Gestation nutrition, tissue exchange and maintenance requirements of heifers.

Forty-six pregnant, crossbred, 2-yr-old heifers of large or small mature size were individually fed a 70% cottonseed hull diet during gestation. Heifers were fed at either 1.0% (nutritionally restricted) or 1.5% (nonrestricted) of body weight from 90 d gestation through parturition. Live weight, from 90 d gestation to parturition, was reduced by 20.5% and 1.0% for restricted and nonrestricted heifers, respectively. Whereas nonrestricted heifers gained maternal protein (3.2 kg) from d 90 through parturition, restricted heifers lost (P less than .05) protein (-5.4 kg) and mobilized twice (P less than .05) as much fat (49 vs 25 kg). Percentage of empty body protein increased 13.1 and 9.3% in restricted and nonrestricted heifers, respectively, whereas fat decreased (P less than .05) 31.7% (restricted) and 23.5% (nonrestricted) from 90 d to parturition. Daily metabolizable energy for maintenance (MEm) was greater for large than for small mature size heifers (169 vs 158 kcal/kg.75). Efficiency of ME use for gain was greater for small than for large mature size heifers (36.5 vs 31.2%). Efficiency in early gestation (45.4%) was greater than in late gestation (29.4%) and averaged 33.8% for 90 d gestation to parturition. Maintenance ME requirements increased 25% and efficiency of ME use decreased 35% with advancing stage of gestation. Nutritional restriction of heifers reduced (P less than .05) calf birth weight (27.3 vs 30.2 kg) and decreased gestation lengths (275 vs 282 d) compared with nonrestricted heifers. This research indicates that nutritional restriction of beef heifers alters birth weight, repartitions maternal tissues and changes rate of tissue mobilization.