5. Simulated performance of two genotypes under various milking and supplementation policies

5.1 Experimental design
5.2 Comparative simulated performance

The analysis of the previous section was carried out under the management regime prevailing in the real system with the objective of establishing the validity of the model for the system under study. This validated model is now used for experimentation to investigate the probable impact on the real system of several interventions, namely various milking strategies under different supplementation regimes for lactating T and ST cows. Specifically, the objectives of the experimentation are:

-To compare the simulated performance of the two genotypes under alternative production strategies. This comparison is done on the basis of different measures of performance, as the ranking of strategies might vary according to the criteria used.

- To estimate the response of key performance variables to various input levels.

- To estimate optimum input and output levels based on economic criteria.

5.1 Experimental design

The variables in this experiment are the level of milk offtake for human consumption and the level of supplementation. A 4 × 6 factorial experiment is conducted, i.e. four levels of milk offtake and six levels of supplementation, the details of which are given in Table 5. For example, experiment number 15 tests a 40% milk offtake for human consumption and a supplementation level of 1.5 kg/d per lactating cow. Experiment number 1 is the baseline run described in Section 4. The breeding, weaning and sales policies assumed in the experimental runs are also identical to those used in the baseline run ¹⁶. In addition to this general specification of the experimental runs, two other controls over milking are effective in the model.

¹⁶ As in the baseline run, the evolution of the herd is examined over a time horizon of 15 years, and 10 replications are made for each experiment. Again, each replication begins with a "good" forage year type (see Table 1), after which year types are drawn probabilistically. The sequence of year types drawn is the same for each experimental run (and identical to that drawn for the baseline run, see Appendix Table A.6), so that there is no bias between runs due to different year type sequences applying.

Firstly, in the absence of suckling calves, both for T and to a lesser extent for ST cows, complete milk letdown is not possible. A limited experiment by APRU (1981) showed that extracted milk as a percentage of potential was about 22% and 42% for T and TS cows respectively ¹⁷. The absolute levels of milk let-down potential for both breeds are expected to be higher, and are assumed in the model to be 30% and 60% for T and ST respectively. Operationally, this implies that after weaning only 30% of the potential milk yield of a T cow and 60% of an ST cow can be extracted.

¹⁷ The experiment consisted of oxytoxin treatment of 18 cows of each breed prior to milking, which produced milked-out yields of 3.7 kg for T and 5.9 kg for ST cows. Milk produced without treatment was about 0.83 kg and 2.5 kg for T and ST respectively.

Secondly, regardless of the milking policy in effect, offtake from cows in their first lactation is limited to a maximum of 20% of their yield. This provision is designed to allow young lactating cows, which have a lower milk potential than mature cows, to provide an adequate supply of milk to their calves.

Lactating cows are supplemented for 1 month before calving and during 7 months post-partum, but only when the digestibility of the forage consumed is at 60% or below. Animals that calve during October are thus supplemented during that month and for 7 months afterwards until the end of May. During an average year type, supplements will be provided for every month during this period except December, when the digestibility of the forage consumed is above 60%. Dairy meal concentrate (15% protein, 3% fat, 9% fibre, 1.5% calcium and 0.6% phosphorus) is the supplement considered. It has a metabolized energy content of 12.5 MJ/kg of DM, and presently costs P 190/t ex-Lobatse. However, this concentrate is taken as an example only, and the exact type, quality characteristics and availability of supplements in the different locations which might be considered for dairy development will have to be assessed before such projects are implemented.

Table 5. Combinations of milk offtake and supplementation levels for the 24 simulation experiments ^a.

Milk offtake b (% of total production)	Supplementation level of lactating cows (kg head/d)
	0	0.5	1.5	2.5	5.0	7.5
0	1	2	3	4	5	6
20	7	8	9	10	11	12
40	13	14	15	16	17	18
60	19	20	21	22	23	24

^a Experiment 1 is the baseline run analysed in Section 4 (i.e. no milk offtake and no supplementation).
^b For human consumption.

