initial weight of 17.5 kg for entires, 13.1 kg over 16.8 kg for cryptorchids and 12.6 kg over 16.4 kg for castrates, there being no differences due to sex.

Live weight gain and feed conversion ratio

Weight gains were not affected consistently by protein level (Table 2). Entires gained more than cryptorchids (P < 0.05) and castrates (P < 0.001) and cryptorchids gained more than castrates (P < 0.05). The sex/protein interaction was significant (P < 0.01), gain increasing with dietary protein for entires and cryptorchids but decreasing for castrates.

Table 2. Effects of sexual condition (S) and dietary protein level (P) on weight gains (kg) and feed conversion ratios (kg feed/kg gain) of Zimbabwe lambs.

Parameter and sexual condition			Protein level in diet (g/kg)
			120	140	160	180	SED (S)	Significance
Initial starved live weight
	Entire		28.8	29.2	28.4	28.9
	Cryptorchid		27.9 -	28.7	27.8	28.1
	Castrate		27.2	26.7	27.5	27.2
Final starved live weight
	Entire		41.4	42.5	43.0	43.8
	Cryptorchid		39.9	40.2	41.7	41.4
	Castrate		38.7	38.0	37.7	38.4
Live weight gain
	Entire		12.6	13.3	14.6	14.9		5 **
	Cryptorchid		12.0	11.5	13.9	13.3	0.53	P NS
	Castrate		11.5	11.3	10.2	11.2		SxP *
		SED (P)		0.60
		SED (PxS)		1.05
Feed conversion ratio
	Entire		6.06	6.10	5.56	5.64		S *
	Cryptorchid		5.95	6.63	5.74	6.37	0.306	P NS
	Castrate		5.96	6.69	7.62	7.38		SxP*
		SED (P)		0.350
		SED (SxP)		0.608

** P < 0.01
* P < 0.05
NS P > 0.05

Protein level had no effect on conversion ratio. Entires and cryptorchids had similar conversion ratios which were lower (P < 0.05) than those of castrates. The interaction between sex and level of dietary protein was significant (P < 0.05). Conversion ratio increased with increasing dietary protein in castrates but decreased for entires and cryptorchids.

Dietary protein level had no overall effect (P > 0.05) on any of the carcass variables, while sexual condition had no effect (P > 0.05) on final carcass weight (Table 3). Entire and cryptorchid lambs had similar eye muscle areas and these were greater (P < 0.05) than those of castrates. Castrate lambs had thicker (P < 0.05) backfat. The interaction between sex and protein level was significant only for eye muscle area (P < 0.01). Eye muscle area increased with protein level in entire and cryptorchid lambs but tended to decrease in castrate lambs.

Table 3. Effects of sexual condition (S) and dietary protein level (P) on final carcass weight (kg), eye muscle area (cm) and backfat thickness (mm) of Zimbabwe lambs.

Parameter and sexual condition			Protein level in diet (g/kg)				SED (S)	Significance
			120	140	160	180
Final carcass weight
	Entire		19.1	19.5	20.3	20.4		S NS
	Cryptorchid		19.5	18.6	19.9	20.2	0.37	P NS
	Castrate		19.5	19.7	18.2	19.3		SxP NS
		SED (P)		0.42
Eye muscle area
	Entire		111.3	12.8	12.8	12.8		S *
	Cryptorchid		11.6	11.0	11.8	13.3	0.42	P NS
	Castrate		12.4	13.8	11.0	11.6		SxP **
		SED (P)		0.48
		SED (SxP)		0.83
Backfat thickness
	Entire		3.27	2.92	3.28	3.77		S *
	Cryptorchid		3.68	3.13	3.13	3.29	0.278	P NS
	Castrate		3.84	4.17	3.96	3.74		SxP NS
		SED (P)		0.324

