Environmental and genetic effects on calf weights
Cow body weights
Weights at birth, 3, 6 and 9 months were available for 403 calves; at 9, 12, 15 and 18 months for 306 calves; and at 18, 21, 24 and 30 months for 189 calves, born in the years 1974  1980, 1974  1979 and 1974  1978 respectively. Analyses of variance laid out in Table 37 show the significance of effects on calf weight at these different stages.
The estimated least squares means for body weight at the different stages are shown in Table 38.
Figure 7. Mean calf body weights from birth to 30 months.
Table 37. Analysis of variance of weights at different ages
Table 38. Estimated least squares means of weight from birth to 30 months (kgs)
Figure 7 illustrates mean growth of all calves from birth to 30 months of age. The coefficients of variation of weights at birth, 3, 6, 9, 12, 15, 18, 21, 24 and 30 months were 15%, 17%, 16%, 16%, 19%, 17%, 18%, 17%, 19% and 17% respectively.
Effect of year of birth
Table 37 indicates that significant year effects existed for weights at all ages except 15 months. The weight differences between extreme years at birth, 3, 6, 9, 12, 15, 18, 21, 24 and 30 months was 15%, 12%, 26%, 17%, 14%, 13%, 15%, 21%, 28% and 17% respectively of the mean weights.
Causes of variation between years in this environment can be annual rainfall affecting pasture availability, annual rainfall affecting the disease situation, changes in management techniques, genetic progress etc. Correlations and regressions between the 7 year means for weight at each stage from 3 months to 30 months and the 7 year means for rainfall from birth to each stage were all negative and nonsignificant.
To determine any linear trends in body weights from 1974 to 1978 for 21, 24 and 30 month weights; from 1974 to 1979 for 12, 15 and 18 month weights and from 1974 to 1980 for birth, 3, 6 and 9 month weights; the regression of the least squares constants on year of birth (represented as 1  5, 1  6 and 1  7) were calculated (Table 39).
Table 39. Correlations and regressions of the least squares constants for weight on year of birth
Trait 
Correlation 
Regression 
Birth 
0.65 
0.30 
3 month weight 
0.23 
0.30 
6 month weight 
0.51 
1.84 
9 month weight 
0.38 
1.48 
12 month weight 
0.45 
1.28 
15 month weight 
0.87 
2.67* 
18 month weight 
0.84 
4.40** 
21 month weight 
0.97 
8.91** 
24 month weight 
0.98 
13.09** 
30 month weight 
0.92 
8.99** 
*P = <0.05 **P =< 0.01
The regressions were significant from the 15 month stage onwards and indicate that weight for age increased at each stage by the amounts listed in Table 39, per year over the relevant period.
Effect of month of birth
Table 37 indicates that significant month of birth effects existed for weights at all ages except 12 and 24 months. Figure 8 shows the weights of animals in each of the month of birth groups, corrected for au other effects evaluated.
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. This situation is clearly illustrated in Figure 8.
Correlations and regressions between the 12 monthly means for weight and the 12 monthly means for rainfall from birth are given in Table 40 for the seven ages, 3, 6, 9, 15, 18, 21 and 30 months when the effect of month was significant.
Table 40. Correlations and regressions of weight (kg) at seven ages and rainfall (mm) between birth and that age.
Age 
Correlation 
Regression 
3 months 
0.84 
0.011 
6 months 
0.86 
0.019 
9 months 
0.68 
0.013 
15 months 
0.74 
0.019 
18 months 
0.80 
0.029 
21 months 
0.55 
0.028 
30 months 
0.79 
0.046 
Figure 8. Effect of month of birth on body weight
Table 40 indicates that the correlations and regressions were all positive and significant at these stages.
Effect of dam parturition number
Table 37 indicates that the effect associated with dam parturition number was significant at all ages from 3 months to 18 months. Table 38 shows that at 3, 6, 9, 12, 15 and 18 months calves from first parturition cows were 12%, 7%, 6%, 12%, 10% and 9% lighter than the mean weight of calves from the three other parturition groups.
Effect of sex
Table 37 indicates that the effect of sex was significant at all ages from birth to 30 months. At birth, 3, 6, 9, 12, 15, 18, 21, 24 and 30 months males were 5%, 4%, 7%, 11%, 10%, 11%, 14%, 15%, 12% and 8% heavier than females.
Repeatability of early calf weights
The repeatabilities of calf birth, 3 month and 6 month weights considered as a characteristic of the cow are indicated in Table 41. Data are from 403 calves born to 158 cows.
Table 41. Repeatability of early calf weights.
Trait 
Repeatability 
s.e. 
Birth weight 
0.15 
0.06 
3 month weight 
0.36 
0.06 
6 month weight 
0.33 
0.06 
Phenotypic correlations between calf weights at different ages
The phenotypic correlations available between calf weights are shown in Table 42. These were all positive with correlations between birth and early weights being lower than those between all other later ages.
Table 42. Phenotypic correlations between calf weights
Weights, at calving and when calf weaned at 6 months, were available for 103 individual cows over 165 parturitions. Only cows that had complete data on their linear measurements were used in this analysis so numbers were rather small. Analyses of variance laid out in Table 43 show the significance of effects on cow weight at the two stages.
Table 43. Analysis of variance of cow weights at calf birth and weaning (6 months)
Source 
d.f. 
Mean squares 

Calving 
Weaning 

Origin 
1 
7176* 
6425* 
Cows 
102 
1443** 
1364** 
Year of calving 
5 
648* 
654 
Month of calving 
11 
702** 
1131** 
Parturition calving number 
3 
325 
67 
Sex 
1 
321 
92 
Remainder 
41 
235 
349 
*=P< 0.05; **= P< 0.01.
The estimated least squares means for the two cow body weights are shown in Table 44.
Table 44. Estimated least squares mean of cow weights at calf birth and calf weaning at 6 months (kg)
Variable 
Number 
Weight at calf birth 
Weight at calf weaning 
Overall mean 
165 
232.26 
235.21 
Origin  
Foundation 
103 
220.36 
223.95 
Born on centre 
62 
244.15 
246.47 
Year of calving  
1975 
7 
209.82 
216.32 
1976 
26 
221.16 
229.54 
1977 
6 
216.58 
253.62 
1978 
59 
232.16 
226.43 
1979 
46 
235.72 
246.94 
1980 
21 
278.09 
238.40 
Month of calving  
January 
3 
265.70 
218.57 
February 
6 
211.70 
266.66 
March 
11 
221.01 
250.50 
April 
23 
212.18 
244.64 
May 
36 
221.74 
268.42 
June 
14 
224.10 
231.24 
July 
13 
220.88 
244.93 
August 
15 
217.56 
222.14 
September 
14 
240.59 
231.88 
October 
11 
244.47 
220.99 
November 
13 
221.88 
169.66 
December 
6 
285.27 
222.96 
Parturition number  
1 
48 
225.63 
222.96 
2 
49 
233.64 
231.58 
3 
34 
238.11 
239.84 
4+ 
34 
230.65 
246.45 
Sex of calf  
Male 
92 
229.44 
235.72 
Female 
73 
235.07 
233.70 
The mean cow weight at parturition was 232 kg and at weaning of calf, 235 kg, thus an average of 3 kg was gained during the 6 months suckling period.
The repeatability of cow weight at parturition was 0.78 ± 0.06 and at calf weaning 0.67 ± 0.08. The phenotypic correlation between cow weight at parturition and calf birth weight was 0.37.