Diurnal heat balance cycle model of a lactating cow in the tropics

A.J. Md. Eusof*

Abstract

Diurnal heat balance cycle model of a lactating cow was developed to quantify the sources of heat production and heat loss, and to assess the causes and period of thermal stress in the animal. The analysis was conducted using the system analysis technique and the values represent an average half-bred Friesian cow in Malaysia. Heat production was calculated from the metabolic heat generated and the radiant heat absorbed by the animal; whereas heat loss was computed from sensible or non-evaporative, evaporative and losses through drinking water. Heat produced by a half-bred Friesian cow at peak lactation producing 12.8 kg milk per day was about 15.10 MJ/m²/d. The amount of heat produced by body maintenance, feed ingestion, milk synthesis, walking and standing, and the amount of radiant heat absorbed by the animal were 8.62, 0.88, 4.27, 0.82 and 0.51 MJ/m²/d, respectively. Heat loss through sensible, sweating, respiration, and drinking water, on the other hand, were 4.67, 6.38, 2.03 and 2.13 MJ/m²/d, respectively. The diurnal heat balance cycle showed that the comfortable period for the cow is between 2300 h and 0600 h. The most stressful period appears to be between 1200 h and 1900 h. The study also showed that stress was largely caused by the accumulation of heat stored by the animal and thus to reduce these stresses, the study suggested that the feeding of roughage and milking are to be carried out 5–6 hours before or a couple of hours after the stressful period.

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