Altering Protein and Sugar in Postpubertal Dairy Heifer Diets

Forty-four Holstein heifers (age 15 to 17 mo) were used to study the level of dietary protein as well as added sugar (molasses-mineral supplement; MolMix Prime 32 with Bovatec), on ADG, structural growth, and body condition of postpubertal dairy heifers over 20 wk. Dietary CP levels of 13.5 or 16.4% were balanced to contain similar levels of soluble protein and RUP as a percentage of total CP, and heifers were fed at 2.2% of BW with all diets containing 200 mg/ d per head of lasalocid. An ADG of 1.04 kg/d was found for the control group and 1.32 kg/d for the heifers fed the molasses treatment diets (P < 0.01). Level of protein did not affect ADG consistently. Structural growth and body condition were not different across treatments (P > 0.05). Plasma urea N was higher in heifers fed higher levels of protein (P < 0.09). These values may indicate that the additional protein was not retained by the animals. The altered carbohydrate fraction of the diet coming from the liquid molasses appeared to increase rumen available energy from the diet. The lack of response to increased dietary protein showed that protein alone was not a limiting factor in these diets. Therefore any increase in ruminal microbial protein that may have occurred from the additional soluble carbohydrate was likely not the major cause of the increased rate of gain. Rather it was likely the increased ruminal microbial digestion that improved diet utilization and resulted in increased ADG.