Cattle reproductive performance

Introduction
Age at first calving
Conception rate by month
Calving interval
Length of cow productive life

Introduction

Reproductive performance is the trait of outstanding importance in beef cattle enterprises, where if there is no calf, then there is no economic return. The best cows are clearly those that have their first calf at an early age, have minimum calving intervals, and live a long time. Thus the most important measures of reproductive performance in the female are age at first calving, length of calving interval, and length of cow productive life.

Age at first calving

The mean age at first calving for 101 females born on the station from 1973 to 1978 was 39.8 ± 0.8 months with a coefficient of variation of 14.1%.

The significance of environmental influences on age at first calving are indicated in Table 29, where year of birth had a major effect.

Table 29. Analysis of variance of age at first calving

Source	d.f.	Mean squares
Year of birth	5	356804**
Month of birth	11	12045
Remainder	84	28987

** = P< 0.01.

The estimated least squares means in Table 30 show that age at first calving ranged from 47.76 months for animals born in 1974 to 33.19 months for those born in 1978.

Table 30. Estimated least squares means for age at first calving (months)

Variable	Number	Age at first calving
Overall mean	101	39.78
Year of birth
1973	5	40.42
1974	25	47.76
1975	11	40.30
1976	16	41.73
1977	16	35.31
1978	28	33.19
Month of birth
January	2	41.26
February	4	40.99
March	6	42.53
April	11	39.01
May	17	39.83
June	10	38.55
July	10	40.22
August	12	41.42
September	6	37.37
October	9	38.68
November	10	39.60
December	4	37.95

Conception rate by month

The distribution of 357 conceptions calculated from the recorded calving dates from 1974 to 1979 are indicated in Figure 6. The mean monthly rainfall over the 6 years is also shown.

48% of conceptions, or 9.6% per month, occurred during the major five month wet season from June to October, the remaining 52% or 7.4% per month during the other 7 months of the year.

Figure 6. Effect of month on conception rate.

Calving interval

The mean calving interval for 357 records from 1974 to 1979 was 495 ± 16 days, with a coefficient of variation of 26%. The significance of environmental and genetic influences on calving interval are indicated in Table 31.

Table 31. Analysis of variance of calving interval

Source	d.f.	Mean squares ×10- 2
Origin	1	1820**
Year of calving	5	1732**
Month of calving	11	331
Parturition number	3	938**
Remainder	336	189

** = P< 0.01

The effects of origin of cow, year of calving and the parturition number were significant. The estimated least squares means for calving interval are laid out in Table 32.

Table 32. Estimated least squares means for calving interval (days)

Variable	Number	Calving interval
Overall mean	357	495.0
Origin
Foundation	286	539.7
Born on centre	72	450.2
Year of calving
1974	67	451.1
1975	36	538.8
1976	52	582.8
1977	44	419.2
1978	83	513.5
1979	75	464.5
Month of calving
January	20	518.7
February	11	502.1
March	14	465.4
April	28	510.0
May	68	462.5
June	36	489.1
July	26	552.0
August	30	555.5
September	30	487.7
October	41	458.5
November	35	482.3
December	18	455.8
Parturition number
1	135	547.1
2	97	525.1
3	67	449.4
4+	58	458.3

Year of calving had a significant effect on calving interval. Cows calving in 1974, 1977 and 1979 had a shorter-than-average calving interval, those calving in 1975, 1976 and 1978 had a longer-than-average. There was a positive correlation of 0.56 between calving interval and annual rainfall, with calving interval increasing by 0.2 days for each 1 mm increase in annual rainfall.

Parturition number had an important effect on calving interval. Calving interval decreased from 547 days between the first and second parturitions to 449 between the third and fourth and then increased slightly to 458 days for subsequent parturitions.

Overall, foundation purchased heifers had a significantly longer calving interval (540 days) than those born on the centre (450 days).

Length of cow productive life

As the oldest records refer to heifers purchased in 1972-73, it is too early to completely assess this trait. However in August 1981, the first 40 heifers purchased had averaged 4.3 parturitions, with 23 or 57% still in production. The remaining 17 or 43% had died or been culled. Thus until more information is available it is tentatively suggested that an average of 5.5 parturitions would be achieved per cow. Using the mean calving interval of 495 days, cows would thus remain for an average of 7.5 years in productive life. This complete replacement of the cow herd each 7.5 years would represent a turnover of 13.4%. When age at first calving is added to the productive life, the average cow age on completion of productive life would be 10.8 years.