Effects of different temperature-humidity indexes on milk traits of Holstein cows: A 10-year retrospective study

Abstract

Test-day records (n = 723,091) collected between 2012 and 2021 from 43,015 Holsteins in 157 farms located in Northern Italy were used to study the effects of heat load on milk production and composition a posteriori. The data consisted of milk yield (kg/d), traditional gross composition traits, somatic cell score (SCS), differential SCC (%), milk β-hydroxybutyrate (BHB, mmol/L), milk urea (mg/dL), and milk FA composition (g/100 g milk). Test-day records were then associated to their relative temperature-humidity indexes (THI) calculated using historical environmental data registered by weather stations. Indexes were created using either yearly or summer THI data: the first included the average daily THI (adTHI) and the maximum daily THI (mdTHI) measured throughout the whole year, whereas summer indexes were focused on 3 mo only (June to August) and included the average daily THI (adTHIs), the maximum daily THI (mdTHIs), and the average daily THI of the hottest 4 h of the day (adTHI4h, 12:00 to 4:00 p.m.). All indexes had significant effects on the majority of milk traits analyzed, with in particular adTHI and mdTHI being highly significant in explaining the variation of all traits. Milk yield started to decline at a higher THI compared with protein and fat content. The reduction in fat ceased in the elevated THI experienced during the summer months as demonstrated by adTHIs, mdTHIs, and adTHI4h. The cows had a tendency for increased BHB concentration with elevated THI, suggesting that there is a greater risk of negative energy balance in presence of heat stress. What is more, the concentration of the de-novo fatty acids C14:0 and C16:0 was reduced in higher THI, reflecting the altered mammary gland activity upon elevated heat load and stress. Milk SCS tended to increase with higher adTHIS, mdTHIS, and adTHI4h. The use of yearly indexes is recommended when investigating the effects of heat load on milk composition, while summer indexes are suggested when investigating traits influenced by extreme conditions, such as SCS and milk yield. With global temperatures set to further rise in the upcoming decades, early and easy identification of cows or herds suffering from heat stress, as through changes in milk composition, is crucial for timely intervention. Adapting measures to mitigate such effects of elevated THI on milk yield and composition is a necessity for the dairy industry to prevent detrimental impacts on dairy production.