Urinary biomarkers as indicators of heat stress in dairy cows

One of the most current relevant raising issues for dairy cattle farmers is heat stress, especially in the temperate area. Heat stress occurs in response to high temperature and humidity, prompting animals to trigger metabolic changes to maintain homeothermy. Several metabolites have been identified as potential biomarkers of heat stress in plasma or milk, but only a few suggestions are available for urine. The aim of this study was to investigate the variations of some urinary metabolic biomarkers associated with the occurrence of heat stress as measured by the temperature hu midity index (THI). For this purpose, between April and November 2023, urine samples were collected from 1,077 Holstein cows in 16 herds of northern Italy. Farms were visited multiple times and at each visit different cows were randomly sampled, so that each cow was sampled once. Throughout the entire sampling period, temperature and humidity values were recorded in each farm every 15 minutes and THI was calculated as: THI = (1.8 × AT) – (1.0 – RH) (AT – 14.3) + 32, where AT is the daily average temperature and RH the relative humidity. Urine samples were analysed using the reference methods to determine pH, specific gravity, and concentrations of urea, protein, creat inine, allantoin, and uric acid. Then, the protein-to-creatinine ratio was calculated. A mixed linear model was used to compare thermal comfort vs discomfort. Fixed effects were THI class (<68; 68-71; 72-79; and 80-89), parity, and stage of lactation, and random effects were herd and the residual. The urinary traits were influenced by THI, except for proteins. In particular, when cows were at high risk of heat stress (exposed to THI ≥68), creatinine and allantoin concentration in urine increased, while uric acid and protein-to-creatinine ratio decreased. Urine can be a valuable matrix for monitoring heat stress in dairy cows as sampling procedure is non-invasive. This study received support from the project CoolCow 2023 of the Breeders Association of Veneto Region (ARAV, Vicenza, Italy).