Postpartum interval, ovarian-uterine relationships of beef cattle and effect of monensin fed to prepuberal heifers under natural and cultivated grazing conditions in the Venezuelan llanos

Abstract

A group of three experiments were conducted in the Central Llanos of Venezuela to determine physiological reproductive effects of ovaries transplanted to the uterine muscle or parotid muscle, the effect of feeding monensin on the age, weight at puberty of prepuberal heifers grazing fertilized Aleman grass (Echinochloa polystachya, (H.B.K.) Hitchc) pastures and the postpartum interval from calving to first estrus of Bos indicus and Bos Taurus cows under natural grazing of yaraguagrass (Hyparrhenia rufa (Nees) Staf) pastures. In the first study, the ovarian transplants were successfully accomplished in 94% of the cows (17/18) and in 83% of the heifers (5/6). Autotransplantation of active ovaries (AO) of mature cycling cows to the myometrium of the uterine horn (U) or the parotid muscles (P) with the inactive ovaries (IO) left in situ resulted in more estrus activity (P<.025) than in cows ovariectomized and with an IO transplanted to either site (6/6 vs 3/6). Microscopic and histological findings indicated that all prepuberal ovaries in situ showed follicular development when mature AO were transplanted to either the U or parotid. The same trend was found in prepuberal ovaries transplanted to mature cycling cows. Estrous cycle lengths tended to be shorter in cows and heifers with their own ovaries in situ and an ovary heterotransplanted to the P (15 days [plus or minus] 3.9 vs 22.5 [plus or minus] 7.0 days, respectively). Pregnancy rates were higher (P<.005) in cows with at least on ovary in situ and an ovary transplanted to the parotid (5/6) than cows with at least one ovary in situ and the ovary transplanted in the uterus (0/6). Ovary auto and heterotransplantation contributed to an increase in volume of all ovaries in situ (P<.05) in cows and prepuberal heifers. In the second study, the average daily gain (ADG) did not differ (P>.05) vs M and F heifers which had similar scores. All M heifers (100%) reached puberty (10/10) (P<.075), while puberty was reached in 80% of F heifers (8/10) and in 60% of C heifers (6/10). Administration of monensin to the prepuberal heifers stimulated total ovarian volume significantly (P<.05) over the control heifers but did not differ from F heifers. Fertility was enhanced in M heifers with 20% pregnancy compared to no heifers in the concentrate or grass-fed groups became pregnant...