HEAT STRESS MANAGEMENT IN DAIRY CATTLE : A SYSTEMATIC LITERATURE REVIEW

Abstract

Climate has raised the severity and frequency of incidences of heat stress as a phenomenon that is a significant challenge to dairy production systems in the world. The lactating dairy cows are very vulnerable to the condition with high Temperature-Humidity Index (THI) since they generate much heat metabolically and have little ability to sustain normal thermoregulatory temperature. The available evidence that was published until the date was integrated in the systematic review and meta-analysis to measure the physiological, productive, reproductive, and health effects of heat stress and evaluate the efficacy of mitigation measures. Abiding by PRISMA 2020,  were transferred to qualitative synthesis, and were taken into quantitative meta-analysis. The results indicated that heat stress significantly increased rectal temperature, respiratory rate, but decreased the dry matter intake and milk yield on a per unit of THI basis above the levels of threshold, respectively. The severe heat stress was associated with milk reduction in the conception rate. Mechanistic data showed that the activation of the HPA axis, oxidative stress, systemic inflammation and intestinal permeability contribute to the production and welfare decrease. It was demonstrated that combined environmental cooling systems have the most short-term beneficial impacts on physiological stability and retention of milk yield despite moderate nutritional support, in comparison with nutritional interventions. The long-term adaptive potential of genetic selection and precision livestock farming technologies could be developed. Overall, heat stress has multi-systemic effects that decrease productivity, reproduction, and animal welfare, and thus solutions to mitigate heat stress should be developed that will integrate mitigation strategies and be evidence-based to ensure dairy production in the future amid climatic changes.