Protein Nutrition and Replacement Heifer Development

By Kiernan Brandt

Introduction

With summer coming to an end and weaning for this year’s calves right around the corner, it is not premature for producers to start thinking about next year’s replacement heifers that will be needed to replace any culled or open cows. Current industry guidelines recommend that heifers should be managed so that a majority have achieved puberty prior to the breeding season. Heifers that achieve puberty prior the breeding season have higher pregnancy rates than those that do not, and heifers that have experienced multiple estrous cycles (≥2) prior to breeding are more likely to become pregnant early in the breeding season. By promoting fertility at a young age in advance of the breeding season, producers can ensure heifers calve prior to 2 years of age, which has been shown to increase longevity and productivity over their lifetime.

Background

Weaned heifers designated as replacements should be managed so that they reach 60-65% of their expected mature bodyweight prior to the breeding season, which usually requires an ADG near 1.5 lbs. throughout the post-weaning period. To achieve this positive plane of nutrition, a 600 lbs. heifer would need 9.5 lbs. of total digestible nutrients (TDN) and 1.32 lbs. of crude protein (CP) per day. Heifers should be observed for body condition score (BCS) regularly during development and enter their first breeding season between a BCS 5-6. Achieving a BCS within this range ensures that heifers have accumulated enough energy reserves to keep growing while supporting the additional nutritional requirements of a pregnancy.

A recently completed study at the University of Tennessee investigated the effectiveness and economics of 3 different supplements containing different feedstuff combinations and CP content. Sixty heifers were divided into equal groups of 20 and housed in pens of 5 shortly after weaning and received either corn (10% CP), corn/DDGS (20% CP), or soybean meal/DDGS (40% CP), which was fed 4x/week until the breeding season. These supplements were selected based on their widespread use in supplementation programs industry-wide, and an ADG of 1.5 lbs./day was targeted for all groups. All heifers had ad-libitum access to average quality, mixed-grass hay.

Results

Final pregnancy rate for all heifers was 88% and there was no difference in pregnancy rates between supplement types. This emphasizes that producers can use a wide variety of feedstuffs to develop replacement heifers successfully, in this experiment CP in supplement ranged from 10-40%. Heifers receiving the corn-only supplement (10% CP) grew slower than groups supplemented 20 or 40% CP and were nearly 90 lbs. lighter at the end of the study. This indicates that the low amount protein may have limited the growth of these females compared to their herd-mates. Despite constant increases in supplementation, ADG of corn (10% CP) supplemented heifers was only 0.98 lbs./day, lower than the 1.5 lbs./day target ADG, which was achieved in heifers receiving corn/DDGS (20% CP) or soybean mean/DDGS (40% CP). This decreased performance did not affect sexual development, and there was no difference in the age at which heifers reached puberty across treatments, likely as a result of the positive plane of nutrition maintained across all groups of heifers. Decreased performance by corn supplemented heifers led to an increase in average total supplement consumed on a per head basis, requiring 160 lbs. more supplement than corn/DDGS and 128 lbs. more than soybean meal/DDGS supplemented heifers. Supplement costs were calculated at $288/ton for corn, $300/ton for corn/DDGS and $320/ton for soybean meal/DDGS.

Economics

Although corn was the cheapest supplement on a per-ton basis, increased consumption due to decreased performance made supplementing corn alone the least economical option at $110/head. Supplementing corn/DDGS (20% CP) was the most economical option, with supplement costs averaging $90/head, while heifers receiving soybean meal/DDGS were slightly more expensive at $102/head. These results illustrate the complexities producers must consider when developing supplements for replacement heifer development. Price is the driving factor for most management decisions; however, it is not as easy as simply selecting the cheapest option. Considerations must be made as to how much supplemental CP heifers will need, which will fluctuate depending on the quality of the forage provided, but CP requirements must be met to avoid limiting growth and decreasing performance. In this experiment, a blend of corn/DDGS containing 20% CP was the most economical option for developing replacement heifers between weaning and breeding.

                                                                       TABLE 1

^xy Numbers within rows with different superscripts differ significantly (P < 0.05).

                                                                                TABLE 2

^xy Numbers within rows with different superscripts differ significantly (P < 0.05).