5.2 Comparative simulated performance

5.2.1 Fertility
5.2.2 Mortality
5.2.3 Animal growth
5.2.4 Feed inputs and milk and meat offtake
5.2.5 Herd viability
5.2.6 Overall comparative performance on the basis of energetic efficiency

The simulated performance of the two genotypes under alternative production regimes can now be considered. This comparison is made on the basis of individual production traits, before an overall comparison is made on the basis of energetic efficiency.

5.2.1 Fertility

The effect of supplementation on herd reproductive performance, as measured by annual calving rates, is presented in Table 6. Figure 6 provides a graphical representation of this effect for the two extreme milk offtake rates considered.

The simulated reproductive performance of the ST genotype is clearly higher than that of T, by 2 to 6 percentage points depending on the milking policy in effect and the level of supplementation provided. At low levels of supplementation, increasing the milk offtake rate from 0 to 60% results in a reduction of calving rates by about half a percentage point for T and about one and half percentage points for ST ¹⁸. As supplementation of lactating cows increases from 0 to 7.5 kg/head/d, calving rates increase up to a point, reaching a maximum at about 2.5 - 3 kg for T and 3 - 5 kg for ST animals, and decline there-after. This increase amounts to 1.7 - 2.3% for T and 2.2 - 15.5% for ST, depending on the milk offtake rate. The higher the milk offtake rate (putting cows under greater stress), the higher the relative increase in calving rates as a result of supplementation ¹⁹.

¹⁸ The cause of this reduction in reproductive performance is the extended lactation period from 7 months to 9 and 10 months for T and ST respectively, when milking takes place. The higher milk potential of ST cows implies higher energy demands during this extended lactation period, resulting in greater weight losses and thus greater reduction in their reproductive performance.

¹⁹ Trials were carried out at Musi to find out the effects of supplementary feeding on the reproductive performance of breeding females (APRU, 1981) An average improvement in conception of 7.1 % above the control group was reported. The results also showed that stressed cows had a much higher response in conception to supplementation of 14.4%. The simulated effects reported here are, in general terms, in line with these results.

Table 6. Simulated average annual calving rates (%) under various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	88.48	89.03	89.81	90.00	89.08	87.50
	ST	91.85	92.57	93.52	93.90	93.75	93.05
20	T	88.40	88.93	89.76	89.92	89.23	87.60
	ST	90.58	91.63	92.72	93.67	93.78	93.23
40	T	88.15	88.71	89.62	89.87	89.34	87.72
	ST	90.40	91.47	92.34	93.36	93.84	93.46
60	T	88.02	88.50	89.36	89.84	89.52	87.75
	ST	90.16	90.76	92.10	93.05	94.05	93.65
Average	T	88.26	88.79	89.64	89.91	89.29	87.64
	ST	90.75	91.60	92.67	93.50	93.86	93.35

Figure 6. Simulated effect of supplementation on cow reproductive performance for two milk offtake

Table 7. Simulated average calf survival rates and average annual cow mortality rates for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
		Average calf survival rate to 1 year (%)
0	T	94.31	95.08	95.59	95.97	96.08	96.44
	ST	94.46	95.39	96.01	95.86	95,94	96.16
20	T	94.26	94.95	95.57	95.92	95.95	95.95
	ST	95.33	95.33	95.83	95.97	96.33	96.35
40	T	92.04	92.01	93.54	94.08	94.68	94.73
	ST	93.83	95.06	95.80	95.84	96.29	96.30
60	T	68.94	72.16	73.79	74.93	76.28	77.72
	ST	85.12	87.00	91.53	92.52	93.91	94.48
		Average calf survival rate to 2 years (%)
0	T	92.78	93.72	94.00	94.52	94.60	95.07
	ST	93.00	94.09	94.85	94.92	95.00	95.35
20	T	92.49	92.94	93.35	93.84	93.77	93.90
	ST	93.71	93.89	94.12	94.67	94.79	95.10
40	T	89.02	88.66	90.38	90.46	91.10	91.13
	ST	91.17	92.92	94.08	93.90	94.51	94.25
60	T	64.59	67.91	69.72	71.18	72.28	73.89
	ST	81.01	83.54	87.28	87.38	88.46	88.96
		Average annual cow mortality rate (%)
0	T	1.20	0.56	0.53	0.37	0.30	0.36
	ST	2.00	1.34	0.52	0.46	0.31	0.28
20	T	1.22	0.59	0.40	0.27	0.27	0.27
	ST	3.00	1.87	1.36	0.41	0.39	0.39
40	T	1.30	0.66	0.43	0.20	0.30	0.34
	ST	3.02	1.81	1.24	0.51	0.49	0.38
60	T	1.46	0.64	0.38	0.24	0.27	0.34
	ST	3.20	2.54	1.30	0.59	0.43	0.43

