EFFECTS OF ALFALFA HAY OF VARYING FIBER FED AT 35 OR 50-PERCENT OF DIET ON LACTATION AND NUTRIENT UTILIZATION BY DAIRY-COWS

Five alfalfa hays (four from Arizona and one from California), varying in percentages of ADF (26, 28, 32, and 38% ), were fed to 40 lactating Holstein cows averaging 90 DIM. Cows were in 1 0 groups of 4 cows each; groups were based on 14-d pretreatment milk yield. Each hay was included in TMR to provide 50 or 35% of DM. Diets were fed for ad libitum intake for 70 d. Feeding behavior of 2 cows per treatment was electronically monitored for 14 d. Total tract digestibilities of DM, ADF, and NDF were determined using Cr2O3, and ruminal in situ loss of DM, ADF, and NDF of hays was estimated using 4 cows fitted with ruminal fistulas. Dry matter intake, 3.5% FCM, changes in BW, rectal temperatures, and milk composition (except milk fat) were not affected by ADF in hays or concentrate percentage. However, milk yield decreased as ADF in hay increased, particularly at 50% concentrate. At 50% concentrate, milk yield of cows fed hays of 26 to 28% ADF averaged 30.7 kg/d, and the mean for cows fed 32 and 38% ADF hays was 27.6 kg/d. Milk fat percentages tended to be lower on higher concentrate. Eating time was longer as hay ADF increased and tended to decrease on high concentrate, but there were no significant effects of treatment on number or length of meals. In situ disappearance of DM, ADF and NDF decreased as hay ADF increased, but total tract digestibilities of ADF and NDF were greater in hay of higher fiber content, particularly in cows fed 35% hay Amount of concentrate influences response of lactating cows to hays of varying ADF. Inferences about forage nutritive value from total tract digestibilities of diets with hays of varying ADF may differ from those obtained from ruminal in situ estimates.