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Suckling Effect on Estrus and Blood Plasma Progesterone in Postpartum Beef Cows¹

Montana State University³, Bozeman 59717

Abstract

Progesterone was measured by radioimmunoassay in plasma collected daily from 12 Angus and six Hereford cows to characterize the progesterone profile from calving through the first postpartum estrous cycle. Cows were randomly assigned within breed to be non-suckled (NS), suckled twice daily (S2X) or suckled ad libitum (S). Nutrient intake (TDN) was adjusted according to lactational status so the cows would maintain their 48-hr post-calving body weights through the postpartum interval. The postpartum interval of 20 days in the NS group was less (P<.05) than those of 34 and 38 days in the S2X and S groups, respectively. Intensity of first observed estrus was scored at 2.2 for cows in the NS group and was increased (P<.05) to 3.8 for those in the S2X and S groups. In analyses excluding three cows with silent first estrus, progesterone in postpartum samples (parturition through 18 days postpartum) averaged .8 ng/ml at parturition and ranged between .1 and .9 ng/ml through 18 days postpartum. Progesterone in preestrus samples (–18 through 0 days in relation to first estrus) ranged between .1 and .4 ng/ml from –18 through –5 days, increased rapidly to 1.8 ng/ml at –3 days (preestrus progesterone peak) and returned to .3 ng/ml at –1 and 0 days preestrus. Progesterone in postestrus samples (days 9, 10 and 11 of first estrous cycle) averaged 3.6 ng/ml. Suckling (1) decreased (P<.01) average progesterone concentrations for groups S2X and S compared to group NS in postpartum samples (NS = .4, S2X = .1 and S = .1 ng/ml), (2) did not significantly affect average concentration in preestrus samples (NS = .4, S2X = .3 and S = .5 ng/ml; S2X differs from S, P<.05) and (3) did not significantly affect average progesterone concentrations in postestrus samples (NS = 2.8, S2X = 4.4 and S = 3.6 ng/ml). The magnitude of the preestrus progesterone peak was positively associated with the length of the postpartum interval (r = .69, P<.01) and was higher (P<.05) in cows in the S group than in those in the NS and S2X groups (3.9, .9 and 1.5 ng/ml, respectively). These data show that suckling affects postpartum estrus and plasma progesterone and indicate that progesterone is involved in the reestablishment of postpartum estrous cycles.

¹ Published with the approval of the Director of the Montana Agr. Exp. Sta. Journal Ser. No. 993. This study was a contribution to Western Regional Res. Project W-112, Reproductive Performance in Beef Cattle and Sheep. Appreciation is expressed to C. Hardee Clark and Robert Brown for technical assistance, to Drs. G. D. Niswender and T. M. Nett for supplying antiprogesterone ll-BSA (#337) and to Mrs. Katie Cimmino and Mrs. Joyce Bissel for typing the manuscript.

² Present address: Iowa State Univ., McNay Memorial Res. Center, RR 2, Chariton. Send request for reprints to Montana State Univ.

³ Anim. and Range Sci. Dept.