Fifteen multiparous early lactating Friesian cows weighing 466 ± 16.6 kg were randomly assigned, in a quintuplicated 3 × 3 Latin square design experiment, to study the effect of humic acid supplementation on feed utilization and milk production for 90 days. The basal diet composed of concentrates, berseem clover, and rice straw at 5:3:2, respectively, on dry matter (DM) basis. There were three treatments including the basal diet without additive (control diet), the basal diet supplemented with 20 g humic acid/cow/d (LH treatment), and the basal diet supplemented with 40 g humic acid/cow/d (HH treatment). Humic acid supplementation linearly increased (P < 0.05) milk production, total solids, solids not fat, fat, and energy contents. Additionally, humic supplementation linearly increased (P < 0.05) milk efficiency. Both LH and HH treatments showed greater proportions of unsaturated fatty acids and total conjugated linoleic acid, however, HH treatment decreased C16:0 and increased C18:3n3 and C18:3n6 proportions. Without affecting feed intake, treatments improved (P < 0.05) the digestibilities of DM, organic matter, non-structural carbohydrates, and neutral detergent fiber. Humic supplementation increased ruminal acetic acid concentration, and decreased ruminal protozoal population, pH, ammonia-N and butyric acid contents. The HH increased total ruminal volatile fatty acids (P = 0.029). Additionally, humic supplementation decreased the concentrations of serum urea-N, cholesterol, and triglycerides, while it increased the concentration of serum glucose compared to the control. It is concluded that humic acid supplementation enhanced feed utilization, lactational performance, and milk nutritive value, without negatively affecting the blood chemistry of lactating cows. However, the high dose of humic acid is recommended for practical administration in lactating cow diet.
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