Accounting for the effects of environment on the nutrient requirements of dairy cattle.

The maintenance requirements of the Cornell Net Carbohydrate and Protein System were revised to evaluate the effects of activity, temperature, and humidity. Four dairy heifer situations were simulated (1 = clean and dry, 2 = moderately matted hair coat, 3 = condition 2 plus 10-cm lot mud from November to March, and 4 = condition 1 plus 16-kph wind) to represent typical conditions of the northern and southwestern US. In the northern condition, predicted daily gain was 0.88, 0.60, 0.53, and 0.68 kg/d for the four environmental situations; corresponding values for the Southwest were 0.88, 0.88, 0.78, and 0.88. Environmentally neutral daily gain was 0.94 kg/d to a BW of 603 kg at first calving at 20.3 mo of age. Calving age was increased when environmental stress extended the age at which puberty weight was reached. Calving weight was decreased when environmental stress occurred after conception. Twelve environmental conditions (variable temperature, humidity, and housing) for lactating dairy cows were simulated. At 30 degrees C and no night cooling, predicted milk production decreased 2.6 and 11.9 kg/d at 20 and 80% humidity, respectively. Increased activity reduced predicted milk production to 0.4 to 1.3 kg/d in confinement scenarios and to 0.9 to 7.5 kg in grazing scenarios.