Effects of dietary neutral detergent fiber concentration and alfalfa hay quality on chewing, rumen function, and milk production of dairy cows.

Six ruminally fistulated Holstein cows (80 d postpartum) were used in a 6 x 6 Latin square to evaluate the effects of dietary NDF concentration and alfalfa hay quality on chewing activities, digestive parameters, and productivity of dairy cattle. Cows received one of six diets formulated to provide three concentrations of dietary NDF (31, 34, and 37%) and two sources of first-cutting, long alfalfa hay. Alfalfa hay was harvested either in early bloom (19.4% CP, 38.8% NDF) or midbloom (16.7% CP, 47.6% NDF) stages of maturity. Dietary NDF concentrations were achieved by adjusting the forage to concentrate ratios. Mean ad libitum DMI was 22.3 kg/d. Increased NDF concentration of the diet corresponded to a linear decrease in milk production (from 26.5 to 24.8 kg/d) and a linear increase in the fat content of milk (from 2.68 to 3.30%). Hay quality had no effect on milk production and composition when diets were formulated for specific NDF concentrations. Total chewing time increased from 767 to 796 and 853 min/d as fiber content of the diet increased from 31 to 34 and 37%, mainly because of increased time spent eating. Rumination time adjusted for fiber intake decreased linearly from 59.0 to 54.2 min/kg NDF as fiber intake increased, and it was higher for the early bloom than for the midbloom hay (57.3 vs. 55.5 min/kg NDF). Effects of decreased forage quality because of the increased maturity of the alfalfa hay can be minimized by formulating diets for specific NDF concentration. For diets formulated with barley-based concentrates, dietary NDF concentrations should be higher than currently recommended with allowance for greater proportions of NDF from concentrates.