Energy utilization by mature, nonpregnant, nonlactating cows of different types.

Four types of crossbred cows were used in each of two studies designed to estimate the metabolizable energy (ME) required to maintain weight or energy equilibrium of mature (9-yr-old), nonpregnant, nonlactating cows. The four cow types were chosen to represent cow types having moderate mature size-moderate milk production potential (Angus X Hereford, Hereford X Angus; AHX), large mature size-moderate milk production potential (Charolais X Angus or Hereford; CX), small mature size-high milk production potential (Jersey X Angus or Hereford; JX) and large mature size-high milk production potential (Simmental X Angus or Hereford; SX). In each of two consecutive years, four cows of each type were assigned randomly and fed individually either a low (113 kcal ME X kg-.75 X d-1), medium (178 kcal ME X kg-.75 X d-1) or a high (ad libitum) level of a corn silage-based diet for 140 d. Body composition of each cow was estimated by deuterium oxide dilution techniques at d 0 and 140 of each study and gains were calculated. The ME required to maintain weight or energy equilibrium was estimated for each type of cow. Estimates of ME required to maintain energy equilibrium (from the regression of log heat production, kcal X kg-.75 X d-1 on ME intake kcal X kg-.75 X d-1) were 130, 129, 145 and 160 kcal X kg-.75 X d-1 for AHX, CX, JX, and SX cows, respectively. Similar trends were observed when other models were used. Cow types with higher milk production potential had higher maintenance (kcal X kg-.75 X d-1) than cows with lower milk production potential. Cows of larger size had maintenance requirements (kcal X kg-.75 X d-1) similar to those of smaller size. Possible reasons for these differences as well as their potential impact on cow efficiency are discussed.