Effects of level of feeding and progesterone dose on plasma and faecal progesterone in ovariectomised cows

The effects of two levels of feeding and two doses of progesterone (P4) on plasma and faecal progesterone metabolites (FP4M) were studied using a total of 24 ovariectomised (OVX), non-lactating, Holstein–Friesian cows. Cows were grazed on improved ryegrass/white clover pastures and allowed ad libitum access to pasture or were restricted to grazing for a total of 4 h per day in two 2 h periods. Progesterone (P4) was administered as one or two, simultaneous, intravaginal progesterone devices (CIDR). The cows were adapted to their pasture supply for 2 weeks before the start of the progesterone treatments. The progesterone devices were administered for 11 days and the cows were dosed with slow release chromic oxide capsules during the P4 treatment to allow faecal output (FO) to be estimated. Daily blood samples for P4 assay and weekly samples for blood metabolite assay were collected. Faecal samples were collected per rectum daily and assayed for pregnanes containing a 20-oxo-, 20α- or a 20β-OH group by enzyme immunoassay (EIA). FO was higher (P<0.001) for ad libitum than pasture restricted cows (6.3 vs 4.1 kg DM) but was similar for both doses of P4. The average mass of P4 released from a CIDR device over a 11-day period was higher for cows allowed ad libitum pasture compared with those on restricted pasture (0.64 vs 0.60 g; P=0.04). Plasma P4 concentrations, however, were higher in restricted than ad libitum fed cows (1× CIDR: 1.81 vs 1.41 ng/ml; 2× CIDR: 4.10 vs 3.46 ng/ml). Increasing the progesterone dose significantly (P<0.001) increased both the concentrations and daily totals of the faecal pregnanes assayed and total FP4M. Restricted pasture cows had higher (P<0.001) pregnanes and FP4M concentrations than cows fed ad libitum. Daily total faecal pregnane and FP4M did not differ between feeding levels except for faecal 20α-pregnane which was highest for ad libitum fed cows (P<0.05). results showed that the plasma concentrations of P4 in CIDR-treated OVX cows were negatively associated with the level of feeding. Level of feeding and dose of P4 affected the concentrations of FP4M, but the daily excretion rate of FP4M was not positively influenced by the level of feeding.