INFLUENCE OF PROTEIN DEGRADABILITY AND EVAPORATIVE COOLING ON PERFORMANCE OF DAIRY-COWS DURING HOT ENVIRONMENTAL TEMPERATURES

Two trials compared four treatments in a randomized block design with a 2 x 2 factorial arrangement of treatments using lactating Holstein cows. Factors were two concentrations of protein degradability (high, 61 to 64%; low, 47 to 55% degradable intake protein) and two types of environment (shading versus shading plus evaporative cooling). In trial 1 (24 cows for 55 d), evaporative cooling resulted in increases in production of milk, 3.5% FCM, gross efficiency of conversion of feed to milk, and DM intakes. The diet of greater protein degradability also increased milk production. There were no significant differences in milk composition. In trial 2 (36 cows for 50 d), cows fed less degradable protein produced more milk of higher lactose and lower fat and protein contents. Protein degradability by evaporative cooling interactions were significant for milk production and efficiency of conversion of feed to milk, with the low degradable protein diet resulting in higher milk yields and efficiencies in cooled than in shaded cows. During both trials, cooling decreased respiration rates and rectal temperatures, but protein degradability had no effect. The different production response between trials probably was due to hotter conditions and a higher quality of rumen undegradable protein in trial 2 than in trial 1. Trial 2 data suggest that feeding of low degradable protein improves milk production during heat stress, provided the undegradable protein is of good quality.