Conception rate in Holstein cows treated with GnRH or hCG on the fifth day post artificial insemination during summer

M.P Beltran,J.L.M Vasconcelos

doi:10.1590/s0102-09352008000300009

Abstract

Lactating Holstein cows (n=158), at 213±112 days in milking and averaging 26±9kg of milk per day, were randomly assigned to one of three treatment groups: control (CG, n=52, saline), GnRH (GG, n=55, 100g gonadorelin), and hCG (HG, n=51, 2500IU) given five days after artificial insemination (AI). Rectal temperature was taken at the moment of AI and blood samples were collected five, seven, and 12 days after AI. Pregnancy was determined between 42 and 49 days after AI. Concentration of progesterone (P4) in serum (ng/ml, mean±SE) for CG, GG, and HG were, respectively, 2.7±0.4, 2.5±0.4, and 3.2±0.5 on day 5; 4.8±0.4, 4.2±0.4, and 5.7±0.5 on day 7; and 5.2±0.4, 6.9±0.4, and 8.5±0.5 on day 12 after AI. P4 concentration had proportional increase in serum between days 5 and 7 after AI (CG: 178%, GG: 168%, and HG: 178%), suggesting that the treatments did not induce a luteotropic effect on the existing corpus luteum (CL). Concentrations of P4 increased between days 7 and 12 in cows treated with GnRH and hCG (GG: 164%, and HG: 149%, P<0.01); but not in control cows (GC: 18%, P=0.31), suggesting that a new CL was formed. Treatments with GnRH or hCG increased conception rates in cows with rectal temperature below 39.7°C (CG: 10.1%, n=26, GG: 36.8%, n=27; and HG: 32.8%, n=21), but not in cows with rectal temperature above 39.7°C (CG: 15.2%, n=26; GG: 17.8%, n=28; and HG: 24.4%, n=30). These data suggest that high body temperature masked the positive impact of treatment with GnRH or hCG on day 5 after AI on conception.

Highlights

MATERIAL AND METHODSDairy cows with high milk production have low conception rates, which causes major economic losses (Nebel and McGilliard, 1993), and have lower progesterone (P4) serum concentrations, probably due to higher progesterone clearance (Vasconcelos et al, 1999)
Changes in progesterone concentrations from day 5 to 7 after artificial insemination (AI) were proportional in all three groups, which suggest that a possible luteotropic effect of hCG and GnRH was not detected in treated animals
This finding is in agreement with Schmitt et al (1996a), who evaluated P4 concentration and the original corpus luteum (CL) morphology and functionality on day 17 in cows treated with GnRH or hCG on day 5 after AI, and did not find increase in P4 concentration or cellular alterations that justified the luteotropic effect

Summary

Introduction

Dairy cows with high milk production have low conception rates, which causes major economic losses (Nebel and McGilliard, 1993), and have lower progesterone (P4) serum concentrations, probably due to higher progesterone clearance (Vasconcelos et al, 1999). Low P4 concentration in blood would partially explain the low fertility in high producing dairy cows, as P4 concentrations after artificial insemination (AI) are positively associated with embryo maturity and functionality (Garret et al, 1988), which is critical for inhibiting luteolysis and maintaining pregnancy (Mann et al, 1995; Meyer et al, 1995). It is possible to conclude that part of the embryonic losses in dairy cattle may be due to inadequate maternal luteal function (Lamming et al, 1989; Mann et al, 2001). The use of GnRH (Lewis et al, 1990; Schmitt et al, 1996a) or hCG (Mancio et al, 1999; Santos et al, 2001) after AI increases P4 secretion due to the luteotropic effect (Kerbler et al, 1997; Santos et al, 2001) and/or induction of an accessory corpus luteum (aCL) (Schmitt et al, 1996a; Fonseca et al, 2001; Santos et al, 2001); increasing progesterone concentration and, potentially, conception

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