EFFECT OF CALF ISOLATION ON FOLLICULAR WAVE DYNAMICS, GONADOTROPIN AND METABOLIC HORMONE CHANGES, AND INTERVAL TO FIRST OVULATION IN BEEF-COWS FED EITHER OF 2 ENERGY-LEVELS POSTPARTUM

The effects of postpartum energy intake, restricted suckling, and cow- calf isolation on concentrations of LH, FSH, growth hormone, and insul in-like growth factor-I (IGF-I) and on postpartum anestrous interval w ere determined by randomly allocating beef cows with a mean body condi tion score of 2.3 +/- 0.1 to receive either 80 MJ metabolizable energy (low-energy diet [L]; n = 51) or 120 MJ metabolizable energy (high-en ergy diet [H]; n = 52) per cow per day from calving. At 30 days postpa rtum, cows within diet were randomized to 1) have continued full acces s to their calves from birth to weaning (ad libitum suckling: ADLIB), 2) be suckled once-daily with their calves penned adjacent (restricted suckling, adjacent: RESADJ), 3) be isolated from all calves except fo r a once-daily suckling period (restricted suckling, isolated: RESISO) . The mean postpartum interval was similar (p > 0.10) for L and H cows (62 and 63 days, respectively). RESADJ cows had a shorter (p < 0.05) postpartum interval than ADLIB cows, and RESISO cows had a shorter int erval tp < 0.05) than RESADJ cows, with all effects independent (p > 0 .10) of diet. FSH secretion pattern was not affected by diet, suckling treatment, sequential follicle wave number, or follicle wave retrospe ctively realigned to emergence of first ovulatory wave. Within 5 days of suckling restriction and calf isolation, the number of LH pulses in creased from 0.18 to 0.48 pulses per hour (p < 0.05). Both mean LH and the mean number of LH pulses increased linearly (p < 0.01) during the six follicle waves up to the first ovulatory wave. From 80 days befor e, until the time of, first ovulation, growth hormone decreased tp < 0 .05) while IGF-I increased (p < 0.05), irrespective of treatment. The results indicate that the ''suckling effect'' in beef cows is the majo r factor affecting the duration of the postpartum interval and suggest s that the maternal bond is more important than suckling in regulating LH pulse frequency, the key endocrine factor determining whether or n ot a dominant follicles ovulates. Removal of the suckling effect resul ted in a rapid increase in LH pulse frequency, which was not dependent on level of postpartum nutrition, at least within the nutritional lim its of this study. Mean concentrations of FSH, unlike LH, did not vary with follicle wave number, suggesting that lack of FSH is not a major factor delaying the resumption of ovulation in postpartum beef cows.