Sheep reproductive performance

Introduction
Age at first lambing
Conception rate by month
Lambing interval
Litter size
Annual reproduction rate
Length of ewe productive life

Introduction

Reproductive performance is a trait of outstanding importance in sheep production enterprises, especially when meat production from young animals is the chief aim. In general the more intensive the meat production system, the more desirable the production of large numbers of young per breeding female. Under harsher conditions optimal production levels are achieved by maintaining reasonable levels of performance in the young, while limiting the drain on the resources of the breeding female. The most important components of reproductive performance in sheep are age at first lambing, length of lambing interval, litter size, and length of ewe productive life.

Age at first lambing

The mean age at first lambing for 115 females born on the station from 1974 to 1980 was 18.8 ± 0.8 months. The significance of environmental influences on age at first lambing over this period is indicated in Table 3.

Table 3. Analysis of variance of age at first lambing

Source	d.f.	Mean squares
Year of birth	6	33912
Month of birth	11	19460
Type of birth	1	30382
Remainder	96	18746

Year of birth, month of birth and type of birth had no significant effects on age at first lambing. The estimated least squares means for age at first lambing are laid out in Table 4. The coefficient of variation was 24.4%.

Table 4. Estimated least squares means for age at first lambing (days)

Variable	Number	Age at first lambing
Overall mean	115	575
Year of birth
1974	4	545
1975	16	572
1976	16	624
1977	31	546
1978	20	638
1979	21	622
1980	7	473
Month of birth
January	11	615
February	8	603
March	12	608
April	6	527
May	11	568
June	4	630
July	5	625
August	4	515
September	5	620
October	10	546
November	18	479
December	21	560
Type of birth
Single	96	551
Twin	19	598

Conception rate by month

The distribution of 663 conceptions calculated from the recorded lambing dates from 1975 to 1980 are indicated in Figure 2. The mean monthly rainfall over these 6 years is also shown.

Figure 2. Effect of month on conception rate.

55% of conceptions, or 11% per month, occurred during the main five month wet season from June to October, the remaining 45% or 6. 5% per month during the other seven months of the year.

Lambing interval

The mean lambing interval for 663 records from 1975 to 1980 was 307 ± 14 days. The significance of environmental and genetic influences on lambing interval over this 6 year period is indicated in Table 5.

The effects of origin of ewe, individual ewes, month of lambing and in the case of foundation ewes only, the lambing number, were significant. The estimated least squares means for lambing interval are laid out in Table 6.

Table 5. Analysis of variance of lambing interval

Source	d. f.	Mean squares
Origin	1	48441*
Ewes	190	7706**
Year of lambing	5	7409
Month of lambing	11	27767**
Type of lambing	1	1110
Number of lambing/foundation	1	43841**
Number of lambing/born on centre	2	5469
Remainder	451	5485

* = P< 0.05; ** = P< 0.01.

The effects of year of lambing were not important, but month of lambing had a significant effect on lambing interval. Females lambing in December, January, February and March had longer than average lambing intervals, while those lambing in all other months had shorter than average lambing intervals.

The monthly rainfall from 1975 to 1980 is illustrated in Table 1, the period December to April constituting the major dry season. The correlation between lambing interval and rainfall from one month before previous lambing date to one month before conception was - 0.51, while there was a 0.04 day decrease in lambing interval for each additional 1 mm rainfall.

In the case of foundation purchased ewes whose age was unknown, lambing intervals following their first four parturitions on the station were significantly shorter than those following their fifth and subsequent parturitions. In the case of ewes born on the centre the effect of parturition number was not significant.

The repeatability of lambing interval (i. e. the correlation between an individual's successive records) was obtained from the between and within ewes components of variance. The value of 0.11 ± 0.04 is within the range usually reported for this trait.

Table 6. Estimated least squares means for lambing interval (days)

Variable	Number	Lambing interval
Overall mean	663	307
Origin
Foundation (1)	474	296
Born on centre (2)	189	318
Year of lambing
1975	66	318
1976	122	319
1977	123	305
1978	146	313
1979	107	304
1980	99	280
Month of lambing
January	45	330
February	73	351
March	59	324
April	40	264
May	51	301
June	57	306
July	32	301
August	21	296
September	42	280
October	58	287
November	80	297
December	105	343
Type of lambing
Single	583	304
Twin	80	309
Number of lambing/origin 1
1-4	353	274
5+	121	316
Number of lambing/origin 2
1	77	305
2	55	313
3+	57	333

Litter size

Table 6 shows that the 663 parturitions recorded from 1975 to 1980 were made up of 583 singles and 80 twins, the mean litter size being 1.12.

Annual reproduction rate

The annual reproduction rate is calculated as the mean litter size × 365 ÷ lambing interval. Overall this was 1.33 lambs per breeding ewe per year.

Length of ewe productive life

As the first records of ewes produced on the station start with animals born in November 1974, it is yet too early to assess this trait. In August 1981, the first 40 ewes born between November 1974 and January 1977 had completed an average of 3.7 parturitions, with 17 or 42% still in production. The remaining 23 or 58% had died, been culled or lost. Thus until more information is collected it might be tentatively suggested with the present information that an average of 5 parturitions would be achieved per ewe. Using the mean lambing interval of 307 days, ewes would thus remain for an average of 4.2 years in productive life. This complete replacement of the ewe flock each 4. 2 years would represent a turnover of 24%. When age at first lambing is added to the productive life, the average ewe age on completion of productive life would be 5.8 years.