Sheep body weights

Environmental and genetic effects on lamb weights at seven different ages
Ewe body weights

Environmental and genetic effects on lamb weights at seven different ages

Weights at birth, 2 and 4 months were available for 360 lambs, at 4, 6 and 8 months for 298 lambs and at 8, 10 and 12 months for 209 lambs, born in the years 1977 - 1980. Analyses of variance, laid out in Table 13, showed origin, year and month of birth, type of birth, sex, age of dam and individual dams to have significant effects on lamb body weight at different stages.

Table 13. Analyses of variance of weight at seven different ages using all records available to 12 months

The estimated least squares means for body weight at seven different ages are shown in Table 14.

Figure 3 illustrates the mean growth of all lambs from birth to 12 months. The coefficients of variation of weight at birth, 2, 4, 6, 8, 10 and 12 months were 19%, 22%, 22%, 20%, 18%, 19% and 19% respectively.

Table 14. Estimated least squares means for weight at seven different ages

Figure 3. Mean body weights from birth to 12 months.

Effect of year of birth

Table 13 indicates that significant year effects existed for weights at all ages. Lambs born in 1980 were significantly heavier at all ages from 2 months to 12 months than those born in the three other years, while lambs born in 1978 were significantly lighter. The weight differences between lambs born in these two extreme years at 2, 4, 6, 8, 10 and 12 months were 25%, 53%, 59%, 62%, 77% and 71% respectively of the mean weights.

Causes of variation between years in this environment can be annual rainfall affecting pasture availability, and the disease situation, changes in management techniques, genetic progress, etc. Correlations and regressions between the 4 year means for weight at each stage from 2 months to 12 months and the 4 year means for rainfall from birth to each stage are indicated in Table 15 (Mean birth date taken as July 1).

Table 15. Correlations between and regressions of weight (kg) at 6 ages and rainfall (mm) between birth and that age.

Age	Correlation	Regressions
2 months	-0.77	-0.003
4 months	-0.91	-0.009
6 months	-0.82	-0.011
8 months	-0.74	-0.014
10 months	-0.66	-0.017
12 months	-0.74	-0.018

Table 15 indicates strong negative correlations between weight at all stages and rainfall over the corresponding periods, ranging from 0.66 to 0.91. The regressions of weight on rainfall showed a reduction in weight of 3, 9, 11, 14, 17 and 18 gm for each additional 1 mm rainfall from birth to 2, 4, 6, 8, 10 and 12 months respectively. Table 8 shows the mortality rates from birth to weaning and weaning to 12 months for these four years. The correlation between mortality rate to weaning and weaning weight of survivors was -0.91; and between mortality rate to 12 months and weight at 12 months, -0.92. The weights of survivors at 4 months and 12 months were 142 gm and 237 gm less respectively for each addition-all 1% mortality that had occurred during these periods.

Effect of month of birth

Table 13 indicates that the effects associated with month of birth were significant at all ages except 2 and 12 months. Figure 4 shows the weights of animals in each of the month of birth groups, corrected for all other effects evaluated.

Figure 4. Effect of month of birth on body weight.

The effects of month of birth on later body weights are usually related to the stage in the wet and dry weather cycle at which an animal reaches a given age; and relative rankings of month of birth groups might be expected to change from stage to stage.

Correlations and regressions between the 12 monthly means for weight and the 12 monthly means for rainfall from birth are given in Table 16 for the four ages 4, 6, 8 and 10 months where month of birth effect is significant.

Table 16. Correlations between and regressions of weight (kg) at 4 ages and rainfall (mm) between birth and that age.

Age	Correlation	Regression
4 months	-0.10	-0.001
6 months	-0.29	-0.002
8 months	-0.39	-0.004
10 months	-0.46	-0.005

Table 16 indicates that the correlations and regressions between rainfall and weight at the 4, 6, 8 and 10 month stages were all negative but small.

Effect type of lambing

Table 13 indicates that type of lambing, single or twin, had a significant effect on all weights from birth to 8 months, twin lambs being 21%, 27%, 23%, 21%, 16%, 8% and 5% lighter than singles at birth, 2, 4, 6, 8, 10 and 12 months of age respectively.

Effect of sex

Table 13 indicates that the effect of sex was significant at all ages except 2 months. At birth, 2, 4, 6, 8, 10 and 12 months, males were 9%, 5%, 8%, 18%, 20%, 13% and 17% heavier than females.

Effect of dam parturition number

Table 13 indicates that the effect associated with dam parturition number was significant at the birth and 2-month stages only. At birth, lambs from second parturition dams were 30% heavier and lambs from third and later parturition dams 41% heavier, than lambs from first parturition dams. At 2-months of age the respective differences were 13% and 34%.

