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The Effect of Age, Plane of Nutrition and Progesterone Pre-treatment on Lambing Rate in Nulliparous Females Bred During the Breeding Season
Replacement ewe lambs comprise approximately 30% of the breeding flock, and successful breeding of ewe lambs at 7-9 months of age can increase the lifetime productivity and profitability. However, lambing rate of ewe lambs is 35% lower than their adult flock mates. A 2x2x2 factorial arrangement of treatment study was used to determine the effect of age, plane of nutrition and progesterone pre-treatment on fertility of replacement ewes. Young (5.45 ± 0.09 months) and old (8.75 ± 0.19 months) females (N=110) were assigned to receive a low (n= 57; LP) or high (n= 53; HP) plane of nutrition (0.23 or 0.68 kg/day of a 15% CP, 65% TDN grain concentrate, respectively) and free access to average quality forage for two months prior to beginning of the breeding season. Prior to breeding, females in each nutrition plane were either pre-treated with progesterone containing CIDR devices for 5 days or introduced to rams only at a ram to female ratio of 1:25 for a 35 day breeding period. Analysis of variance was conducted using the GLM procedures of SAS, and the least squares means were computed on reproductive performance variables. Older ewe lambs had a higher estrus response (P=0.04:81.9 ± 7.5 vs. 61 ± 7.3%), conception rate (P= 0.04: 83.6 ± 7.8 vs. 60 ± 8.9%) and tended to have a higher pregnancy rate to first service (P= 0.09: 91.4 ± 7.5 vs. 74.3 ± 7.3%) than young females. Females receiving progesterone pre-treatment had a higher estrus response (P= 0.001: 81 ± 5.9 vs. 61 ± 6.4%), an earlier lambing day and shorter duration from ram introduction to lambing (P= 0.0021: 7.66 ± 1.27 vs. 13.2 ± 1.30 days; P= 0.0216: 151.7 ± 1.32 vs. 155.8 ± 1.38 days, respectively) and tended to have a higher pregnancy rate to second service (P= 0.06: 92.8± 4.9 vs. 80 ± 5.3%). Prolificacy and overall lambing rate was higher in females receiving the HP diet than those receiving the LP diet (P= <0.001: 1.57 ± 0.09 vs. 1.01 ± 0.08; P= 0.0272: 1.10 ± 0.12 vs. 0.71 ± 0.11 respectively), but prolificacy and lambing rate was not affected by age and progesterone pretreatment. In conclusion, a high plane of nutrition prior to breeding increased prolificacy and lambing rate in nulliparous ewes, but a similar effect was not observed by progesterone pre-treatment or by increasing age at breeding.
Keywords:
Ewe Lambs, Fertility, Breeding Performance, Lambing Rate, Prolificacy