** P < 0.01
* P < 0.05
NS P > 0.05

Carcass composition

Dietary protein level had no effect (P > 0.05) on water, fat and protein contents of the carcass (Table 4). Lambs on the 120 g/kg diet produced carcasses with more fat (P < 0.05) than those on the 180 g/kg diet: differences between the other treatments were non-significant (P > 0.05). Entire and cryptorchid lambs produced carcasses with more water and protein and less fat than castrate lambs (P < 0.05). The interaction between sexual condition and dietary protein was significant (P < 0.05) for water, fat and protein contents. Water content in the carcass tended to decrease with protein level in entire lambs. Protein content in the carcass peaked at the 140 g/kg level and then declined in entire and cryptorchid lambs with the opposite being the case in castrates. Fat content in the carcass decreased linearly as dietary protein level increased.

Table 4. Effects of sexual condition (S) and dietary protein level (P) on carcass composition (%) of Zimbabwe lambs.

Parameter sexual and condition			Protein level in diet (g/kg)				SED (S)	Significance
			120	140	160	180
Water content
	Entire		50.8	55.7	52.9	51.5		S *
	Cryptorchid		51.7	50.7	54.0	53,0	0.79	P NS
	Castrate		49.1	50.5	49.6	51.2		SxP *
		SED (P)		0.91
		SED (SxP)		1.57
Fat content
	Entire		26.8	20.2	23.6	24.8		S *
	Cryptorchid		25.3	26.3	33.3	33.5	0.96	P NS
	Castrate		29.1	28.4	28.8	26.5		SxP*
		SED (P)		1.11
		SED (SxP)		1.92
Protein content
	Entire		15.2	17.3	16.7	15.7		S *
	Cryptorchid		15.5	17.3	16.7	15.7	0.27	P NS
	Castrate		14.8	13.7	13.8	14.8		SxP *
		SED (P)		0.30
		SED (SxP)		0.53

** P < 0.01
* P < 0.05
NS P > 0.05

Discussion

Sexual condition had no effect on pre-weaning weight gains indicating that the effects of androgens in the entire and cryptorchid lambs were minimal during this period. In other studies, when lambs were weaned at 116 d, entire and cryptorchid lambs had heavier weaning weights than castrates (Wilson et al, 1970). The difference in the results may be explained by the fact that in the current study lambs were weaned at 70-84 d.

The superior performance of entire and cryptorchid lambs compared to castrates in weight gain, feed conversion ratio, eye muscle area, carcass protein and lower carcass fat confirms earlier results in sheep (Turton, 1962; Hudson et al, 1968) and cattle (Wilson et al, 1974; Albaugh et al, 1975; Kiwanuka, 1987). These results support the view that entire males grow more rapidly, utilize feed more efficiently and produce a higher-yielding carcass with less fat, more protein and more edible product (Seideman et al, 1982).

Although hormonal levels were not measured in this study, it is generally accepted that the better performance of entire and cryptorchid animals compared to castrates is due to the higher levels of androgens, especially testosterone, produced by entires and cryptorchids (Galbraith and Topps, 1981; Seideman et al, 1982; Kiwanuka, 1987). Cryptorchidism, while it renders the animals more docile and infertile, does not appear to alter markedly the levels of circulating androgens (Wilson et al, 1970).

Overall, the level of protein in the diet had no effect on the performance of lambs, except for carcass fat which tended to decrease as dietary protein increased. The significant interaction between the level of protein in the diet and sexual condition resulted in dietary protein level having a significant effect on the performance of entire and cryptorchid lambs performance improving with an increase in dietary protein up to 140 g/kg of diet.

An increase in post-ruminal protein supply has been shown to increase protein deposition at the expense of fat in lambs both in negative (Fattet et al, 1984) and in positive energy balance (Andrews and Orskov, 1970). As less energy is needed to lay down a unit mass of protein than fat it is not surprising that the amount of feed consumed per kg gain tended to decrease with increasing protein level.

