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 performa nce of sheep was studied in 50 Booroola x Assaf crossbred ewes, of whi ch 32 were carriers of the Fec(B) (Booroola) allele and 18 were not ca rriers. 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 starc h (RUS) were obtained during the 29 day treatment period by feeding gr ain 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 dieta ry treatments (Day 0). Ovulation rate (OR) was estimated by laparoscop y. 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). Wit hin Fec(B) allele carriers, all WC-fed ewes had multiple ovulations, c ompared with 61% of EC-fed ewes (P less-than-or-equal-to 0.005). OR va lues were 3.29 +/- 0.27 and 2.46 +/- 0.31 corpora lutea (CL) per ewe o vulating 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 prolific acy (1.20 and 1.44 lamb per ewe lambing, in the same order). In a seco nd experiment, aimed to assess the effect of dietary RUS on circulatin g 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 le vels were higher in EC- than in WC-fed ewes before and 3 h after a mea l (P less-than-or-equal-to 0.05). EC feeding may be associated with hi gher peripheral insulin concentration but does not enhance OR. Data ar e used to show that: (1) provision of about 100 g RUS day-1 exerts a p ositive effect on the OR and prolificacy of Fec(B) carriers; (2) dieta ry treatments may affect OR in sheep of different ovulation potential differently; (3) no positive relationship was found between circulatin g plasma insulin concentration and OR.