Metabolizable protein supply while grazing dormant winter forage during heifer development alters pregnancy and subsequent in-herd retention rate

Two studies were conducted to evaluate effects of post-weaning management of British crossbred heifers on growth and reproduction. In Exp. 1, 239 spring-born, crossbred heifers were stratified by weaning BW and allotted randomly to 1 of 2 treatments. Treatments were fed at rate equivalent to 1.1 kg/d while grazing dormant forage and were: (1) 36% CP containing 36% RUP (36RUP), or (2) 36% CP containing 50% RUP (50RUP). Supplementation was initiated in February (1995 and 1996) or November (1997 and 1998) and terminated at the onset of breeding season (mid-May). Body weights were taken monthly up to breeding and again at weaning. After timed AI, heifers were exposed to a breeding bull for 42 ± 8 d. In Exp. 2, 191 spring-born, crossbred heifers were stratified to treatments by weaning BW. Heifer development treatments were: (1) pasture developed and fed 0.9 kg/d of a 36% CP supplement containing 36% RUP (36RUP); (2) pasture developed and fed 0.9 kg/d of a 36% CP supplement containing 50 % RUP (50RUP); (3) growing diet in a drylot (DRYLOT). Heifers receiving 36RUP and 50RUP treatments were developed on dormant forage. Supplementation was initiated in February and terminated at the onset of a 45-d breeding season in May. Heifer BW and hip height were taken monthly from initiation of supplementation until breeding and at pregnancy diagnosis. In Exp. 1, BW was similar (P > 0.27) for 36RUP and 50RUP heifers at all measurement times. However, pregnancy rates were greater (P = 0.02; 80 and 67 %) for 50RUP heifers than 36RUP heifers. In exp. 2, breeding BW was greater (P < 0.01) for the DRYLOT heifers than 36RUP or 50RUP developed heifers. However, pregnancy diagnosis BW was similar (P = 0.24) for all heifer development treatments. Pregnancy rates tended to be greater (P = 0.10) for 50RUP heifers than 36RUP and DRYLOT. However, retention rate after breeding yr 3 and 4 was greatest (P < 0.01) for 50RUP heifers. This study indicates increasing the supply of metabolizable protein by increasing the proportion of RUP in supplements fed to heifers on dormant forage prior to breeding has shown to have an economic advantage while increasing pregnancy rates and cow herd retention rate compared to developing heifers on a concentrate diet in a drylot.