Estrous cycle characteristics of postpubertal gilts approaching anestrus due to limited dietary energy intake

Twenty-nine Duroc x (Yorkshire x Landrace) gilts, approximately 9 mo of age and each having experienced at least three estrous cycles, were allotted randomly to five dietary treatments. Each treatment provided a different daily intake of metabolizable energy: .25, .5, .75, 1.0 Mcal (severely restrictive [RES]; chosen to cause cessation of estrous cycles) or 3.4 Mcal (control; designed for maintenance of estrous cyclicity). Blood samples were drawn by venipuncture every 3 d and sera were assayed for progesterone (P4) concentration. Estrous cycle length was estimated by defining d 0 as the last day when serum P4 concentrations were < 1 ng/mL before increasing to > 1 ng/mL. Gilts were considered acyclic when serum P4 levels remained < 1 ng/mL for four consecutive samples (9 d). Anestrus occurred after 78.7 +/- 6.1 d in all RES gilts (n = 23) and in one control on d 24. Compared with the control treatment, RES treatments resulted in longer (P < .05) average estrous cycles (21.0 +/- .2 vs 20.2 +/- .3 d) and greater (P < .05) mean serum P4 concentrations (13.7 +/- .4 vs 11.4 +/- .8 ng/mL). There were no differences in P4 or cycle length measurements among RES treatments. Cycle duration and serum P4 concentrations of the final estrous cycle before anestrus were similar to those of previous cycles. It is concluded that severe energy restriction caused elevated serum P4 concentrations. Neither cycle length nor serum P4 concentration was notably altered as gilts neared anestrus.