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Effects of climate on milk yield, fat percent, feed intake, and changes of body weight were determined for first lactation Holsteins at Beltsville, MD. Cows calving in January and February were larger in milk yield (17%), fat percent (7%), gross efficiency (14%), and breeding efficiency (-32 days open) than cows calving in July and August. Cows calving during the other 8 mo were intermediate in these traits. Cows exposed to 20 days of maximum temperature >27 C in the first 100 days of lactation averaged 27% higher in gross efficiency (kg milk/Meal of estimated net energy) than cows exposed to 40 to 87 days >27 C. Four measures of climate (maximal and minimal temperatures and dew point 0700 and 1900 h) accounted for nearly as much influence on milk yield as 12 measures of climate. The percentage of the variation in milk yield accounted for by four climatic variables, plus feed and body weight, ranged from 11 to 62 with the most for cows calving in July and August and least for those calving in the spring or fall. Climatic conditions appeared to have the greatest influence in the first 60 days of lactation. During this period, high temperatures restricted feed intake causing a rapid utilization of body reserves and high losses in body weight. Conversely, cold temperature stimulated feed intake resulting in higher yields and gross efficiency than for moderate or high temperatures. After about 60 days of lactation, feeding was the primary environmental variable limiting performance irrespective of climate.
McDowell et al. (Sat,) studied this question.