A trial was conducted to evaluate the accuracy of the 1996 NCR beef model to predict DMI and ADG of pregnant cows under western Canadian conditions. Over two consecutive years, 90 Angus (587±147 kg) cows assigned to 15 pens (N=6) were fed typical diets ad libitum, formulated to stage of pregnancy. Data collection included pen DMI and ADG (corrected for pregnancy), calving date, calf weight, body condition scores and ultrasound fat measurements, weekly feed samples and daily ambient temperature. DMI and ADG for each pen of cows in each trimester was predicted using the computer program Cowbytes based on the 1996 NRC beef model. The results indicate that in the 2nd and 3rd trimester of both years the model under predicted (P≤0.05) ADG based on observed DMI. Ad libitum intake was over predicted (P≤0.05) during the 2nd trimester, and under predicted (P≤0.05) during the 3rd trimester of pregnancy. A second evaluation was carried out assuming thermal neutral (TN) conditions. In this case, it was found that during the 2nd and 3rd trimesters there was an over prediction (P≤0.05) of ADG relative to observed. Under these same TN conditions, the ad libitum intake of these cows was under predicted (P≤0.05) for both the 2nd and 3rd trimesters. These results suggest current energy equations for modelling environmental stress, over predict maintenance requirements for wintering beef cows in western Canada. The results also suggest that the cows experienced some degree of cold stress, but not as severe as modelled by the NRC (1996) equations. Further research is required to more accurately model cold stress felt by mature cattle, and their ability to acclimatise to western Canadian winter conditions.

A trial was conducted to evaluate the accuracy of the 1996 NCR beef model to predict DMI and ADG of pregnant cows under western Canadian conditions. Over two consecutive years, 90 Angus (587±147 kg) cows assigned to 15 pens (N=6) were fed typical diets ad libitum, formulated to stage of pregnancy. Data collection included pen DMI and ADG (corrected for pregnancy), calving date, calf weight, body condition scores and ultrasound fat measurements, weekly feed samples and daily ambient temperature. DMI and ADG for each pen of cows in each trimester was predicted using the computer program Cowbytes based on the 1996 NRC beef model. The results indicate that in the 2nd and 3rd trimester of both years the model under predicted (P≤0.05) ADG based on observed DMI. Ad libitum intake was over predicted (P≤0.05) during the 2nd trimester, and under predicted (P≤0.05) during the 3rd trimester of pregnancy. A second evaluation was carried out assuming thermal neutral (TN) conditions. In this case, it was found that during the 2nd and 3rd trimesters there was an over prediction (P≤0.05) of ADG relative to observed. Under these same TN conditions, the ad libitum intake of these cows was under predicted (P≤0.05) for both the 2nd and 3rd trimesters. These results suggest current energy equations for modelling environmental stress, over predict maintenance requirements for wintering beef cows in western Canada. The results also suggest that the cows experienced some degree of cold stress, but not as severe as modelled by the NRC (1996) equations. Further research is required to more accurately model cold stress felt by mature cattle, and their ability to acclimatise to western Canadian winter conditions.