The effect of level of feed intake on progesterone clearance rate by measuring faecal progesterone metabolites in grazing dairy cows

The objective of the present study was to determine the effect of level of feed intake of pasture on P4 clearance rates in dairy cows. Twelve non-lact ating Holstein-Friesian cows aged 4-9 years were randomly allocated to a re stricted or ad libitum group. The ad libitum group had unrestricted access to irrigated pasture, whereas the restricted group had access for only 2h p er day. Each animal was drenched orally twice daily with a chromic oxide ca psule to allow daily feed intake to be estimated from faecal output (FO). E ndogenous progesterone (P4) production was eliminated by subcutanously impl anting a capsule containing 6 mg of a potent GnRH-agonist (deslorelin) into the ear of each animal 3 weeks before inserting a CIDR device containing 1 .9 g P4 into the vagina. Two luteolytic PGF2 alpha were given 10 days later . Each device was removed after 11 days and residual P4 measured. Daily pla sma samples were assayed for P4. Faecal samples were also taken daily and a ssayed for pregnanes (FP4M) containing a 20-oxo-, a 20 alpha- or a 20 beta -OH group with EIAs. The average daily dry matter (DM) intake of pasture was higher for cows in the ad libitum group (15.9 versus 6.3 kg DM, P = 0.001). Their plasma P4 co ncentrations were lower (1.08 versus 1.71 ng/ml, P = 0.05), even though the average residual P4 content of the used CIDR devices was not affected by f eed intake (1.20 versus 1.25 g, P > 0.05). The concentrations of FP4M were not affected by level of feed intake (20-oxo-: 3.3 versus 1.7, 20 alpha-: 3 .5 versus 3.7, 20 beta-: 2.1 versus 3.2 mug/g DM). Daily excretion rates of 20-oxo- and 20 alpha- were higher in ad libitum cows (20-oxo-: 17.8 versus 4.3 mg per day, P = 0.05, 20 alpha-: 18.2 versus 8.9 mg per day, P = 0.001 ), but daily yield of faecal 20 beta- was not affected by feed intake (11.9 versus 8.6 mg per day, P = 0.5). These results show that there was a negat ive relationship between feed intake and plasma P4 concentrations in these CIDR-treated GnRH-downregulated Holstein cows. Concentrations of FP4M were not affected by level of feed intake or FO, but daily excretion rate of FP4 M was associated with the volume of faeces. (C) 2001 Elsevier Science B.V. All rights reserved.