GnRH in the infundibular stalk-median eminence is related to percentage body fat in carcasses of beef cows.

Mature Hereford cows (n=28) were used to determine the effect of percentage body fat on secretion of LH and content of GnRH in the infundibular stalk-median eminence (ISME). Cows were fed to maintain, lose, or gain weight to achieve body condition scores (BCS; 1=emaciated; 9=obese) of 3 to 7. Then cows were fed to maintain weight and body condition. Before slaughter, estrus was synchronized using two injections of prostaglandin F2α(PGF) 11 d apart. Five d after the second PGF injection, cows were given 100 μg of GnRH (im) and serum samples were obtained. LH was quantified using RIA. The anterior pituitary and ISME were obtained within 45 min of death. Anterior pituitary weight and LH concentration, total GnRH in the ISME, total carcass fat, and percentage carcass fat were determined. BCS of cows at the time of slaughter influenced percentage carcass fat (P<.001), total GnRH in the ISME (P<.02), and maximum LH after GnRH treatment (P<.09), but did not influence pituitary weight or concentration of LH in the pituitary. Content of GnRH in the ISME averaged 76 ± 12, 32 ± 14, 27 ± 13, and 24 ± 13 ng for cows with BCS of 3, 5, 6, and 7, respectively. BCS was correlated (P<.001) with percentage carcass fat (r=.94) and total fat in the carcass (r=.92). Total GnRH in the ISME was negatively correlated (P<.005) with BCS (r=−.54), percentage carcass fat (r=−.55), and total carcass fat (r=−.49). Maximum LH after GnRH treatment was negatively correlated with BCS (P<.05; r=−.38) and percentage carcass fat (P<.05; r=−.32). We conclude that reduced secretion of LH associated with a decrease in body fat in cows may be due to reduced release of GnRH from the ISME.