Repeatability of early lamb weights

The repeatabilities of lamb birth, 2 month and 4 month weights considered as a characteristic of the ewe are indicated in Table 17. Data are from 360 lambs born to 146 ewes.

Table 17. Repeatability of early lamb weights

Trait	Repeatability	s.e.
Birth weight	0.22	0.07
2 months weight	0.18	0.07
4 months weight	0.24	0.08

Phenotypic correlations between lamb body weights at different ages

The phenotypic correlations available between lamb weights are shown in Table 18. These were all positive, with correlations between birth and early weights being lower than those between all other later ages.

Table 18. Phenotypic correlations between lamb body weights

Trait	2 months	4 months	6 months	8 months	10 months	12 months
	weight	weight	weight	weight	weight	weight
Birth weight	.41	.29
2 months weight		.78
4 months weight			.84	.72
6 months weight				.86
8 months weight					.85	.72
10 months weight						.86

Ewe body weights

Weights, at lambing and when lamb weaned at 4 months of age, were available for 144 ewes, together with the birth and 4 month weights of their 360 lambs, produced in 316 parturitions. In order to facilitate analyses and obtain correlations between ewe and lamb weights, the ewe weights in the 44 lambings producing twins, were used twice in the analyses. The data referred to lambings over the four years 1977 - 1980. Analyses of variance laid out in Table 19, showed origin, year of lambing and individual ewes to have significant effects on weights at lambing, lamb weaning and their mean. Additionally, month of lambing had a significant effect on weight at lamb weaning and mean weight.

Table 19. Analyses of variance of weights of adult ewes

Source	d.f.	Mean squares × 102
		Weight at lambing	Weight at weaning	Mean weight
Origin	1	71424**	57971**	63299**
Ewes	145	1904**	2284**	1745**
Year of lambing	3	12261**	22386**	17876**
Month of lambing	10	682	2238**	696**
Type of lambing	1	566	865	357
Sex	1	199	56	246
Number of lambing/foundation	1	180	106	180
Number of lambing/born on centre	2	647	431	550
Remainder	195	475	516	269

**= P< .01.

The estimated least squares means for the three measures of ewe body weight are shown in Table 20.

The mean ewe body weight at lambing was 23.8 kg, and at weaning of lamb 23.3 kg, thus an average of 0.5 kg was lost during the 4 months suckling period.

The repeatabilities of ewe weights at parturition, at weaning of lambs 4 months later and the mean of the two, are indicated in Table 21.

Table 20. Estimated least squares means for weights of adult ewes (kgs)

Variable	Number	Weight at lambing	Weight at weaning	Mean weight
Overall mean	360	23.82	23.33	23.47
Origin
Foundation (1)	225	25.55	24.88	25.10
Born on centre (2)	135	22.09	21.77	21.85
Year of lambing
1977	45	22.94	21.53	22.39
1978	86	22.60	21. 04	21.60
1979	102	23.12	23.29	22.95
1980	127	26.62	27.46	26.96
Month of lambing
January	26	24.35	21.14	22.82
February	34	24.44	23.27	23.91
March	38	23.80	23.85	23.73
April	21	23.68	23.76	23.65
May	28	24.81	22.73	23.75
June	29	22.68	22.06	22.15
July	6	22.39	21.31	21.91
August	-	-	-	-
September	16	24.83	25.57	24.48
October	39	23.28	25.33	23.97
November	64	23.78	23.64	23.65
December	59	23.99	23.95	24.20
Type of lambing
Single	272	23.58	23.03	23.28
Twin	88	24.07	23.63	23.67
Sex
Male	198	23.72	23.28	23.36
Female	162	23.93	23.38	23.59
Number of lambing/origin 1
1-4	93	25.74	24.74	25.29
5+	132	25.37	25.03	24.92
Number of lambing/origin 2
1	32	21.18	21.05	21.04
2	39	22.59	22.21	22.35
3+	64	22.51	22.05	22.16

Table 21. Repeatability of ewe body weights

Trait	Repeatability	s.e.
Weight at parturition	0.57	0.05
Weight 4 months after parturition	0.60	0.05
Mean weight	0.70	0.04

These repeatability estimates were within the range normally reported for ewe body weights.

The phenotypic correlations between ewe weights and lamb preweaning weights are shown in Table 22.

Table 22. Phenotypic correlations between ewe and lamb weights

Trait	Lamb birth weight	Lamb 2 months weight	Lamb 4 months weight
Ewe weight at parturition	0.20	0.38	0.39
Ewe weight 4 months after parturition	0.06	0.27	0.32
Mean ewe weight	0.18	0.36	0.39

The correlation of ewe weight with lamb pre-weaning weights was about double that between ewe weight and lamb birth weight.