Direct and indirect nutritional factors that determine reproductive performance of heifer and primiparous cows

Abstract

Pregnancy rate is a major determinant of population dynamics of wild ungulates and of productivity of livestock systems. Allocation of feeding resources, including stocking rates, prior to and during the breeding season is a crucial determinant of this vital rate. Thus, quantification of effects and interaction among multiple factors that affect pregnancy rate is essential for management and conservation of pasture-based systems. Pregnancy rate of 2982 heifers and primiparous cows was studied as a function of animal category, average daily gain during the breeding season, stocking rate, pasture type and body weight at the beginning of the breeding season. Data were obtained from 43 experiments conducted in commercial ranches and research stations in the Pampas region between 1976 and 2015. Stocking rate ranged from 200 to 464 kg live weight ha-1, which brackets values for most of the grazinglands in similar regions. Age at breeding was 14–36 months (24.6±7.5 months); initial breeding weights were 129–506 kg and 194–570 kg for heifers and primiparous cows. Pregnancy rate was modeled with an apriori set of explanatory variables where proximate variables (breed, body weight at start of breeding, weight gain during breeding and category) were included first and subsequently modeled as functions of other variables (pasture type, supplementation and stocking rate). This modeling approach allowed detection of direct and indirect effects (through nutrition and body weight) of factors that affect pregnancy rate. Bos taurus breeds (N = 1058) had higher pregnancy rate than B. Taurus x B. indicus crossbreed (N = 1924) females. Pregnancy rate of heifers and primiparous cows grazing in natural grasslands decreased with increasing stocking rate, but no effect of stocking rate was detected in cultivated and improved pastures. Pregnancy rate increased with increasing average daily gain during the breeding season. Use of cultivated or improved natural pastures promotes higher pregnancy rate, as well as allows an increase in stocking rate at the regional level. Body weight at the start of the breeding season is the primary determinant of pregnancy rates in heifer and primiparous cows.

Effect of the performance, calving date and lactation period on the probability of pregnancy in beef cows

“…Vieira et al (2005), working with a herd of Nellore in the cerrado, found quadratic behaviour for the reproductive performance of the cows, with the pregnancy rate increasing up to the seventh calving, and decreasing as the number of births increased. When evaluating the determinant effects of pregnancy in heifers and primiparous cows in 43 experiments conducted on farms, Eloy et al (2022) found greater ease of pregnancy in heifers associating lower primiparous pregnancy with calving stress and the effects of the first lactation while still growing. When evaluating both growing and adult cows, Bitencourt et al (2020) found that adult cows were 66 % more reproductive than growing cows, and when stratified by calving precocity within the calving season, adult cows, even when calving at the end of the season, showed better reproductive performance than growing cows, even though the latter gave birth earlier.…”

“…Early pregnancy in the breeding season increases the overall production efficiency of herds (Eloy et al, 2022). However, cows that calve early in the season, in addition to better reproductive performance, tend to be more efficient due to their consuming better-quality pasture (Castilho et al, 2018) during lactation, weaning heavier calves (Bitencourt et al, 2020) and producing more kilogram of calf per kilogram of cow at weaning (Vaz et al, 2020).…”

“…Ideal nutrition levels promote better reproductive performance in beef cows, while at non-ideal feeding levels, weight loss and a negative daily variation in weight are typical (Trindade et al, 2016). Beef cows under inadequate nutrition conditions deplete their body reserves, losing body condition score (Vieira et al, 2005), resulting in a low ovulation rate due to the decrease in glucose and insulin levels and growth factor I (IGF-I) non-potentiating godonotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) responsible for ovulation (Eloy et al, 2022). Therefore, for better reproductive response during mating, the cow must be in a nutritional state consistent with a positive daily variation in weight which will favor the hormonal trigger responsible for reproductive success.…”

“…Therefore, for better reproductive response during mating, the cow must be in a nutritional state consistent with a positive daily variation in weight which will favor the hormonal trigger responsible for reproductive success. For heifers and primiparous cows, body weight at the beginning of the breeding season and average daily gains during the breeding season were determinants that explain the pregnancy rate (Eloy et al, 2022). However, multiple factors can determine the pregnancy rate and may be closely related to body weight, animal category or breed (Eloy et al, 2022).…”

“…Due to the complexity inherent in each production system, reproduction in beef cattle has yet to be fully understood, and it is important to quantify how much each factor might influence the reproductive performance of a herd. Information on how management affects nutrition and reproduction, especially in pasture conditions, is still scarce (Eloy et al, 2022).…”