Effect of low levels of exogenous progesterone on puberty in beef heifers.
Twenty eight prepuberal Charolais-Angus cross heifers approximately 15 months old and averaging 334 kg were implanted intravaginally with either a placebo (control, 9 heifers) or progesterone containing implant (treated, 19 heifers). Implants were left in place 3 days. Seventeen heifers (8 controls and 9 treated) retained implants. Plasma progesterone concentrations were < .4 ng/ml in control heifers and 1.5 +/- .59 ng/ml in treated heifers while implants were in place. Seven heifers, all in the treated group, were observed in estrus within 4 days after implant removal. All but one heifer observed in estrus had elevated plasma progesterone concentrations 12 days after implant removal. None of the heifers not observed in estrus had elevated progesterone levels at this time. This study indicates that low levels of exogenous progesterone (1 to 2 ng/ml plasma for 3 days) will induce estrus in prepuberal heifers following its withdrawal.
Sheffield
L G
LG
Clemson University Clemson, SC USA.
Ellicott
A R
AR