EFFECTS OF RUMINALLY INERT FAT AND EVAPORATIVE COOLING ON DAIRY-COWS IN HOT ENVIRONMENTAL TEMPERATURES

Under hot summer conditions of Tucson, Arizona, 24 Holstein cows ((X) over bar = 80 d of lactation) were assigned for 56 d to four treatment s in a randomized block design with a 2 x 2 factorial arrangement of t reatments. Factors were 1) medium [4.6% of dry matter (DM)] versus hig h (7.4% of DM) amounts of dietary fat and 2) corral shade only versus shade equipped with evaporative cooling. The high fat diet contained 3 % prilled fatty acids. The efficiency of the conversion of feed to mil k tended to be better for cows fed prilled fat than for cows fed mediu m dietary fat, but other lactation measurements were unaffected. Cows with access to evaporative cooling had greater milk yields than did co ws with access to shade only. Frilled fatty acids did not depress the percentage of milk protein, but reduced short- and medium-chain fatty acids (C-6:0 to C-14:0) in milk fat and increased palmitic acid. Diges tibilities of DM, organic matter, elude protein, acid detergent fiber, neutral detergent fiber, and starch were unaffected by amount of fat or by cooling method, but prilled fatty acids tended to decrease appar ent digestibility of fatty acids. No differences were observed among t reatments in respiration rates or rectal temperatures. When rectal tem peratures were determined, cows were crowded, which probably negated d etection of an effect of evaporative cooling. Evaporative cooling incr eased milk yield of cows in hot weather, but the addition of 3% fatty acids did not increase yield, and no interactions were observed.