Effect of High Temperature and Dietary Fat on Performance of Lactating Cows

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Abstract
Two Latin squares balanced for carry-on effects were used for evaluation of produc- tive performance of 12 Holstein cows fed high levels of fat under environmental tem- peratures: cool 15-24 C and hot 32.2 C, 60% relative humidity. The cows were fed 1.25 kg alfalfa-grass hay per 100 kg body weight and one Of three concentrates: con- trol (without added lipids), oil (10% soy- bean oil), or fat (10% hydrogenated veg- etable fat, iodine value 48) replacing corn on a weight basis. Elevated temperature resulted in a marked depression in yield of milk and milk fat, SNF, and protein, of body weight, feed intake, rumen VFA, acetic :propionic ratio, and gross efficiency ; water intake and rectal temperature increased. Significant ration effects were observed only in the ease of FC!V( production, where the oil-fed cows excelled. Environmental effects were inde- pendent of ration effects, except in water intake, tureen total VFA, and acetic acid levels, where interactions were noted. Sig- nificant period effects from repeated ex- posures to heat stress were noted only in FCM yield, water consumption, and rumen propionate. The drastic environmental stress imposed in this study served to mask many of the anticipated ration effects.