It would seem that entire and cryptorchid lambs, with a greater propensity to deposit protein due to higher levels of androgens, can utilize extra protein in the form of UDP. Castrate lambs do not require protein additional to that supplied by microbial protein, given adequate levels of RDP. The results of the present and earlier studies (Andrews and Orskov, 1970; Fattet et al, 1984) indicate the possibility of manipulating carcass composition through dietary protein level. On the basis of feed conversion ratios and desirable protein and fat contents in the carcass it may be necessary to use higher dietary protein levels for entire and cryptorchid than for castrate lambs. The choice of the dietary protein level would depend mainly on economic factors. There seems to be no reason for using protein levels greater than 120 g/kg for castrate lambs.

Acknowledgements

We are grateful to R M Chiwara for taking care of the animals, E Rusike for the chemical analyses and J Kangai of the Biometrics Bureau, Agricultural Research Centre, for the statistical analysis.

References

Albaugh A, Carroll F D, Ellis K W and Albaugh R. 1975. Comparison of carcasses and meat from steers, short scrotum bulls and intact bulls. Journal of Animal Science 41: 1627-1631.

Andrews R P and Orskov E R. 1970. The nutrition of early weaned lambs. Journal of Agricultural Science, Cambridge 75: 11-18.

Corbett J L, Furnivalo E P. Southcott W H. Park R J and Shorthose, W R. 1973. Induced cryptorchidism in lambs. Effects on growth rate, carcass and meat characteristics. Animal Production 16: 157-163.

Fattet I, Hovell F D de B. Orskov E R. Kyle D J. Pennie K and Smart R I. 1984. Under nutrition in sheep. The effect of supplementation with protein on protein accretion. British Journal of Nutrition 52: 561-574.

Galbraith H and Topps, J H. 1981. Effects of hormones on growth and body composition of animals: a review. Nutrition Abstracts and Reviews 51B: 521-540.

Glimp H A. 1971. Effect of sex alteration, breed, type of rearing and creep feeding on lamb growth. Journal of Animal Science 32: 859-862.

Hudson L W. Glimp H A, Woolfolk P G. Kemp J D and Reese C M. 1968. Effect of induced cryptorchidism at different weights on performance and carcass traits of lambs. Journal of Animal Science 27: 45-47.

Kiwanuka V B. 1987. The effects of breed and sexual condition on growth characteristics and composition of carcasses of cattle. M. Phil. Thesis. University of Zimbabwe, Harare, Zimbabwe.

O'Donovan W M and Elliot R C. 1971. Developmental changes in the bodies of Dorper sheep. 1. Changes in live body-weight, body proportions and composition of weaned Dorper lambs given different amounts of food. Rhodesian Journal of Agricultural Research 9: 65-75.

Seideman S C, Cross H R. Oltjen R R and Schanbacher B D. 1982. Utilization of the intact male for red meat production: A review. Journal of Animal Science 55: 826-840.

Topps J H and Oliver J. 1978. Animal foods of Central Africa. Technical Handbook No. 2. Rhodesian Agricultural Journal, Harare, Zimbabwe.

Turton J D. 1962. The effect of castration on meat production and quality in cattle, sheep and pigs. Animal Breeding Abstracts 30: 447-456.

Wilson L L, Ziegler J H. Rugh M C, Watkins J L, Merritt T L, Simpson M J and Kreuzberger F L. 1970. Comparison of live, slaughter and carcass characteristics of rams, induced cryptorchids and wethers. Journal of Animal Science 31: 455-458.

Wilson L L, Varela-Alvarez H. Rugh M C and Borger M L. 1972. Growth and carcass characters of rams, cryptorchids, wethers and ewes subcutaneously implanted with zeranol. Journal of Animal Science 34: 336-338.

Wilson L L, Rugh M C, Ziegler, J H and McAllister T J. 1974. Live and carcass characteristics of Holstein castrated, short scrotum and intact males. Journal of Animal Science 39: 488-492.

Dorper sheep in Zimbabwe