Maximum calving rates occur at the optimum liveweights for reproductive performance. The simulated optima are at about 600 kg and 630 kg liveweight for T and ST cows respectively. The model assumes that cows with liveweights above these levels will have a reduced reproductive performance. The outlet of increased energy intake through supplementation is first in increased milk yields, but once the milk yield potential is achieved the residual energy is absorbed in liveweight gains. The lower the milk yield potential and milk offtake rate, the higher this residual energy for liveweight gain. Thus, it should be expected that, as supplementation levels increase, optimum liveweight for reproductive performance is reached for T cows before ST cows and, within a genotype, at lower milk offtake rates. This occurred in the simulation and is demonstrated in Figure 6.

Figure 7. Simulated effect on calf survival for various milk offtake rates.

5.2.2 Mortality

The simulated effect of different supplementation levels on mortality is shown in Table 7, and in Figures 7 and 8 for calves and cows respectively. When up to 20% of the milk produced is removed for human consumption, the effect on the survival of animals to 2 years is relatively small. However, over that level the impact on calf mortality is exponential, reducing the survival rate of calves to 2 years by about 21 to 28 and 6.4 to 12 percentage points for T and ST calves respectively, depending on the level of supplementation provided.

Higher milk offtake rates also result in increased cow mortality, particularly for ST cows at low supplementation levels. At a supplementation level below 1.5 kg/head/d, the mortality rate of ST cows almost doubles as milk offtake rates increase from 0 to 60%.

The effect of milking on mortality can be shown more clearly in marginal terms. For example, the survival rate of T calves to 2 years is reduced by 0.02 to 0.06 percentage points (depending on the level of supplementation) for each additional percentage point of milk offtake, as milk offtake increases from 0 to 20%. However, as milk offtake increases from 40 to 60%, the same rate is reduced by 0.86 to 1.22 percentage points for each additional percentage point of milk offtake. Thus the survival rate of T calves to 2 years decreases almost 25 times faster when over 40% of milk is removed than when milk offtake is from 0 to 20%.

Figure 8. Simulated effect of supplementation on cow mortality for two milk offtake rates.

As seen from Figures 7 and 8, supplementation substantially improves the simulated survival rates of both calves and cows. Almost all this improvement takes place as supplementation increases from 0 to 2.5 kg/head/d. After that level the improvement is minimal.

The much higher milk potential of ST as compared with T cows is reflected in the better survival rates of ST calves to 2 years. For example, when the 60% milk offtake policy applies, the survival rate of ST calves is higher than that of T calves by as much as 16 percentage points. However, this substantial increase in ST calf survival rates is not achieved without cost. As might be expected, ceteris paribus the higher milk potential of ST cows must result in an overall lower body condition as compared with T cows, and therefore in higher cow mortality rates. The simulation results support this hypothesis. As shown in Figure 8, the mortalities for ST are markedly higher than for T cows at supplementation levels up to 2.5 kg/head/d. Above that level the difference between the two genotypes is insignificant.

