The time to oestrus was shorter and the proportion of female lambs higher when ewes were fed a diet high in omega-6 (n-6, 70% oats, 8% cottonseed meal, 22% silage) compared with omega-3 (n-3, 100% silage) fatty acids (FAs) prior to joining (Gulliver et al. 2013). As diets varied in components other than FAs, these observations may not be attributable to dietary FA variation alone. The aim of the current study was to determine whether the time to oestrus and lamb sex ratio was altered by diets varying only in FA content.
Ewes (n=30) were fed a basal diet (wheat straw 74.5%, lucerne hay 10.5%, molasses 5.9%, urea 2%, minerals 1.4%, casein 1%DM) with either linseed, sunflower or palm oil (4.7%DM) added as sources of n-3, n-6 or saturated FA (SFA), respectively (n=10 ewes/treatment). Diets were fed for six weeks prior to joining, oestrus was synchronised and ewes joined at the second oestrus. Behavioural oestrous was detected using crayon harnessed rams. Blood was collected prior to and following consumption of experimental diets for FA analysis. Lamb sex was determined at birth.
The time to oestrus (p=0.16) and, lamb sex ratio (p=0.60), was not significantly different when ewes were fed the n-6, SFA or n-3 diets (18.65±0.77, 19.00±0.77 and 19.70±0.77 days from CIDR pull and 69±14%, 58±15% and 50±13% female lambs, respectively). A subgroup analysis of time to oestrus (p=0.06) and lamb sex ratio (p=0.31), between the n-6 and n-3 groups only, was also not significant, although the direction of the effect was the same as that observed previously (Gulliver et al. 2013). The effect of experimental diets on plasma FA status and, further experiments with larger numbers of animals, will provide additional information on the effects of n-6 and n-3 on time to oestrus and lamb sex ratio.