Energy level in winter diets of Fallow deer: effect on plasma levels of insulin-like growth factor-I and sex ratio of their offspring.

To determine the effect of dietary energy level in the first winter on subsequent puberty onset, pregnancy, growth, and secretion of progesterone and insulin-like growth factor-I (IGF-I) in Fallow deer, prepubertal deer were fed either a high (H) or a low (L) energy diet in a randomised complete block design. June-born female Fallow deer were fed ad libitum either 12.5MJ/kg DM (H, n=29) or 10MJ/kg DM (L, n=29) in pelleted rations once per day during the winter (approximately 4-10 months of age) preceding puberty. Blood samples were collected twice weekly during the peripubertal period. During the winter feeding period, DM intake was similar for both groups but average daily gain was greater (P<0.05) for deer fed the H versus L diet. Onset of puberty was not affected (P>0.10) by dietary treatment. Concentrations of progesterone in plasma did not differ (P>0.10) between dietary treatments before or after puberty, increasing after puberty in both groups, and reaching maximal levels 8-12 days after the onset of puberty. Concentrations of plasma IGF-I increased (P<0.05) before puberty in both groups reaching maximal levels 3-4 days before the onset of puberty but did not differ (P>0.10) between H and L diets before or after puberty. Of the 28 does fed the H diet that calved, 75% of the calves born were male versus 46% in the L diet (P<0.05). In conclusion, increased plasma IGF-I concentrations were associated with the onset of puberty in Fallow deer regardless of the level of dietary energy intake during the preceding winter. Increased dietary energy during winter does not alter pregnancy rates but does alter sex ratio of calves born.