Table 8. Simulated growth to 7 and 18 months (average for males and females) for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
		Average 7-month weaning weight (kg)
0	T	190.5	191.1	191.6	191.8	191.9	192.0
	ST	218.2	221.2	223.7	224.5	224.6	224.7
20	T	176.8	176.8	177.6	178.2	178.4	178.7
	ST	204.0	209.9	214.1	215.4	215.9	216.0
40	T	156.7	157.6	158.6	159.6	160.0	160.2
	ST	187.1	192.8	197.8	199.8	200.6	201.1
60	T	117.8	119.1	120.2	120.9	122.4	123.0
	ST	153.4	158.5	164.5	167.0	170.2	171.6
		Average 18-month weight (kg)
0	T	322.2	322.4	322.5	322.6	322.8	323.0
	ST	350.5	351.7	352.7	353.1	353.1	353.2
20	T	311.0	311.0	311.1	311.2	311.2	311.4
	ST	336.7	343.0	344.7	345.6	345.6	345.7
40	T	294.8	295.0	295.0	295.8	295.8	295.9
	ST	323.5	329.1	331.6	332.6	332.9	332.9
60	T	268.6	269.4	269.8	269.9	270.0	270.2
	ST	298.7	300.5	305.0	305.7	307.8	309.0

5.2.3 Animal growth

Average 7-month weaning weights and 18-month weights for both males and females are shown in Table 8 and Figures 9 and 10. The milk offtake rate has a substantial effect on 7-month and 18-month weights at any level of supplementation. Weaning weights are reduced by about 70 kg and 60 kg for T and ST calves respectively, as milk offtake rates increase from 0 to 60%. Similarly, 18-month weights are reduced by about 53 kg and 47 kg for T and ST animals respectively, again as milk offtake rates increase from 0 to 60%. As expected, because of the lower milk potential of T relative to ST cows, the effect of milking on calf growth is more severe in the case of T calves ²⁰.

²⁰ Weaning and 18-month weights for T animals at a 60% milk offtake rate, expressed as a percentage of corresponding weights when no milk is removed, amount to about 63% and 83% respectively. The corresponding figures for ST animals are much higher, about 74% and 87% respectively.

The marginal effect of milking on calf growth increases as higher milk offtake rates apply. As the milk offtake rate increases from 0 to 20%, weaning weights decrease by about 0.5 to 0.7 kg (depending on the genotype and the supplementation level) for each additional percentage point of milk removed. On the other hand, the marginal decreases in weaning weights when the milk offtake rate increases from 40 to 60% are about 1.4 kg to 1.9 kg (again depending on the genotype and the supplementation level). On average, weaning weights when the milk offtake rate is over 40% thus decrease almost three times faster than when it is 0 to 20%.

Supplementation has relatively little effect on weaning and 18-month weights, particularly for T calves and for both genotypes when a low milk offtake rate applies (below 20%). Supplementation has a substantial effect at higher milk offtake rates, particularly on the growth of ST calves, due to the higher milk potential of their dams, which are capable of realizing a higher fraction of their potential at higher supplementation levels. At a 60% milk offtake rate weaning and 18-month weights of ST calves increase by about 18 kg and 10 kg, compared with about 5 kg and 2 kg for T calves, as supplementation rises from 0 to 7.5 kg/head/d. Again, as was also observed for the effect of supplementation on fertility and mortality, the marginal contribution of supplementation to calf growth diminishes at higher supplementation levels.

Figure 9. Simulated effect of supplementation on weaning weights for various milk offtake rates.

5.2.4 Feed inputs and milk and meat offtake

Figure 4 summarized the inputs and outputs of the livestock production system under study. The production process started with an initial herd which evolves into a final herd at the end of a 15-year simulation period, with intermediate inputs and outputs in the form of the feed consumed and the milk and meat produced ²¹.

