Cattle adapted to tropical and subtropical environments: genetic and reproductive considerations

Abstract

Efforts to understand biological functions and develop management schemes specific to Bos indicus-influenced cattle raised in tropical and subtropical environments are critical to meet the increasing global demand for protein. In the United States, B. indicus breeds are mostly used to generate B. indicus × B. taurus crosses with increased thermal and parasite tolerance, while retaining some productive characteristics of B. taurus cattle. Although crossbreeding represents a proven strategy to improve cattle adaptation almost immediately, research has also attempted to identify B. taurus genetics that can withstand subtropical and tropical climates. Reduced milk production and delayed reproductive maturation appear to be related with tropical adaptation of B. taurus breeds, as a means to conserve energy under stressful conditions and limited nutrition. Moreover, longevity may be the ultimate adaptation response to unfavorable environments, and retention of bulls and heifers from proven cows is the recommended strategy to improve longevity in B. indicus-influenced herds. Besides selection for longevity, other aspects should be considered when planning reproductive management in tropical and subtropical regions. Bos indicus and B. taurus breeds have multiple differences pertaining to reproductive function, including age at puberty, ovarian dynamics, and pregnancy development. Nutritional strategies such as the stair-step regimen, and use of exogenous progesterone (P4) inserts are options to hasten puberty attainment of late-maturing B. indicus-influenced heifers. Yet, limited pharmacological alternatives are available for reproductive management of B. indicus-influenced females in the United States, which rely on GnRH-based protocols not specifically designed to the reproductive function of B. indicus breeds. In contrast, hormonal protocols based on exogenous P4, estradiol esters, and equine chorionic gonadotropin are available for use in B. indicus females in South America. These include protocols tailored to prepubertal heifers, anestrous cows, and cycling nulliparous or parous females, which often yield pregnancy rates of 50% to fixed-time artificial insemination. The global dairy industry also faces similar challenges in increasing demand and production as the beef industry. Selection of cows capable of sustaining optimal milk yield, reproductive success, and health status in hot and humid conditions is essential for optimal dairy production in subtropical and tropical regions.

Keywords: Bos indicus; genetics; physiology; reproduction; tropical and subtropical environments.

Figure 1.

Trajectory plots of the additive genetic variance for intramuscular fat across latitude within the overall West region (data divided into West and East regions at 99°W) and subdivided West regions (furthest West subregion with boundary at 104.55°W). Adapted from Liberona et al. (2020).

Figure 2.

Shedding and regrowth of winter hair curve (solid lines) of Angus cows, from 2 different maternal lineages, in subtropical areas modeled as $A\sin [B(t-C)]+D$ by month (t). A = height of peaks in the oscillatory function above a centralized baseline in the Cartesian plane; B = the angular frequency, that is, the rate of change of scores across a complete fundamental cycle of the process, C = horizontal shift in the cycle and D = represents the vertical offset of the function. Upper panel (blue lines) represents maternal lineage group 2, and lower panel (red lines) represents maternal lineage group 5. In both panels, dotted lines represent 68% confidence bands. Adapted from Riley et al. (2015a).

Figure 3.

Plots of heritability (h²) and permanent environmental variance (c²) for exit velocity as a proportion of the phenotypic variance across age of Bos indicus × Bos taurus calves. Adapted from Littlejohn et al. (2018).

Figure 4.

Effects of recipient breed (Angus or Brangus) on pregnancy failure at day 28 of gestation. Cows received diets to provide 100% or 70% of their daily nutrient requirements. Dietary treatments were designed based on the specific requirements assumed for Angus and Brangus breeds, according to the National Academies of Sciences, Engineering, and Medicine (2016). A recipient breed × diet interaction was detected (P < 0.01), and values with different superscripts (a,b) differ (P < 0.05). Adapted from Fontes et al. (2019).

Figure 5.

Stair-step, compensatory growth model for development of Bos indicus-influenced beef heifers. Left panel depicts different rates of BW gain of heifers weaned at approximately 4 mo of age and subjected to 1 of 3 nutritional treatments: high BW gain (HG), low BW gain (LG), or stair-step BW gain (SS-1). Right panel depicts the cumulative percentage of heifers that attained puberty throughout the study. Adapted from Cardoso et al. (2014).

Figure 6.

Pregnancy rates (%) to AI according to the interaction between THI (low: ≤65, med: 65 to 70, high: ≥70) and percentage of time (min) 9 to 11 d before breeding when PCT39 was classified as high (≥22.90%) or low (<22.90%). Values within parenthesis report pregnancy cows divided by total cows receiving AI. Within THI group, *P < 0.03. Adapted from Polsky et al. (2017).