Nutrition and reproductive performance of dairy cattle 1. The effect of level of feeding in late pregnancy and around the time of insemination on the reproductive performance of first lactation dairy heifers

Abstract

ABSTRACTOne hundred first lactation Friesian dairy heifers were used to investigate the effect of level of nutrition n i late pregnancy and for a limited period around the time of insemination on reproductive performance. The heifers were housed throughout the period of the trial in cubicle yards and all individuals were given controlled amounts of food.Fifty animals were offered a ‘high’ (H) level of feeding in late pregnancy (for 10 weeks before calving) and 50 were offered a ‘low’ (L) level. After calving, the heifers were re-randomized with half of each pre-calving group being offered either the H or L level of feeding in early lactation (weeks 6 to 18), thus forming four subgroups of 25 heifers: HH, HL, LH and LL. The level of feeding in late pregnancy for all groups increased towards calving. The high plane heifers consumed 83·6 MJ metabolizable energy (ME) per day on average compared with 64·6 MJ ME per day for the low plane heifers, a proportional mean difference of 0·29 (P < 0·001). After calving, heifers consumed an average of 86 MJ ME per day in week 1 rising to 126 MJ ME per day by week 5. Between weeks 6 to 18 of lactation, the high plane group consumed an average of 146·8 MJ ME per day compared with 119·8 MJ ME per day for the low plane group, a proportional mean difference of 0·23 (P < 0001).There were no significant effects of feeding treatments on milk yield. Fat yield in weeks 6 to 18 was increased by the high level of feeding in lactation (P < 0001), while protein yields in weeks 1 to 5 and n i weeks 6 to 18 were increased by high plane feeding in pregnancy (P < 0·05) and lactation (P < 0·05), respectively. Heifers receiving the high level of feeding in late pregnancy gained significantly (P < 0·001) more weight than those on the lower level of feeding. All groups lost similar amounts of weight during the first 5 weeks after calving, but there were marked differences in the rate of live-weight gain between weeks 6 to 11 of lactation (P < 0·05 to P < 0·001). In particular, the HL group had the lowest rate of live-weight gain, and this was significantly lower than that of the HH group (P < 0·01).First ovulation occurred significantly earlier in heifers receiving the high level of feeding in pregnancy P ( < 0·01), but there was no effect of treatment on ovulation detection rate, time to first oestrus, time of first insemination or the proportion of wrongly-timed inseminations. Heifers receiving the high level of feeding in lactation had a successful pregnancy rate to first insemination of 0·42 compared with 0·63 for those on the lower level of feeding (P < 0·05), with the greatest difference observed between the HL group (0·72) compared with the HH group (0·35) (P < 0·01). Embryo loss rate was low in the HL group (008) compared with that in the HH group (0·26; P < 0·l). The net result was that the HH group had a median interval from calving to successful pregnancy of 95 days with an interquartile range of 82 days, whereas the figures for the HL group were 75 and 23 days (P < 0·001). Live-weight change at artificial insemination (AI) and level and change in milk yield at AI had independent effects on reproductive performance and as a consequence net energy output showed the strongest relationship with days to successful pregnancy. Heifers with the highest net energy outputs per day at AI had the longest intervals to successful pregnancy (r = 0·408; P < 0·001).