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1 Manitoba Agriculture and Food, Teulon, Manitoba, Canada R0C 3B0
2 Department of Animal Science, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2
The objective of this research was to characterize the production responses of lactating dairy cows during and after short-term, moderate heat exposure, and to determine whether evening (p.m.) feeding would alleviate the associated production losses. In a two-period, cross-over design, eight mature lactating cows were fed a total mixed ration at either 0830 or 2030 h. Each 15-d period consisted of a 5-d thermoneutral phase, a 5-d heat stress phase and a 5-d thermoneutral recovery phase. Mean daily vaginal temperature and respiration rate increased by 0.6 ± 0.04°C and 27 ± 1.3 breaths/ min, respectively, during short-term heat exposure. Daily dry matter intake, milk yield and solids-not-fat were depressed by 1.4 ± 0.13 kg, 1.7 ± 0.32 kg and 0.07 ± 0.023%, respectively, during heat exposure. During the recovery phase, dry matter intake remained depressed, milk protein declined by 0.05 ± 0.020%, and daily milk yield exhibited a further decline of 1.2 ± 0.32 kg. Time of feeding had no effect on vaginal temperature, respiration rate, dry matter intake, water intake, milk yield, fat-corrected milk, protein percent, solids-non-fat percent or somatic cell count during heat exposure or during the recovery period that followed. Fat percent was, however, significantly lower in p.m.-fed animals. These data indicate that short-term, moderate heat stress, which occurs during the spring and summer months in Canada and the Northern United States, will significantly decrease production in the lactating cow. Shifting from morning to evening feeding did not alleviate production losses associated with this type of heat stress.
Key Words: heat stress • time of feeding • vaginal temperature • milk yield
Submitted on February 14, 2000
Accepted on November 13, 2000
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