143 Short- and long-term effects of heat stress in cow-calf pairs adapted to tropical and subtropical regions
Authors: Moriel Philipe , Izquierdo Vinicius S , Vendramini Joao M B 
Published: September 2024

Abstract
Chronic exposure to high temperatures and humidity can limit beef cattle performance in tropical/subtropical environments, regardless of breed. Despite better heat adaptability compared with Bos taurus cattle, heat-stressed Bos indicus cattle also experience an increase in body temperature above the physiological limit. As body temperature increases, cascades of behavioral and physiological events are triggered to lessen heat load but often at the expense of productive outcomes. Contrary to other species and feedlot beef cattle, the effects of heat stress on physiology and performance of grazing cow-calf pairs have not been evaluated in detail, particularly for Bos indicus cattle. Preliminary data showed opposite outcomes to heat stress in Bos indicus vs. Bos taurus cattle, limiting the implications of the existing data generated from studies using beef and dairy Bos taurus cattle. This presentation will share some nutritional and management practices to mitigate heat stress in grazing beef cattle. Practices mentioned herein include altering the timing of body weight (BW) growth pattern (stair-step, SST) and immunomodulatory feed ingredient (IFI) supplementation for replacement beef heifers, and maternal access to shade to reduce heat load. For 100 d before the breeding season, Bos indicus-influenced beef heifers grazing warm-season grasses received a constant supplementation amount or SST strategy (50 d of low then 50 d of high supplementation), with or without IFI. Under severe heat stress conditions, IFI supplementation reduced heifer vaginal temperature but did not increase their growth and reproduction, whereas the SST strategy lowered internal body temperature, enhanced growth performance and pregnancy success compared with a conventional constant supplementation practice. Our group is also focused on the critical need to determine the specific effects of heat stress mitigation during pre- and postnatal phases on physiology and performance of Bos indicus-influenced cow-calf pairs. Study 1 evaluated the impacts of maternal access to artificial shade from 83 d prepartum until 50 d postpartum on body thermoregulation, performance, and physiology of beef heifers and their first offspring. Removing maternal access to shade led to greater maternal respiration rate, vaginal and body surface temperatures and reduced maternal body condition score and calf BW at birth. Nonetheless, offspring born from cows with no access to shade were remarkably heavier during a 60-d post-weaning period and had decreased plasma indicators of inflammation and positive effects to humoral immune response. Study 2 is a multi-year, USDA/NIFA-funded study designed to capitalize on the ability of cattle to reprogram physiological/immunological systems following exposure to stress during gestation. Study 2 combined gestational and postnatal heat abatement to enhance the overall growth/immune/reproductive performance of grazing Bos indicus-influenced cow-calf pairs. Partial data on long-term growth and reproductive performance, physiology, metabolome, and microbiome of both cows and their female offspring will be shared in this presentation.