Plasma progesterone profiles and fertility status of anestrus Zebu cattle treated with norgestomet-estradiol-eCG regimen

Zebu cattle are notorious for poor fertility characterized by late maturity and long intercalving intervals attributed to a variety of factors, includ ing genetic, nutritional and climatic. The aim of the present investigation , therefore, was to induce fertile estrus in acyclic pubertal heifers and p ostpartum anestrous Zebu cows by hormonal intervention. Pubertal Hariana an d Sahiwal anestrous heifers (n=51) and postpartum cows (n=55) were either a ssigned a placebo (controls, n=6 for each breed and parity) or treated with 10-d norgestomet (3 mg) subcutaneous ear implants, with an initial injecti on of 3 mg, im norgestomet + 5 mg estradiol valerate, followed by 500 IU eC G at implant withdrawal (NOR-treated groups). Jugular venous plasma samples were obtained from a total of 28 animals (controls : 4 heifers and 4 cows; NOR-treated : 12 heifers and 8 cows) on Days 0 (implant insertion), 3, 7, 9 and Day 10 (implant withdrawal), every 12 h on Days II and 12, and then o nce daily on Days 17, 24 and 31. All the samples were assayed for progester one. Almost all (97%) heifers and 81% cows were induced to estrus, the majo rity (92% heifers and 79% cows) within 120 h of implant removal. Synchrony of the induced estrus was better in cows, but interval to estrus and estrus duration were significantly longer in heifers (P<0.05). Post-treatment fer tility, based on Day 28 nonreturn rate, first service, and overall concepti on rates, was better in heifers (78.9, 60.5 and 73.7%, respectively) than c ows (77.1, 48.6 and 62.9%, respectively), but the differences were signific ant only for the overall pregnancy rate (71.8% for heifers and 51.2% for co ws; P<0.05). Low pre-treatment plasma progesterone values (<0.5ng/mL) were consistent with ovarian inactivity, confirming the true anestrus status of experimental animals. Controls failed to exhibit estrus and maintained low progesterone concentrations throughout the study. In treated animals, high progesterone values from Day 17 onwards suggested ovulatory estrus. These e arly luteal phase progesterone concentrations in nonpregnant (P=0.06) and n onpregnant, nonreturn (P<0.05) animals were low in comparison with those of pregnant animals. Good fertility resulting from breeding according to estr us, inspite of variable intervals to estrus and estrus duration, advocates its advantage over fixed-time insemination in norgestomet-treated anestrous Zebu cattle. (C) 1998 by Elsevier Science Inc.