The Effects of Nitrogen and Forage Source on Feed Efficiency and Structural Growth of Prepubertal Holstein Heifers

Abstract

Eighty Holstein heifers averaging 189.6 ± 6.8kg of BW were used to evaluate the effects of forage level and rumen degradable nitrogen source on feed efficiency and structural growth. A randomized complete block design was used with heifers blocked according to BW (≤ 136.0kg and > 136.0 kg) and assigned to 1 of 4 treatment diets in a 2 x 2 factorial design. Treatments were constructed with 2 levels of forage (65 or 75%) and 2 nitrogen sources. Forage sources were a mixture of corn silage and chopped timothy hay. Nitrogen sources were either soybean meal (SBM) or a slow-release, polymer-coated urea product (Optigen 1200, CPG Nutrients, Syracuse, NY), which was fed at 1.8% of diet DM on low-forage diets and at 1.3% of diet DM on high-forage diets. Average daily gain and feed efficiency did not differ between rations of different forage level or nitrogen source, averaging 0.87 ± 0.05kg and 7.4 ± 0.5, respectively, across treatments. Similarly, no differences were observed in change of withers height, hip height, hip width, or heart girth. No differences were observed in plasma urea nitrogen, which averaged 12.3 ± 0.4 mg/dL across treatments. Results of this experiment suggest that feeding moderately different levels of forage along with either SBM or a polymer-coated urea product does not result in any significant differences in feed efficiency or structural growth. Polymer-coated urea can be used in heifer diets to effectively replace SBM as a nitrogen source in either low- or high-forage nations.