Use of an enzyme immunoassay to determine concentrations of progesterone in caprine plasma and milk.

Concentrations of progesterone (P4) were determined using enzyme immunoassay kits on plasma and milk samples obtained on the same days from 18 lactating dairy goats. Progesterone profiles documenting anestrus, short estrous cycles, normal estrous cycles, a prolonged follicular phase, and prolonged luteal phases were established. When plasma P4 concentrations were used as an accurate indication of the presence or absence of functional luteal tissue, milk P4 concentrations agreed with plasma determinations in 79.4% of the 465 samples tested. Milk samples could not be used to make a definitive decision because of marginal values in 11.2% of the determinations. Milk P4 concentrations were high when plasma P4 concentrations were low in 6.2% of the paired samples, especially those obtained around the time of estrus when peripheral P4 concentrations were changing rapidly. The remaining 3.2% of milk samples had low milk P4 concentrations when plasma P4 concentrations were high. Composite milk from 8 does in estrus or 8 does in the luteal phase was not consistently different from strippings in butterfat percentage or P4 concentration.