Effects of Pre-Breeding Nutritional Management, Weight Changes, and Age on Ewe Lamb Fertility

The fertility of ewe lambs can be substantially lower than adult flock mates. This study evaluated the effects of pre-breeding nutritional management and weight changes on ewe lamb fertility. Dorset X Texel (DT; n = 126, 6.6 ± 0.1 months, 36.9 ± 0.7 kgs) and Katahdin (KD, n= 69, 4.8 ± 0.2 months, 23.3 ± 0.8 kgs) ewe lambs were given either a high (H) or low (L) level of grain supplementation (0.23 and .68 kgs, 14% crude protein ration/head/day, respectively) for 2 months prior to breeding, and body weights were recorded biweekly. Following synchronization of estrus, pregnancy diagnosis was conducted 1 month after ram introduction and lambing data was recorded. An ANOVA was conducted to determine the impact of main effects and interactions. The effect of age and weight gain on reproductive response within breed were also classified based on age (Old, O; Young, Y), level of weight gained (Light, Moderate, Heavy), and weight at breeding (Low (L), Medium (M), High (H)). Additionally, an ANCOVA was used to determine if breeding weight and weight gain differed in lambs experiencing a positive or negative reproductive outcome with breeding age and breeding weight as covariates. Overall fertility did not differ with breed or age. Ewe lambs on the high nutrition plane grew faster, gained more weight and had higher breeding weights (P < 0.05) than those on the low level of supplementation. The higher level of supplementation resulted in greater conception (81.1 ± 0.1% vs 53.8 ± 0.1%) and pregnancy rates (49.2 ± 0.1% vs 32.3 ± 0.1%) in DT but not in KD lambs (Breed X Supplementation, P = 0.01, P =0.03 and P < 0.01, respectively). The proportion of lambs that expressed estrus, became pregnant to the first service and lambed increased as breeding weight increased from L to M to H (P < 0.05; 36.5 ± 0.1, 66.1 ± 0.1, 82.6 ± 0.1 %; 21.1 ± 0.1, 44.9 ± 0.1, 62.9 ± 0.1% and 41.9 ± 0.1, 63.4 ± 0.1, 70.4 ± 0.1 %, respectively). A positive pregnancy outcome was associated with greater weight gains (P = 0.03) and higher weights at breeding in DT but not KD ewe lambs (Outcome X Breed, P = 0.02). In conclusion the fertility of ewe lambs can be improved by increasing the plane of pre-breeding nutrition, but the level of supplementation required and subsequent impact can vary with breed.