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

Abstract

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-lactating Holstein–Friesian cows aged 4–9 years were randomly allocated to a restricted or ad libitum group. The ad libitum group had unrestricted access to irrigated pasture, whereas the restricted group had access for only 2 h per day. Each animal was drenched orally twice daily with a chromic oxide capsule to allow daily feed intake to be estimated from faecal output (FO). Endogenous progesterone (P4) production was eliminated by subcutanously implanting 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 PGF2α were given 10 days later. Each device was removed after 11 days and residual P4 measured. Daily plasma samples were assayed for P4. Faecal samples were also taken daily and assayed for pregnanes (FP4M) containing a 20-oxo-, a 20α- or a 20β-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 concentrations 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 feed 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, 20α-: 3.5 versus 3.7, 20β-: 2.1 versus 3.2 μg/g DM). Daily excretion rates of 20-oxo- and 20α- were higher in ad libitum cows (20-oxo-: 17.8 versus 4.3 mg per day, P=0.05; 20α-: 18.2 versus 8.9 mg per day, P=0.001), but daily yield of faecal 20β- was not affected by feed intake (11.9 versus 8.6 mg per day, P=0.5). These results show that there was a negative 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 FP4M was associated with the volume of faeces.

Introduction

Nutritional management plays an important role in the postpartum reproductive performance of dairy cattle, especially the overall energy balance of the animal. Both feed intake (FI) and body tissue reserves are used as energy source for body function and milk production. Progesterone (P4) has been recognised as the hormone that maintains pregnancy (Mee et al., 1991, Taylor and Rajamahendran, 1994). Attention has been focused on the dynamics of P4 production and metabolism, and how nutrition influences these parameters (Butler, 1998). Most of the research that has addressed the influence of nutrition on the concentrations of P4 and oestradiol has focused on production. However, little is known about the production rate of P4; or whether the changes in plasma concentration are related either to differences in production rate, or to alternations in the rate of its catabolism.

The concentration of P4 in blood is related to production and clearance rate. Aspects of nutrition have been shown to affect both rates. Most of the research that has focused on P4 production has suggested that subnormal P4 levels in energy-deficient cows are sometimes a result of decreased CL function (Carstairs et al., 1980, Ducker et al., 1985, Villa-Godoy et al., 1988). Cows in negative energy balance and low levels of P4 may be at a greater risk for premature luteolysis than cows with normal P4 levels and a positive energy balance. Low level of energy in the diet also can affect P4 concentrations as shown by Smith (1988) that a low serum glucose concentration was found to decrease LH pulse frequency and amplitude and result in diminished P4 production by the CL.

The P4 clearance rate from the body is considered to have an influence on the circulating concentration and may in turn influence the conception rate of cows. The splanchnic extraction rate of P4 was found to be about 90% in sheep to account for approximately 27% of whole body metabolic clearance (Bedford et al., 1974). The blood flow rate to the liver may be modified by FI (Symonds and Prime, 1989), and thus be expected to substantially alter splanchnic clearance rates. Cumming et al. (1971) were the first to recognise that modifying the level of FI also altered the plasma concentration of P4. The cause of this response was thought to be an increase in hepatic blood flow with increased FI and consequently an increase in the metabolism of P4 by the liver. An inverse relationship between feed level and concentrations of peripheral plasma P4 was attributed to differences in the clearance rate of P4 rather than to changes in its entry rate into systemic blood circulation (Williams and Cumming, 1982, Parr et al., 1993). There is limited information regarding the effect of level of feeding on P4 metabolism in dairy cattle. It has been suggested (Vasconcelos et al., 1998, Sangsritavong et al., 2000) that high feed consumption could alter the clearance rate of P4 in the dairy cows.

The main route of excretion of P4 is through the liver and then as bile to the faeces. Therefore, the analysis of faecal P4 metabolites (FP4M) has emerged as an appropriate method for monitoring ovarian function, as already shown in cattle (Desaulniers et al., 1989, Larter et al., 1994, Schwarzenberger et al., 1996). Our previous study (Rabiee et al., 2001a) showed that by measuring faecal P4 metabolites, it was possible to investigate P4 clearance rate in dairy cows. The objective of the present study was to determine the effect of level of FI of pasture on P4 clearance rates in dairy cows. In this study, an external source of P4 was administered to cows implanted with a potent GnRH-agonist, deslorelin, to investigate the effect of level of feeding on plasma P4 and the concentrations and daily yield of FP4M.