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

Test-day records (n = 723,091) collected between 2012 and 2021 from 43,015 Holstein cows at 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 somatic cell count (%), milk ß-hydroxybutyrate (BHB, mmol/L), milk urea (mg/dL), and milk fatty acid composition (g/100 g of milk). Test-day records were then associated with 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 yearly indexes included the average daily THI (adTHI) and the maximum daily THI (mdTHI) measured throughout the whole year, and the summer indexes focused on 3 mo only (June-August) and included the average daily summer THI (adTHIs), the maximum daily summer THI (mdTHIs), and the average daily THI of the hottest 4 h of the day (adTHI4h; 1200-1600 h). 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 a greater risk of negative energy balance in presence of heat stress. Furthermore, the concentration of the de novo fatty acids C14:0 and C16:0 was reduced in higher THI, reflecting 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, whereas summer indexes are suggested when investigating traits influenced by extreme conditions, such as SCS and milk yield. With global temperatures expected to further rise in the upcoming decades, early and easy identification of cows or herds suffering from heat stress, such 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.

How heat stress conditions affect milk yield, composition, and price in Italian Holstein herds.

An edited data set of 700 bulk and 46,338 test-day records collected between 2019 and 2021 in 42 Holstein-dominated farms in the Veneto Region (North of Italy) was available for the present study. Information on protein, fat and lactose content, somatic cell count, and somatic cell score was available in bulk milk as well as individual test-day records, whereas urea concentration (mg/dL), differential somatic cell count (%), and milk yield (kg/d) were available for test-day records only. Milk features were merged with meteorological data retrieved from 8 weather stations located maximum 10 km from the farms. The daily and weekly temperature-humidity index (THI; wTHI) and maximum daily (MTHI) and weekly temperature-humidity index were associated with each record to evaluate the effect of heat stress conditions on milk-related traits through linear mixed models. Least squares means were estimated to evaluate the effect of THI and, separately, of MTHI on milk characteristics correcting for conventional systematic factors. Overall, heat stress conditions lowered the quality of both bulk milk and test-day records, with fat and protein content being greatly reduced, and somatic cell score and differential somatic cell count augmented. Milk yield was not affected by either THI or MTHI in this data set, but the effect of elevated THI and MTHI was in general stronger on test-day records than on bulk milk. Farm-level economic losses of reduced milk quality rather than reduced yield as consequence of elevated THI or MTHI was estimated to be between $23.57 and $43.98 per farmer per day, which is of comparable magnitude to losses resulting from reduced production. Furthermore, MTHI was found to be a more accurate indicator of heat stress experienced by a cow, explaining more variability of traits compared with THI. The negative effect of heat stress conditions on quality traits commences at lower THI/MTHI values compared with milk yield. Thus, a progressive farmers' income loss due to climatic changes is already a reality and it is mainly due to deterioration of milk quality rather than quantity in the studied area.