Improving the energy status of dairy cows during the early post-partum (PP) period by adding a safe dietary supplement such as live yeast culture (YS) may have a positive effect on reproductive function. The objective was to examine potential benefits of YS supplementation on PP energy status and fertility indices of dairy cows managed to have low or high body condition score (BCS, 1-5 scale) at calving. Forty (10 primiparous and 30 multiparous) Holstein/Friesian dairy cows were blocked by yield, parity, BCS, and predicted calving date. Within each block, cows were randomly allocated to a 2 x 2 factorial arrangement of treatments which were: BCS at calving (low < or =3.5 or high > or =3.75; n=20) and YS supplementation (2.5g/cow/day for pre-calving and 10g/cow/day for post-calving x 10(8) CFU of Saccharomyces cerevisiae/g) (supplemented or control; n=20). Daily milk yield was recorded and weekly milk composition, BCS and BW were assessed from calving to week 10 PP. Forage (100% grass silage pre-calving; 50% grass silage, 50% maize silage post-calving; ad libitum) intake was recorded individually. Concentrate (2kg of pre-calver nuts+/-YS for pre-calving and 8 kg of lactating nuts+/-YS for post-calving) feeding was controlled individually. Estimated energy balance PP was calculated on a weekly basis individually as the difference between the net energy (NE) intake and the sum of NE for maintenance and milk production. Insulin and IGF-I concentrations were determined on days 14 and 7 pre-calving and 1, 5, 15, 25 and 35 post-calving. Daily ovarian ultrasonography was performed from day 10 PP to monitor the size and development of the first dominant follicle (>10mm in diameter with absence of other large growing follicles), first ovulatory follicle and days to first ovulation PP. Pre-ovulatory peak of serum oestradiol concentration was determined during the 2 days before ovulation day. Cows with high BCS (over-conditioned) at calving ingested less NE, produced more milk NE output, and consequently had a significantly (P<0.05) exacerbated negative energy balance in comparison with low BCS cows (moderately conditioned) during early lactation. Higher (P<0.05) insulin concentrations and a tendency for higher (P=0.06) pre-ovulatory peak oestradiol concentrations in low BCS group were detected in the early PP period. Supplementing the diet with YS had no effect (P>0.10) on NE intake, NE milk output or energy balance. On the other hand it increased (P<0.01) insulin concentration and tended to increase (P=0.07) pre-ovulatory peak oestradiol concentrations and the size of first ovulatory follicle (P=0.09) early PP. Feeding YS had no effect on energy status of lactating dairy cows with high or low BCS at calving, whilst it improved serum insulin concentration, pre-ovulatory peak of oestradiol and the size of first ovulatory follicle in the early PP period. These observed effects of YS supplementation require to be substantiated with further research.
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