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Cows that were subjected to hot environmental temperatures yielded less milk (3.1 kg/d) on a diet high in CP (18.4%) and of medium degradability (65%) than on diets high in CP of low degradability (59%) or medium in CP (16.1%). The high CP diets were associated with decreased DMI and higher water intake, ruminal NH4, and blood urea. Negative effects on yield from the high CP, medium degradability diet were not observed at moderate temperatures. Evaporative cooling of cows in hot weather resulted in a greater milk yield response to low versus medium rumen-undegradabale protein diets than for uncooled cows. Evaporative cooling of cows also affected response to protein quality. For cooled cows, high Lys diet (soybean, fish, and blood meals) increased milk yield 14% over that with low Lys diet (corn gluten meal), but, for uncooled cows, a high Lys diet only increased yield by 9%. Percentage of CP, degradability, and protein quality had no effect on body temperatures or respiration rates of lactating cows. Some, but not other, reports showed that supplementation of 2 to 2.5% fat to diets fed under hot summer conditions resulted in less yield response than when fat was added at moderate temperatures. In several studies, fungal cultures (3 to 5 g/d) in the diet decreased body temperatures and respiration rates in hot, but not cool, weather. Increased milk yields and cellulose digestibility also resulted from dietary fungal cultures in some, but not all, trials. The mechanism of action exerted by fungal cultures on body temperature and respiration rate is unclear.

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Trials were conducted to evaluate effects of a fermentation extract of Aspergillus oryzae (AO) on milk production and composition, diet digestibility, and rectal temperature changes in lactating dairy cows. Treatments were incorporated as a top dressing at the morning feeding and consisted of control (90 g/d of ground sorghum) or AO (3 g of culture + 87 g of ground sorghum daily). Twenty-four mid-lactation Holstein cows were paired for production in Lactation Trial 1 (LT-1). In Lactation Trial 2 (LT-2), 46 cows (20 primiparous and 26 multiparous) in early lactation were used. Trials lasted 12 wk. In LT-1, AO supplementation increased milk yields only at 2 (P less than .05) and 8 wk (P less than .10) of treatment. Rectal temperatures were lower (P less than .05) for cows fed AO for 4 of 10 readings made during summer. Supplementation of AO culture in LT-2 (early lactation cows) increased milk production and feed efficiency (P less than .05). Inner ear temperatures tended to be lower (P less than .11) for cows fed AO. Digestion trials, conducted at the end of lactation trials, used Cr2O3 as an indigestible marker. In Digestion Trial 1, digestibilities were not significantly (P greater than .10) affected by AO supplementation. However, in Digestion Trial 2, AO increased (P less than .05) digestibilities of DM, OM, CP, NDF, and ADF. Length and number of meals were not affected (P greater than .10) by feeding AO. In summary, milk yields, efficiency of milk production, and nutrient digestibilities were higher for early lactation cows fed a high-concentrate diet supplemented with 3 g of AO/d. Mid-lactation cows fed a lower-energy diet were less responsive to AO than early lactation cows, though similar trends were shown.