Abstract
This study aimed to determine whether dietary manipulation of starch and fat could affect fertility in breeding ewe lambs. The hypothesis tested was that a high starch (insulin promoting) diet would increase the rate of onset of oestrous activity and conception compared to high fat (insulin suppressing) diet. Two diets, a high starch and a high fat diet, were fed to ewe lambs (n=102 per treatment) for a continuous 60-day period, (30 days prior to and after joining with the ram), and a sub-sample of 39 ewes per treatment were blood sampled for insulin analysis. Joining body mass pre-joining growth rate did not differ between the treatment groups. There was no overall effect nutrition on plasma insulin concentrations, though concentrations in the high starch group increased by day 18 compared to day O (P<0.005). Irrespective of nutritional treatment, joining plasma insulin concentration had no effect on conception rates. There was no effect of nutrition on the proportion of ewes cycling or the number of twin pregnancies. There was an increase in the conception rate of the cycling ewes in the high starch group (P<0.05). Higher body mass at joining, irrespective of diet, increased cycling rate (P<0.01), conception rate (P<0.05) and twinning rate (P<001). These findings indicate that manipulation of starch and fat in the diet of breeding ewe lambs does not affect their fertility, and that achieving a target body mass, pre-joining, is a more effective strategy to increase conception rates.