Thermal and Productive Responses of Grade Dairy-Cows to Dietary Supplementation during Summer

Abstract

This study was conducted in the University of Khartoum Experimental Farm to investigate the thermoregulation of low (25- 37.5%) and high (50-62.5%) grade dairy cows during a transitional period between winter and summer and to investigate the effects of ration supplementation on thermoregulation and milk production during proper summer conditions. The normal ration of the cows was supplemented to fulfill the nutrient requirements of maintenance and milk production in each grade. Regression analysis showed that the rectal temperature (Tr) increased on the average by 0.03 and 0.05 ºC in low and high grade cows, respectively, when the mean ambient temperature (Ta) increased by 1 ºC. High grade cows maintained higher Tr values than low grade cows. However, the difference in Tr values between low and high grade cows was not significant either before or during supplementation of the cows' ration. A significant difference (P<0.05) in respiratory rate RR between low and high grade cows was obtained during supplementation. However, the difference in RR between low and high grade cows before the supplementation was not significant. Regression analysis revealed that RR increased on the average by 0.8 and 1.1 min-l in low and high grade cows, respectively when the mean ambient temperature increased by 1 °C. Diurnal measurements of Tr and RR under direct solar radiation showed that at the minimum Ta (35°C) and minimum solar radiation (185 Wm-2) at 07.00hrs, low and high grade cows maintained same Tr x I value (38.7 °C). Minimum respiration rates were 37 and 48 min-l in low and high grade cows, respectively. Rectal temperature and respiration rate were increased in both grades as the ambient temperature and solar heat load increased towards midday. Maximum Tr values were 39.3 and 39.4°C, in low and high grade cows, respectively, recorded at maximum solar radiation 404 wm-2 and Ta 41.2 t at ll.00h. Maximum RR values were 50 and 63 min-l in low and high grade cows, respectively, recorded at 13.00 hrs at maximum Ta 41.6 °C and solar radiation load = 361 Wm-2. The study revealed that ambient humidity had a minor influence on thermoregulation of low and high grade dairy cows. Regression analysis of the relationship between Tr and RR and temperature humidity index showed that ambient temperature was the most effective climatic variable that influenced the thermoregulatory responses of low and high grade dairy cows under the conditions of the experiment. The regression analysis and the dirnal changes of Tr and RR indicated that high grade cows were more sensitive to environmental variables, Ta and solar radiation, than low grade cows. The heat tolerance of low and high grade cows was evaluated by two tests the 'Iberia Heat Tolerance Coefficient' using Tr measurements and coefficient of adaptability in which both Tr and RR measurements were used. The tests revealed a lower degree of heat tolerance in high grade cows. The study showed that low and high grade cows could maintain their maximum milk (17.9 and 25.1 1bs/cow/day, respectively) at mean ambient temperature below 26.6°C (18.2- 34.9°C). The milk yield of both grades started to decline as the mean ambient temperature approached 29.7 °C (18.9 - 39.4°C). The milk yield of low and high grade cows decreased by 0.20 and 0.24 lb, respectively, as the mean ambient temperature increased by 1°C. The supplementation of the cows' ration revealed a significant increase in milk yield of low and high grade cows. However, high grade cows were more responsive to supplementation low grade cows. The higher milk yield of the high grade cows under extreme summer conditions suggests the suitability of high grade cows for milk production under Sudan conditions. However, the lower heat tolerance and thermal sensitivity of high grade cows suggest that grading up should not proceed beyond the 62.5% level under the climatic conditions of northern Sudan.