²¹ The only inputs quantified by the simulation model are those of feed requirements. The production alternatives considered here would in addition require fixed expenditures for infrastructure (e.g. equipment for feeding and milking), as well as variable inputs such as labour. Quantification of these other "less variable" inputs does not necessarily require the use of a model and can be done straightforwardly.

These inputs and outputs are presented in Tables 9.10.11. 12 and 13 for all the 24 production alternatives considered. There is some increase in the total quantity of forage consumed for higher levels of supplementation and some decrease for higher levels of milk offtake. However, these differences are not the result of different consumption levels per animal. The explanation lies in the size of the whole herd under the various production alternatives (see Appendix Table A.7). The average forage consumption for the average animal in the system studied amounts to about 2850 kg/year or about 7.9 kg/d.

Figure 10. Simulated effect of supplementation on 18-month progeny weights for various milk offtake

Available quantities of supplements per head per day are maximum levels which a lactating cow has at its disposal for consumption. Whether these maximum quantities are totally consumed depends on the energy outlets that lactating cows have. As Table 10 shows, total consumption of supplements by ST cows is higher than for T cows, reflecting the higher energy outlets of ST cows due to their higher milk yield potential. At high supplementation levels T cows, after satisfying their energy demands for milk production and increasing liveweight to the extent allowed by their genetic potential, do not have any other use for the extra supplements available to them. Thus the saturation point for T cows is somewhere between 2.5 and 5.0 kg/head/d, whereas the corresponding saturation point for ST cows is somewhere between 5.0 and 7.5 kg/head/d.

This observation is made on the basis of a comparison between total annual supplement consumption of the two genotypes at levels of 2.5, 5.0 and 7.5 kg/head/d. ST consumption levels increase by about 100% as the available quantity of supplements increases from 2.5 to 5.0 kg/head/d, implying that all available supplements are consumed. However, T consumption levels increase by about 66%, implying that T animals reach a saturation point at about 4.2 kg/head/d. Similarly, for ST cows consumption levels increase by about 40% as the available quantity of supplements increases from 5.0 to 7.5 kg/head/d, implying that they reach a saturation point at about 6.9 kg/head/d ²².

²² Supplement utilization is not uniform throughout the year, as shown in the example in Appendix Table A.8. The months of heaviest use are October, November and February to May. Utilization during December and January is relatively small due to the usually very high quality of forage on offer at that time.

The annual total milk and liveweight offtake under the different production alternatives considered are presented in Tables 11 and 12. The effect of supplementation in increasing milk yields and consequently milk offtake is evident. The increase for T cows is relatively small, reflecting the low milk yield potential of this genotype. However, for ST cows milk offtake increases by almost 50% as the quantity of available supplements increases from 0 to 7.5 kg/head/d. An overall comparison of the two genotypes confirms the superiority of ST cows as milk producers. The greatest difference between the two genotypes occurs at high supplementation levels, when ST cows are able to achieve their higher potential milk yields.

Table 9. Simulated average annual forage consumption (t) by the herds for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Herd genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	344.7	353.7	356.6	359.6	355.1	352.6
	ST	355.7	374.2	385.1	388.2	387.5	385.3
20	T	333.7	347.4	354.3	355.1	349.9	348.3
	ST	358.9	356.3	380.2	387.8	390.6	390.7
40	T	316.3	331.6	341.2	343.6	341.4	338.5
	ST	344.0	347.3	367.3	380.1	387.2	384.9
60	T	265.9	282.0	287.8	294.8	298.4	297.7
	ST	303.6	324.0	342.0	359.7	366.0	368.8

Table 10. Simulated average annual supplement consumption (kg) by the herds for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Herd genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	0	3339	10127	16730	25804	29978
	ST	0	3381	10552	16864	34585	43343
20	T	0	3379	10377	17067	26556	31199
	ST	0	3295	10524	17909	35953	49779
40	T	0	3332	10270	17009	26811	31510
	ST	0	3242	10280	17691	36041	50337
60	T	0	3273	9942	16719	26913	31242
	ST	0	3279	10136	17748	35880	50503

