Response of lactating cows to supplemental unsaturated fat and niacin

Responses to supplemental unsaturated fat or niacin in a 2 x 2 factorial arrangement were evaluated with 16 lactating Holstein cows in a replicated Latin square with 4-wk periods. Total mixed diets contained (dry matter basis) 25% alfalfa hay, 25% corn silage, and 50% concentrate mixture. Diets were the control or the control supplemented with unsaturated fat (3% of dry matter of the total mixed diet from extruded soybeans, which replaced soybean meal and portions of the corn and barley in other concentrates), niacin (12 g/d of nicotinic acid), or both unsaturated fat and niacin. Milk production increased with the addition of fat but was unaffected by the addition of niacin (31.9, 35.1, 32.2, and 35.5 kg/d). Milk fat percentages were unaffected by the addition of fat or niacin to the diet, but proportions of both unsaturated and long-chain fatty acids increased with supplemental fat and were further increased with niacin in addition to fat; however, niacin was ineffective without fat. Milk protein percentages were lower with supplemental fat but were unaffected by niacin. Dry matter intake; ruminal volatile fatty acids, pH, and ammonia; serum urea; plasma glucose; and plasma amino acids were unaffected by supplemental fat or niacin. The first-limiting amino acid for milk protein synthesis was lysine for control diets and methionine for diets with supplemental fat. Supplementation of the diet with an unsaturated fat source increased milk production and the proportion of unsaturated fatty acids in milk, but niacin supplementation had no substantial influence on milk production and only a minor influence on milk fatty acid content.