Growth and Reproductive Performance of Holstein Heifers as Affected by Ruminal Degradation of Supplemental Protein

Abstract

Reproductive performance of primipa-rous and multiparous dairy cows has been improved by increasing the proportion of undegraded intake protein (UIP) in the diet, especially when fish meal (FM) was supplemented. This positive response has not been documented with dairy heifers, in general, or under grazing conditions, in particular. Therefore, the objective of this study was to determine the effects of increasing the UIP level in the supplement on growth and reproductive performance of Holstein heifers in a 104-d grazing trial. A total of 63 heifers (BW ± SD = 317 ± 14 kg) were allotted at random to nine paddocks (7 heifers each) to allow for three replications (paddocks) per treatment. One heifer was later found as a freemartin and, therefore, was excluded from the study. The pasture [81.8% OM, 11.1% CP, and 61.1% NDF (DM basis)] was composed mainly of tall fescue (Festuca arundinacea) and Kentucky bluegrass (Poa pratensis) with a minor proportion of other forages [e.g., white clover (Trifolium repens)]. The heifers were group-fed one of three supplements (1.9kg DM/d per heifer) based on corn and dry molasses; the supplements also contained soybean meal, FM, or both protein sources to allow for 0, 50, or 100% of supplemental CP from FM. The heifers had ad libitum access to water and mineral-vitamin-molasses blocks. They were synchronized for estrus [two injections (5mg each of prostaglandin F2) on d 45 and 56], artificially inseminated on d 59, allowed to graze without the supplement on d 80, and tested for pregnancy via rectal palpation on d 104. Over the 80 d of feeding the supplements, there were no differences (P>0.05) among treatments for ADG (0.87, 0.82, and 0.94 kg), gain-to-supplement ratio (0.42, 0.39, and 0.46), or conception rate (28.6, 38.1, and 35.0%). The absence of significant gain or reproductive responses in this study might have been due to the poor quality forage, the limited amount of supplement fed, or both. Low quality forage also can negatively affect reproduction by decreasing efficiency of ruminal N utilization caused by limited energy supply.