THE EFFECT OF RUMINAL STARCH DEGRADABILITY IN THE DIET OF BOOROOLA CROSSBRED EWES ON INDUCED OVULATION RATE AND PROLIFICACY

The effect of ruminally degradable starch on the reproductive performance of sheep was studied in 50 Booroola x Assaf crossbred ewes, of which 32 were carriers of the Fec(B) (Booroola) allele and 18 were not carriers. The sheep were group-fed, according to body weight (BW), diets composed of pea straw, soybean concentrate and corn grain offered in two meals per day. Dietary differences in ruminally undegradable starch (RUS) were obtained during the 29 day treatment period by feeding grain as whole corn (WC, 1.4 g RUS day-1 kg-1 BW) or extruded (EC, 0.4 g RUS day-1 kg-1 BW). Oestrus was synchronized by inserting progestagen pessaries at Day 8, and withdrawing pessaries and injecting 400 IU of pregnant mare serum gondaotrophin at Day 20 after initiation of dietary treatments (Day 0). Ovulation rate (OR) was estimated by laparoscopy. The Fec(B) allele was associated with a significant increase in OR (2.88 vs. 1.40, P less-than-or-equal-to 0.0001) and prolificacy (1.85 vs. 1.32 lamb born per ewe lambing, P less-than-or-equal-to 0.09). Within Fec(B) allele carriers, all WC-fed ewes had multiple ovulations, compared with 61% of EC-fed ewes (P less-than-or-equal-to 0.005). OR values were 3.29 +/- 0.27 and 2.46 +/- 0.31 corpora lutea (CL) per ewe ovulating and prolificacy was 2.13 +/- 0.22 and 1.57 +/- 0.23 lamb per ewe lambing in WC- and EC-fed ewes, respectively (P less-than-or-equal-to 0.09). In non-carrier ewes, dietary treatments did not affect OR (1.44 and 1.36 CL per ewe ovulating in WC- and EC-fed ewes) or prolificacy (1.20 and 1.44 lamb per ewe lambing, in the same order). In a second experiment, aimed to assess the effect of dietary RUS on circulating insulin levels, blood was sampled before and at hourly intervals for 10 h after a meal from ten ewes fed rations with WC or EC. Insulin levels were higher in EC- than in WC-fed ewes before and 3 h after a meal (P less-than-or-equal-to 0.05). EC feeding may be associated with higher peripheral insulin concentration but does not enhance OR. Data are used to show that: (1) provision of about 100 g RUS day-1 exerts a positive effect on the OR and prolificacy of Fec(B) carriers; (2) dietary treatments may affect OR in sheep of different ovulation potential differently; (3) no positive relationship was found between circulating plasma insulin concentration and OR.