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

Abstract

23Pregnancy rate is a major determinant of population dynamics of wild ungulates 24 and of productivity of livestock systems. Allocation of feeding resources, including 25 stocking rates, prior to and during the breeding season is a crucial determinant of this vital 26 rate. Thus, quantification of effects and interaction among multiple factors that affect 27 pregnancy rate is essential for management and conservation of pasture-based systems. 28 Pregnancy rate of 2982 heifers and primiparous cows was studied as a function of animal 29 category, average daily gain during the breeding season, stocking rate, pasture type and 30 body weight at the beginning of the breeding season. Data were obtained from 43 31 experiments conducted in commercial ranches and research stations in the Pampas region 32 between 1976 and 2015. Stocking rate ranged from 200 to 464 kg live weight/ha, which 33 brackets values for most of the grazinglands in similar regions. Age at breeding was 14-34 36 months (24.6 ± 7.5 months); initial breeding weights were 129-506 kg and 194-570 35 kg for heifers and primiparous cows. Pregnancy rate was modeled with an apriori set of 36 explanatory variables where proximate variables (breed, body weight at start of breeding, 37 weight gain during breeding and category) were included first and subsequently modeled 38 as functions of other variables (pasture type, supplementation and stocking rate). This 39 modeling approach allowed detection of direct and indirect effects (through nutrition and 40 body weight) of factors that affect pregnancy rate. Taurine (Bos taurus breeds, N = 1058) 41 had higher pregnancy rate than B. Taurus x B. indicus crossbreed (N = 1924) females.42Pregnancy rate of heifers and primiparous cows grazing in natural grasslands decreased 43 with increasing stocking rate, but no effect of stocking rate was detected in cultivated and 44 improved pastures. Pregnancy rate increased with increasing average daily gain during 45 the breeding season. Use of cultivated or improved natural pastures promotes higher 3 46 pregnancy rate, as well as allows higher in stocking rate at the regional level. Body weight 47 at the start of the breeding season is the primary determinant of pregnancy rates in heifer 48 and primiparous cows. 49 50 4 70changes in economic and technological factors that affect productivity (IBGE 2020). Low 71 pregnancy rates prevent full development of the livestock sector in many regions of the 72 world, and it may be associated with poor pasture management, overstocking and lack of 73 differential nutritional management for animal categories with different requirements. 74Stocking rate is considered the most important decision in grazing management because 75 it affects the forage base, herbage allowance, intake and animal performance 76 (Sollenberger, Agouridis, Vanzant, Franzluebbers & Owens 2012). Nutritional limitation 77 during periods of high requirement can compromise development and delay puberty of 78 heifers, as well as inhibit ovulation of cows (Rocha & Lobat...