Table 11. Simulated annual milk offtake (kg) from the herds for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Herd genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	0	0	0	0	0	0
	ST	0	0	0	0	0	0
20	T	13057	13903	14477	14608	14620	14640
	ST	26024	28668	33502	36593	38181	38483
40	T	21273	22986	23927	24066	24107	24126
	ST	38156	41194	47758	52253	55498	55693
60	T	29839	31735	32813	33083	33187	33197
	ST	48729	55014	62261	68763	72582	72869

Table 12. Simulated average annual liveweight offtake (kg) from the herds for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Herd genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	14485	15046	15327	15582	15620	15650
	ST	15162	15783	16370	17050	17180	17245
20	T	13902	14471	14869	15012	15097	15137
	ST	14840	14857	15820	16363	16793	16943
40	T	12878	13438	14048	14317	14384	14410
	ST	13846	14347	15051	15757	16378	16455
60	T	8831	9809	10331	10735	11094	11218
	ST	11343	12177	13446	14094	14736	15089

Total annual milk offtake figures are not proportional to the corresponding milk offtake rates, as might be expected. As noted earlier, higher milk offtake rates are associated with lower reproductive rates, higher mortality rates, and generally lower liveweights of lactating cows. The combined effect of all these factors is smaller average breeding herds (see Appendix Table A.7) and lower milk yields per lactating cow at higher milk offtake rates.

Higher milk offtake rates are directly reflected in much lower liveweight offtakes, particularly at low levels of supplementation, as shown in Table 12. When no supplementation is in effect, liveweight offtake decreases by almost 40% for T and 26% for ST animals as the milk offtake rate increases from 0 to 60%. At high supplementation levels, the effect of milking is still high for T (28% reduction) but very small (4% reduction) for ST animals. This is again the result of the low milk yield potential of T cows compared with ST. Regardless of the quantity of supplements available, when 60% of the milk is removed the residual milk available to calves from T cows is inadequate. As seen in Sections 5.2.2 and 5.2.3, this results in both high calf mortalities and slower growth, the combined effects of which are low liveweight offtake levels.

Table 13. Simulated average annual changes in herd biomass (kg) for various supplementation levels and milk offtake rates ^a.

Milk offtake rate (%)	Herd genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	513	617	752	825	915	928
	ST	418	637	825	976	1173	1263
20	T	402	558	692	776	851	864
	ST	249	392	621	760	1024	1141
40	T	232	407	594	681	760	758
	ST	128	248	537	684	961	1066
60	T	10	78	198	412	536	516
	ST	-179	78	267	498	761	894

^a Compared with baseline herd at the beginning of the 15-year simulation.

The above results highlight the fact that a thorough comparison between the two genotypes and between the different production alternatives requires the simultaneous consideration of both milk and meat output. This simultaneous consideration will be undertaken in Section 5.2.6 in terms of energetic efficiency, and finally in Section 6, where the economic trade-offs between these outputs for the various production alternatives considered are analysed.

5.2.5 Herd viability

In addition to the level of outputs from the system, reflected in the quantities of milk and meat produced, the desirability of different policies must be examined within the context of long-term herd viability. For example, although a policy of high milk offtake might be associated with a higher overall income, it may also increase the probability of system failure to unacceptable levels.

In systems where forage on offer varies markedly from year to year, there is always a probability (however small) of an extended dry season occurring for two or more years running. Milk yields drop substantially during such periods of drought, there is a higher than usual calf mortality and, depending on the length of the drought period, the consequences for the whole herd can be catastrophic. Management will usually react to the prospect of a catastrophe by selectively disposing of the less productive animals and perhaps by strategic supplementation of the remaining breeding herd. Such a policy ("drought policy") is available within the general management options of this simulation model.

