BODY ENERGY RESERVES INFLUENCE THE ONSET OF LUTEAL ACTIVITY AFTER EARLY WEANING OF BEEF-COWS

The influence of body energy reserves on the onset of luteal activity and concentrations of LH and IGF-I in serum was evaluated in postpartu m anestrous beef cows after early weaning. Multiparous Hereford and He reford x Angus cows (n = 24) were fed during gestation to establish bo dy condition scores between 3 and 6 (BCS, 1 = emaciated; 9 = obese) at parturition. Concentrations of progesterone in plasma were determined weekly for 5 wk postpartum (PP). Anovulatory cows and their calves (n = 19) were confined in stalls on d 40 +/- 3 PP. Jugular cannulas were inserted on d 44 +/- 3 PP, and calves were weaned (d 0) the following day. Blood samples were collected from all cows for 4 h (every 10 min ) before weaning and on d 1, 2, 4, 6, 8, and 10 after weaning and LH w as quantified. Progesterone was quantified in daily blood samples unti l d 10, and in samples taken twice weekly until d 46. Within 25 d afte r weaning, 100% of the cows with a BCS greater than or equal to 5 at w eaning (n = 7) had initiated luteal activity, whereas only 43% (P < .0 1) of the cows with BCS < 5 (n = 12) had luteal activity. Mean serum I GF-I concentrations were correlated with BCS (r = .50; P < .05). Frequ ency of LH pulses was influenced (P < .01) by body condition at weanin g but was not influenced by day after weaning. The number of LH pulses at weaning, serum IGF-I, and the interval to the onset of ovarian act ivity after early weaning of anestrous beef cows were influenced by BC S.