Environmental factors affecting the growth and viability of crossbred sheep and goats on range grazing in Zimbabwe

C.T. Khombe

Makoholi Experiment Station
Private Bag 9182 Masvingo Zimbabwe

Introduction
Material and methods
Results
Discussion

Summary

Analyses were conducted on birth weight, weaning weight, pre-weaning growth and viability of 237 kids and 179 lambs born over a four-year period, 1981 to 1984. All kids were born in the dry season while lambs were born in the wet and dry season each year. In goats, year of birth was important (P<0.05) for both weaning weight and pre-weaning gain, kids born in 1983 having the lowest liveweights: year of birth had no significant effect on birth weight and viability. In sheep, year of birth had a large effect (P<0.05) on all traits studied. Lambs born in earlier years were lighter at birth but grew more rapidly until weaning than those born in later years. Differences in viability were significant among years, lambs born in 1981 and 1984 having the highest mortality while all those born in 1982 survived. In both species males grew faster and were heavier at weaning than females. Singles were heavier at birth and grew faster than twins. Differences in viability due to litter size at birth and sex of offspring were small. Goats were more prolific but had lighter offspring than sheep. Survival rates were similar for both species.

Introduction

Studies have been carried out on the productivity of cattle and sheep in Zimbabwe over the years and adoptions of recommendations from research findings have resulted in improvement in production (Donkin, 1974; Bembridge, 1975). However very little work has been carried out on goats although the demand for goat meat appears high (Cross, 1974). Recent studies suggest that goats outnumber sheep in communal areas (Mombeshora, Agyemang & Wilson, 1985). The potential economic advantages to smallholder farmers of small ruminants over cattle have been emphasized (McDowell & Bove, 1977).

The main objective of this study was to determine environmental factors affecting pre-weaning growth and viability of crossbred sheep and goats on range. The second objective was to provide comparative information on productivity of the two species.

Material and methods

The study was conducted at Makoholi Experiment Station in southern Zimbabwe. The station is at an altitude of 1200 m with a mean annual precipitation of 500 mm which falls between November and May. Consequently, the quality and quantity of herbage is seasonal.

The foundation goat herd comprised indigenous x Boer does. These were mated to purebred Boer bucks over the period of this study, 1981 to 1984, and replacement does were generated from the same herd. All goats were mated in the wet season to kid during the dry season (July and August).

Breeding ewes were German Merino x indigenous crosses which were mated to indigenous rams until 1982 and then to Dorper rams in subsequent years. There were two breeding seasons. All ewes were mated to lamb in the dry season (July and August) and those that failed to conceive were remated to lamb the following wet season (December and January).

All offspring were weighed within 12 hours of birth and at monthly intervals thereafter. They were weaned at five months of age and details of birth, weaning and deaths were recorded.

The data for each species were analysed separately to evaluate environmental influences on pre-weaning growth and viability. A further analysis was carried out to enable comparison between species. Only progeny born in the dry season were included in the latter analysis.

Results

GOATS

Year of birth had a significant (P<0.05) effect on both weaning weight and pre-weaning gain, kids born in 1983 having lower weights and gains than those born in the other years. Year of birth did not have a significant (P>0.05) influence on viability although the difference between the extreme years, 1983 and 1984, was large (13 per cent). Kids born as singles were 0.5 kg heavier at birth than those born as twins (P<0.05). No other litter size differences were important (Table 1).

Sex differences in birth weight and viability were not important, although males were heavier and experienced greater mortality than females. Males grew faster and were heavier at weaning (P<0.05).

SHEEP

Year of birth had a large effect on all traits studied. Lambs born in earlier years were lighter at birth than those born in later years but lambs born in earlier years grew more rapidly and were heavier at weaning (Table 2).

All lambs born in 1982 survived until weaning while those born in 1981 and 1984 had higher mortality than those born in 1983. Thus viability in sheep appeared more sensitive to the environmental influences of year than the same trait in goats.

Table 1. Environmental influences on pre-weaning growth (kg) and viability (per cent) of goats

Variable		Birth weight		Weaning weight		Pre-weaning gain		Viability
		n		n		n		n
Overall mean		237	3.0	128	15.4	128	12.2	237	86.1
Year of birth:	1981	24	2.9	-	-	-	-	24	83.3
	1982	50	3.1	19	17.8b	19	14.8b	50	88.0
	1983	76	2.9	51	12.5a	51	9.0a	76	78.9
	1984	87	3.0	58	17.1b	58	14.2b	87	92.0
Litter size:	single	63	3.3a	31	16.3	31	12.6	63	88.9
	twin	174	2.8b	97	15.1	97	12.1	174	85.0
Sex:	male	119	3.0	48	16.0a	48	12.9a	119	84.0
	female	118	2.9	80	14.4b	80	11.0b	118	88.1

Within variables means in the same column without a common superscript differ significantly (P < 0.05)

Table 2. Environmental influences on pre-weaning growth (kg) and viability (per cent) of sheep

