Despite outstanding advancements in hormonal tools and strategies regarding the synchronization protocols that maximize the reproductive efficiency of grazing females in the tropics [[1], [2], [3], [4]], there are still some gaps to be fulfilled that could bring additional benefits to the beef cattle herd and, therefore, to the reproductive performance and profitability of cow-calf operations. Ovulation and pregnancy rates, as well as pregnancy loss remain the target variables to be improved to optimize the reproductive efficiency of cow-calf operations [[5], [6], [7]]. In the tropics, the breeding season often starts during the transition of the dry to the rainy periods, meaning that the reproducing females are likely going through a period of limited amount of nutrients from the forages.

Others have demonstrated that the nutritional status of a beef female impacts its reproductive function [8,9], as key hormones and metabolites associated with the nutritional plan and/or BCS of the females are required for a proper reproductive performance of the herd [10,11]. In fact, Carvalho et al. [11] demonstrated that Nellore cows that had a greater BCS at calving and at the beginning of the breeding season had a greater reproductive function and serum IGF-I when compared with cows that calved and entered the breeding season at a lower BCS. In fact, our research group has been reporting different strategies to maximize the reproductive performance of beef and dairy cows with a low BCS (≤2.75) [12,13], but additional efforts are warranted to identify further options to be used in commercial cow-calf beef operations. It is noteworthy to mention that not only BCS, but also the parity of the cows in the herd may impact the likelihood of a greater or lower reproductive performance in a cow-calf operation. Primiparous Nellore cows often have a greater BCS loss during the post-partum period when compared with secundiparous (two parities) and multiparous cows (≥3 parities) [11], indicating that strategies to adjust this nutritional deficit in primiparous might have a greater contribution to the operation.

Cows in negative energy balance and losing BCS often have an inadequate luteinizing hormone (LH) pulse frequency that affects the growth of the dominant follicle, estrus expression, ovulation rates, corpus luteum size, pregnancy maintenance, and pregnancy rates [14]. Hence, it is imperative to find alternatives to support the reproductive performance of the herd entering the breeding season during a period of feed scarcity (amount and quality). Gonadotropins, such as equine chorionic gonadotropin (eCG), present LH activity and have been included in progesterone-estradiol-based (P4-E2) synchronization protocols to improve follicle growth [15], size of the dominant follicle [15], estrus expression [15] as well as ovulation and pregnancy rates [[15], [16], [17]] in grazing B. indicus females. The standard dose of eCG determined to be effective to enhance pregnancy in suckling Nellore anestrous cows is 300 IU [5] and increasing the eCG dose from 300 to 400 IU resulted in similar pregnancy rates and growth of the ovulatory follicle in B. indicus grazing females [2,18,19]. Recently, Pugliesi et al. [20] demonstrated that pregnancy rates and size of the dominant follicle were improved in primiparous cows when the standard eCG dose (300 IU) was split on two days of the synchronization protocol. Based on this rationale, we hypothesized that (1) increasing the dose of eCG administered to grazing Nellore cows and (2) splitting the full dose over two days would impact follicle growth, estrus expression, and overall pregnancy rates in grazing Nellore cows assigned to a timed-artificial insemination (TAI) protocol. Therefore, our objective was to evaluate the effects of increasing and splitting the dose of eCG on reproductive performance of grazing primiparous and multiparous Nellore cows assigned to a TAI with different BCS.