Effects of feeding treatment on growth rates and consequences on performance of primiparous Holstein dairy heifers

The objective of this study was to investigate the effects of feeding rearing programs aiming a first calving between 20 and 27 months (mo) of age on growth, reproduction and production performance of Holstein cows at nulliparous and primiparous stages. Our hypothesis was that, in a seasonal autumn calving strategy, it was possible for late-born heifers in the season to catch up with the growth of heifers born earlier and be inseminated at the same time-period, at a 370 kg body weight (BW) minimum. This would result a first calving age at about 21 to 22 mo, without impairing their later performance. To answer this question, an experiment was run, involving a total of 217 heifers over 3 years. These heifers were split into 3 groups: the first group received a control feeding treatment (SD), the second one an intensive-plane diet (ID1) from birth to 6 mo, and the last group an intensive-plane diet until 1 year of age. Groups SD and ID1 comprised heifers born from September until end of November; ID2 was composed of heifers born later. The present study showed that late-born heifers (ID2) could catch up with the growth of the other thanks to feeding treatment, although there were still 42 kg lighter than both SD and ID1 ones at first calving. There was no difference in reproductive performance of the heifers between the groups. Once primiparous, the cows reared with the ID2 treatment tended to produce less milk than SD and ID1 (about -400 kg over the lactation), and there were no difference regarding milk quality, feed intake, body condition score, or BW. A classification on age at first service was created a posteriori leading to 3 classes with heifers first inseminated at about 12.5 mo (AFS12.5), 14.0 mo (AFS14.0), 15.5 mo (AFS15.5) of age. Heifers in AFS12.5 had a faster growth than those in AFS14.0 and AFS15.5. Once primiparous, the AFS12.5 cows tended to produce less milk at peak than AFS14.0 and AFS15.5 (about -1.5 kg/d) although there was no difference regarding total milk production over the lactation. There was no difference between these groups regarding milk quality, feed intake, body condition score, or BW. All these results support the conclusion that it is possible, through feeding treatment, to help late-born heifers to catch up with the growth of other. This leads to an earlier first calving but do not impair their reproductive and productive performance.