Variable		Birth weight		Weaning weight		Pre-weaning gain		Viability
		n		n		n		n
Overall mean		179	3.3	96	23.6	96	20.1	179	87.9
Year of birth:	1981		44	3.0a	-	-	-	44	75.0b
	1982	33	3.2a	23	24.5a	23	21.5a	33	100.0a
	1983	55	3.5b	48	23.7a	48	20.0b	55	90.4a
	1984	47	3.5b	25	22.8b	25	19.2b	47	83.0b
Season of birth:	wet	81	3.4	45	25.6a	45	21.9a	81	87.7
	dry	98	3.3	51	21.9b	51	18.6b	98	88.8
Litter size:	single	83	3.4a	35	24.7a	35	20.6	83	91.7
	twin	96	3.3b	61	23.1b	61	19.8	96	84.3
Sex:	male	49	3.5a	29	24.7a	29	20.9a	49	81.6
	female	130	2.9b	67	21.4b	67	18.3b	130	90.8

Within variables means in the same column without a common superscript differ significantly (P < 0.05)

Lambs born in the wet season had similar birth weights and viability to those born in the dry season. However lambs born in the wet season grew more rapidly after birth and were significantly (P<0.05) heavier at weaning. Single lambs were heavier at birth and grew faster than twins (P<0.05). Singles also had higher survival rates but the difference was not significant.

A comparison of Table 1 with Table 2 shows that litter size differences in sheep were larger than those observed in goats. Male lambs were superior in growth but experienced 9.2 per cent higher mortality than females, a trend similar to that observed in goats.

SPECIES COMPARISON

Sheep were heavier than goats at birth. They also grew faster and were heavier at weaning. Preweaning survival rates were similar although goats were more prolific (Table 3).

Table 3. Comparison between species pre-weaning growth, viability and prolificacy

Parameter	Species
	Combined		Sheep		Goats
	n		n		n
Birth weight (kg)	318	3.1	81	3.4a	237	3.0b
Weaning weight (kg)	178	18.1	50	24.9a	128	15.4b
Pre-weaning gain (kg)	178	14.3	50	19.7a	128	12.2b
Viability (%)	318	86.2	81	86.4	237	86.1
Litter size (number)	416	1.37	179	1.16a	237	1.58b

Means in the same row without a common superscript differ significantly (P < 0.05)

Discussion

The observed increase in birth weight of lambs from 1981 to 1984 could have been due to the change in the sire breed used. Lambs born in 1981 and 1982 were sired by indigenous rams while those born in subsequent years were sired by the larger Dorper rams. Post-natal growth and pre-weaning viability declined from 1982 to 1984 probably as a result of progressive deterioration in forage due to drought conditions. Lambs born in the wet season had heavier weaning weights than their counterparts born in the dry season, although birth weights were similar. Thus birth weight appeared less sensitive to environmental influences of year and season of birth than subsequent growth. The lack of a significant difference in viability between seasons was not expected and could have been due to a reduced population of internal parasites as a result of the drought. Donkin (1974) and Asiedu (1983) found that lambs born in the wet season experienced higher mortality than those born in the dry season.

The significant effect of year of birth on pre-weaning growth rate and weaning weight in goats has also been reported by other workers (Moulick & Syrstad, 1970; Darokhan & Tomar, 1983; Mavrogenis, Constantinou & Louca, 1984). In the present study differences were mainly due to the poor performance of kids born in 1983. However goats appeared less sensitive to environmental effects of year of birth, suggesting their superior adaptation to harsh conditions (du Toit, 1972; McDowell & Bove, 1977; Mombeshora, Agyemang & Wilson, 1985).

The higher liveweight and growth rate of singles over twins in both species agrees with other studies (Darokhan & Tomar, 1983; Mavrogenis Constantinou & Louca, 1984). Viability among twins was lower than that among singles, also in agreement with published results (Highs & Jury, 1970; Smith, 1977). Hight & Jury (1970) suggested that physiological starvation, resulting in small and weak offspring at birth, was the major cause of death among twins.

Male offspring grew faster and were heavier at weaning than females, as has been shown by several workers (Smith, 1977; Arrowsmith & Ward, 1983; Mavrogenis, Constantinou & Louca, 1984). Sex differences were not significant for survival although males experienced higher mortality. Laster & Gregory (1973) observed that males, because of their superior birth weight, experienced higher mortality as a result of dystocia.

Goats were more prolific and were lighter in weight than sheep, as has been reported by several workers. (Animal Production Research Unit, 1980; Gall, 1981; Arrowsmith & Ward, 1983; Wilson & Durkin, 1983; Wilson, Peacock & Sayers, 1984). The higher liveweight in sheep could have been due partly to the high proportion of singles. This is supported by the fact that at birth differences between species were greater among twins (0.4 kg) than singles (0.1 kg) suggesting a species x litter size interaction.

Although sheep were 9.5 kg heavier at weaning than goats, the higher twinning rate in goats reduced the differences in weaner production per dam giving birth to 4 kg. Arrowsmith & Ward (1983) found that although sheep were heavier than goats, weaner production per dam mated was 17 per cent higher in goats due to their higher twinning rate and fertility. Therefore productivity of goats can be expected to be higher if conception rate proves to be higher than that among sheep in this environment.