The experiments presented so far were conducted without any drought policy in effect, so that the impact of nutritional stress is reflected directly in the performance of the different production alternatives considered. However, the accounting part of the model records the incidence of nutritional stress, the occurrence of which is determined at the beginning of each month of simulation and is defined as a situation in which the average liveweight condition of the whole herd is very low (e.g. liveweights are below 300 kg and 323 kg for mature T and ST females respectively) and the quality of forage on offer for that month is below the level sufficient for liveweight maintenance. Such situations imply continuation of liveweight losses for the whole herd for that month with, in turn, an expected increase in mortality.

The average intervals between severe nutritional stress situations are presented in Table 14 and Figure 11. Out of the 24 experiments conducted for each genotype, 3 for T and 5 for ST herds proved to be catastrophic: in other words, all animals in the herd died of starvation (indicated by an asterisk in Table 14). These catastrophies took place during replication 6, when a sequence of 3 consecutive below average years occurred (see Appendix Table A.6).

In general, severe nutritional stress situations, as defined earlier, occur more frequently at higher milk offtake rates and also more frequently for the ST genotype. For example, when no supplementation is given the frequency of nutritional stress-in the T system increases from once every 16.5 years to once every 4.5 years as milk offtake rates increase from 0 to 60%. The corresponding frequencies for the ST genotype are 12.5 years and 2.7 years. Although not shown here, the severity of nutritional stress, as measured by the quantities of strategic supplements that would have been required to alleviate its consequences, is higher at higher milk offtake rates and also higher for the ST genotype.

Supplementation of lactating cows substantially alleviates nutritional stress by reducing its frequency at any one milk offtake rate. At the maximum supplementation rate of 7.5 kg/head/d, nutritional stress did not occur at any milk offtake level with either genotype.

The economics of strategic supplementation is itself a topic warranting a separate study and is not covered here. However, the above analysis was undertaken to gain an appreciation of the long-term consequences of different intervention policies within the context of a viable production system.

5.2.6 Overall comparative performance on the basis of energetic efficiency

So far the performance comparisons of the different production alternatives considered in this analysis have been based on individual measures of performance, namely herd reproduction, mortality, animal growth, milk and meat output, herd viability and input requirements. Ranking of production alternatives on the basis of single measures of performance is not feasible, as the rank of a given alternative depends on the criterion used. An overall performance index is thus required.

Table 14. Simulated average interval (years) between severe nutritional stress situations for various supplementation levels and milk offtake rates ^a.

Milk offtake rate (%)	Genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	16.5	19.0	45.0	300.0	-	-
	ST	12.5	14.0	37.0	150.0	-	-
20	T	12.5	13.5	25.0	150.0	-	-
	ST	6.5*	7.5	13.0	18.0	300.0	-
40	T	8.0*	9.0	14.0	37.0	-	-
	ST	5.5*	6.0*	7.5	13.5	75.0	-
60	T	4.5*	5.5*	9.5	21.0	-	-
	ST	2.7*	3.0*	4.0	5.5	20.0	-

^a The cases indicated by an asterisk were catastrophic; that is, an unfavourable sequence of below average years occurred, during which all animals in the simulated herd died of starvation.

In the long term, intermediate measures of performance such as fertility, mortality and animal growth are directly reflected in the overall outputs from the system, i.e. milk and meat offtake and the capital value of the herd at the end of the period. For the purposes of constructing an overall measure the different production alternatives can be evaluated on the basis of outputs and corresponding inputs. Tables 9, 10, 11, 12 and 13 presented annual inputs and outputs of the production process and also the net change in the herd biomass (expressed in annual terms) over the 15-year simulation period. These figures are now used as the basis for an overall comparison between the different production alternatives.

The approach, in the construction of an overall performance index, is to compare the total outputs from the system with the total inputs. In order to sum up the individual components of inputs and outputs, these must be expressed in the same units of measurement. On the input side the quantities of forage and supplements consumed annually are expressed in MJ of metabolizable energy. Similarly, on the output side the annual milk and meat offtakes and the annual change in herd biomass are expressed in MJ on the basis of their energy content.

