Effects of plane of nutrition during the first month of pregnancy on conception rate, foetal development and lamb output of mature and adolescent ewes

Embryo transfer studies have demonstrated that high plane feeding in early pregnancy is detrimental to the successful establishment of pregnancy in adolescent but not mature ewes. To further examine this relationship with ewes that conceive naturally and to investigate its effects on foetal development, 102 Greyface and Texel x Greyface ewes (body condition score (BCS) 3.8) and 114 Texel x Greyface adolescent ewes (BCS 3.3) were allocated to one of three treatments following a synchronized mating. From day 1 to 31 of pregnancy, animals were offered grass nuts (10.2 MJ metabolizable energy (ME) per kg dry matter (DM); 163g crude protein per kg DM) at one of three levels to supply approximately 2.0 (H); 1.0 (M) and 0.6 (L) of their predicted ME requirements for maintenance. Increasing the postmating plane of nutrition resulted in significant increases in ewe live weight (P < 0.001) and BCS (P < 0.001) during the treatment period. When diets were standardized for all animals during mid pregnancy (days 31 to 105), L ewes gained more live weight (P < 0.05) and BCS (P < 0.001) than H ewes. Early pregnancy nutrition had no effect on the conception rate of mature ewes; however adolescent ewes on the H and M treatments had lower conception rates (P < 0.05) than those offered the L diet (0.59, 0.61 and 0.82 respectively). As a result, L adolescent ewes tended to have a higher mean total lamb birth weight per ewe mated (P = 0.09) although lamb output at weaning was not influenced by plane of nutrition. Lambs born from adolescent mothers had lower lamb birth weights (P < 0.001) and a shorter mean head length, crown-rump length and thoracic girth (P < 0.001). Foetuses from H ewes had longer hind legs than those from L ewes on day 83 of gestation (P < 0.05) and at term (P < 0.05). The results of this study suggest that allowing adolescent but not mature ewes to lose live weight and body condition can increase the proportion of productive ewes following a natural service.