REFERENCE : Armengol-Gelonch R et al. Impact of phase of the estrous cycle and season on LH surge profile and fertility in dairy cows treated with different GnRH analogs (gonadoreline vs buserelin). Theriogenology 2017,91,121-126.

SOME BACKGROUND

In bovine reproduction, GnRH is primarily used to induce ovulation of a dominant follicle (and its subsequent luteinization) or to increase the synthesis capacity of a luteal tissue (corpus luteum or luteinized follicular cyst). These effects are mediated by the LH thus released. These effects justify its use at the beginning of an Ovsynch-type synchronization protocol. Furthermore, the release of LH following injection of a GnRH is less during environmental heat stress.

HOW DID THEY DO IT ?

The study involves 793 dairy cows (41.5 kgs of milk per day) raised in an environment characterized by a THI of 80 and 40 in the warm (May to October) and cold (November to April) seasons of the North East of Spain), treated in a synchronization protocol with 100 mg of gonadorelin diacetate tetrahydrate (Ovareline CEVA) or 10 mcg of buserelin diacetate (Receptal MSD).

THEIR RESULTS

• No difference is observed between the two types of gonadotropin with respect to their effects on the amount of LH release they induce in warm or cold season or in proestrus vs. diestrus.
• Ovareline induces the appearance of a peak earlier than Receptal (1.5 h vs 2.3 h)
• The peak of LH is significantly higher in the proestral phase than in the estral phase (8.2 ng vs 4.7 ng). This reflects the “inhibitory” effect of progesterone and the “facilitating” effect of estradiol.

• Heat stress has no effect on the LH peak (nor on the interval between injection and peak or on the ovulation rate) but reduces the total amount of LH released. This effect would be mediated by cortisol. The type of GnRH and the phase of the cycle do not influence this effect of heat stress.
• Fertility was decreased during the heat stress period (33 vs 44% at D60). No effect of gonadotropin type was observed.
• Fertility was found to be lower in pluriparous compared to primiparous (29 vs 49%).

MORALITY :

The choice of gonadotropin is less important than its indication.