Formally, define:

f = quantity of forage consumed annually in t (Table 9);

x = quantity of supplements consumed annually in kg (Table 10);

e_f = average metabolizable energy (MJ/t of forage consumed); based on the values of Table 1, the average digestibility of the forage of the five year types (weighted by their respective probabilities of occurrence) is 0.50; further, taking the average energy content of forage as 18 MJ/kg and the ratio of metabolizable to digestible energy as 0.82, the average metabolizable energy content of forage equals 7380 MJ/t;

e_x = metabolizable energy of the supplements consumed, which equals 12.5 MJ/kg;

q₁ = annual quantity of milk offtake in kg (Table 11);

q₂ = annual quantity of liveweight offtake in kg (Table 12);

D Q = change in total herd biomass (kg) for the whole 15-year period expressed annually (Table 13);

e₁ = net energy content of milk. As discussed in Section 3.4, this equals 3.5 MJ/kg and 3.3 MJ/kg for T and ST milk respectively;

e₂ = net energy released from the mobilization of body tissues; assuming 20 MJ/kg of body weight and a coefficient of efficiency for its utilization in different body functions of 0.82, the result is a net energy content of 16.4 MJ/kg of body weight.

Based on the above, the energetic efficiency of the different production alternatives considered can be obtained from the relationship

where c is the energy equivalent of the total output expressed as a percentage of the total metabolizable energy utilized by the production system.

Figure 11. Simulated effect of supplementation on the incidence of severe nutritional stress for various milk offtake rates.

Table 15 and Figure 12 present the energetic efficiencies obtained using the above relationship for the various production alternatives considered. For both genotypes energetic efficiency increases monotonically for higher milk offtake levels at any given level of supplementation. This implies that at higher milk offtake rates the energy loss from lower liveweight offtake is less than the increase in energy output in the form of milk. It is clear from the values of Table 15 that the increase in total energy output from higher milk offtake rates is in favour of the ST genotype. At a 60% milk offtake rate the energetic efficiency of the ST genotype is higher than that of the T genotype by as much as 3 percentage points. At the other extreme, ST cows are marginally inferior to T in a beef production system (i.e. no milking).

Table 15. Simulated herd level energetic efficiency (%) for various supplementation levels and milk offtake rates.

Milk offtake rate (%)	Genotype	Supplementation level (kg/head/d)
		0.0	0.5	1.5	2.5	5.0	7.5
0	T	9.67	9.65	9.56	9.40	9.21	9.13
	ST	9.73	9.60	9.48	9.35	9.14	8.97
20	T	11.38	11.33	11.15	10.94	10.73	10.59
	ST	12.59	12.91	12.94	13.01	12.55	12.08
40	T	12.40	12.36	12.24	12.01	11.66	11.52
	ST	13.99	14.41	14.56	14.61	14.13	13.58
60	T	12.71	12.88	12.79	12.52	12.09	11.93
	ST	15.35	15.73	16.20	16.21	15.67	14.99

The energetic efficiency is equally sensitive to the level of supplements provided. Except in the case of the 60% milk offtake rate, the maximum energetic efficiency for T cows occurs at zero supplementation. Even in this exceptional case, the optimum supplementation level on the basis of energetic efficiency, is only 0.5 kg/head/d. For the ST genotype maximum energetic efficiencies are achieved at a supplementation level of 2.5 kg/head/d ²³. Providing supplements above that level decreases energetic efficiency, such that the percentage increase in total energy output is less than the corresponding percentage increase in the level of supplements consumed. Whether this energetically efficient supplementation level is also economically efficient depends on the prevailing relative prices between meat, milk and supplements. A comparison between the different production strategies on the basis of economic efficiency is the topic of the following section.

²³ For both genotypes the energetic efficiency optima occur well below their saturation levels of supplement consumption (4.2 and 6.9 kg/head/d) as obtained in Section 5.2.4.

Figure 12. Simulated effect of supplementation on energetic efficiency for various milk offtake rates.