Calf mortality
Mortality rates in relation to genetic improvement
Cow mortality
Causes of death
Mortality data based on 516 calves born over a seven year period were available. The significance environmental influences on pre-weaning mortality (subdivided into two periods, birth to 3 days and 3 days to 6 months) and post-weaning mortality from 6 months to 12 months of age are indicated in Table 33.
Table 33. Analysis of variance of mortality rates
|
Source |
d.f. |
Mean squares × 103 |
|||
|
B - 3d |
3d - 6m |
B - 6m |
6m - 12m |
||
|
Origin |
1 |
13 |
92 |
36 |
11 |
|
Year of birth |
6 |
19 |
211* |
224 |
28 |
|
Month of birth |
11 |
17 |
116 |
106 |
10 |
|
Parturition number |
3 |
20 |
183 |
244 |
3 |
|
Sex |
1 |
21 |
45 |
4 |
41 |
|
Remainder |
493 |
23 |
90 |
107 |
19 |
* = P< 0.05
The estimated least squares means for mortalities over these periods are laid out in Table 34.
Pre-weaning mortality. The mean mortality from birth to weaning at 6 months was 9.6%, with 2.9% occurring in the first 3 days of life and 6.7% from 3 days to 6 months. The only significant effect was that of year of calving on deaths from 3 days to 6 months. The effect of parturition number approached significance, calves born to heifers having a higher mortality rate than those born to subsequent parturitions.
Table 34. Estimated least squares means for mortality rates
|
Variable |
Mortality rates (%) |
|||
|
Birth - 3 days |
3 days - 6 months |
Birth - 6 months |
6 months - 12 months |
|
|
Overall mean |
2. 87 |
6.69 |
9.56 |
1.32 |
|
Origin |
||||
|
Foundation |
1.93 |
9.21 |
11.14 |
2.18 |
|
Born on centre |
3.82 |
4.16 |
7.98 |
0.45 |
|
Year of calving |
||||
|
1974 |
1.79 |
-2.67 |
-0.89 |
-0.38 |
|
1975 |
6.45 |
-1.00 |
5.44 |
-0.32 |
|
1976 |
1.69 |
3.88 |
5.57 |
-0.43 |
|
1977 |
0.94 |
9.21 |
10.15 |
-0.13 |
|
1978 |
3.31 |
4.75 |
8.06 |
5.15 |
|
1979 |
1.79 |
13.30 |
15.08 |
3.92 |
|
1980 |
4.16 |
19.36 |
23.52 |
1.41 |
|
Month of calving |
||||
|
January |
1.26 |
8.14 |
9.40 |
5.10 |
|
February |
7.65 |
2.63 |
10.28 |
-1.58 |
|
March |
0.03 |
-3.62 |
-3.59 |
2.49 |
|
April |
5.02 |
3.00 |
8.02 |
0.14 |
|
May |
2.83 |
6.30 |
9.13 |
1.36 |
|
June |
4.68 |
9.32 |
14.00 |
3.13 |
|
July |
2.38 |
9.35 |
11.73 |
1.39 |
|
August |
0.54 |
12.96 |
13.50 |
-0.20 |
|
September |
0.79 |
18.89 |
19.68 |
-0.29 |
|
October |
0.92 |
4.45 |
5.36 |
1.37 |
|
November |
4.84 |
1.65 |
6.50 |
-0.15 |
|
December |
3.53 |
7.20 |
10.73 |
3.06 |
|
Parturition number |
||||
|
1 |
2.79 |
13.21 |
16.00 |
0.63 |
|
2 |
1.70 |
4.02 |
5.72 |
1.28 |
|
3 |
1.83 |
4.78 |
6.61 |
0.99 |
|
4+ |
5.17 |
4.73 |
9.91 |
2.36 |
|
Sex |
||||
|
Male |
2.22 |
7.65 |
9.87 |
0.40 |
|
Female |
3.53 |
5.73 |
9.26 |
2.24 |
Post-weaning mortality. The mean mortality from 6 months to 12 months of age was 1.3%. There were no significant influences found in the analyses.
An appropriate definition of mortality rate in the context of genetic progress is the percentage of females that die before calving. Here applying the post-weaning mortality rate of 1.3% per 6 months, to the period to first calving, it is approximately 17%. The rearing proportion is the proportion of births that produce a heifer that survives and is fertile. The rearing proportion here is about 0.40. This means that once in 2.5 calvings a cow produces a heifer calf that will reach lactation in the herd. The average length of reproductive life is 5.5 calvings: therefore, approximately 45% of the females born are required as replacements to maintain herd size. High mortality rates are inconsistent with genetic progress, and when they can be reduced, a higher intensity of selection can be achieved.
Genetic improvement per unit of time is more important than per animal generation, and high mortality rates have a marked effect on generation interval. The average age at first calving is 39.8 months, and the average calving interval 16.2 months. To replace herself, a cow must calve 2.5 times; thus, the average generation interval is 80.3 months or 6.7 years, within the normally reported range for tropical indigenous breeds.
Data on cow mortality over the 6 years from 1975 to 1980 are presented in Table 35. The overall mortality rate was 3% per year.
Table 35. Cow mortality
|
Year |
Number of cows |
Number of deaths |
Cow mortality (%) |
|
1975 |
97 |
1 |
1.0 |
|
1976 |
121 |
3 |
2.5 |
|
1977 |
101 |
3 |
3.0 |
|
1978 |
102 |
5 |
4.9 |
|
1979 |
143 |
6 |
4.2 |
|
1980 |
151 |
3 |
2.0 |
|
Overall |
715 |
21 |
2.9 |
The main causes of mortality in cattle have been grouped into four classes: -diseases of the alimentary tract; other diseases; accidents; and unidentified causes and snake bites. Diseases of the alimentary tract includes all digestive problems such as gastrointestinal parasitism, "other diseases" includes skin problems, nutritional problems, trypanosomiasis and diseases of the respiratory systems. Over the period from birth to 2½ years, the proportions of deaths falling into each group are shown in Table 36.
Table 36. Mortality rate related to cause
|
Class |
% (birth to 2½ years) |
|
Alimentary |
20 |
|
Other diseases |
23 |
|
Accidents |
9 |
|
Unidentified |
48 |
