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

The dairy cow experiences impaired health, productivity, fertility, and overall welfare when outside her thermo-neutral zone. The thermo-neutral zone is defined as the range of temperatures within which no additional energy is spent to heat or cool the body and typically falls between 5°C and 25°C for dairy cows. Heat stress occurs when this range is exceeded and physiological mechanisms fail to dissipate an adequate amount of heat to maintain the internal body temperature. Genetic selection for high-yielding cows has further impaired the animal to adequately respond to elevated temperatures and because high milk production requires increased feed intake and greater metabolic activity, even more internal heat is produced. Furthermore, quality of milk in terms of protein and fat content following exposure to elevated heat loads may be impaired, and milk yield decreases under such conditions. Therefore, considering the projected increase in average annual temperatures with climate change, dairy cows are expected to suffer from heat stress more frequently, which could have potentially detrimental impacts on the dairy sector.

Several other factors beyond temperature also affect heat exchange, including thermal radiation, air flow, and air moisture content. Although temperature is the primary driving force of heat exchange, it is generally agreed that temperature alone is not an adequate indicator of the environmental impact, because other factors can influence the perception of heat. The temperature-humidity index (THI) combines the two most substantial of these factors, namely ambient temperature and relative humidity, to provide a single measure that is able to easily approximate and assess the level of heat stress experienced by the cow. As milk traits can be affected at different THI levels, it can be hypothesized that the extent to which they vary changes across seasons and time of day (e.g., morning vs. afternoon). Furthermore, apart from the effect of the THI in explaining changes in milk yield and composition, the diverging effects of different THI indexes on milk unconventional traits such as the fatty acid composition and biomarkers of health, such as somatic cell score (SCS) and β-hydroxybutyrate (BHB), has yet to be investigated. Therefore, the cited retrospective study aimed to quantify the effects of THI indexes on milk yield and composition to gain a better understanding of how heat stress may affect the dairy industry. For this purpose, different THI indexes were created both with various temporal components and with different combinations of environmental temperature and humidity.

Test-day records (n = 723 091) collected between 2012 and 2021 from 43 015 Holstein cows at 157 farms were used to study the effects of heat stress on milk production and composition. The data consisted of milk yield (kg/d), gross composition traits, SCS, differential somatic cell count (%), milk 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 THIs, 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 and the maximum daily THI, measured throughout the whole year, with the summer indexes focussing on three months only. These included the average daily summer THI, the maximum daily summer THI, and the average daily THI of the hottest four hours of the day (12:00–16:00).

All indexes had significant effects on the majority of the milk traits analyzed, with, in particular, the average daily THI and the maximum daily THI being highly significant. Milk yield started to decline at a higher THI compared with the milk protein and fat content. The reduction in fat ceased in the elevated THI experienced during the summer months, as demonstrated by the average daily THIs, the maximum daily THIs, and the average daily THI between 12:00 and 16:00. The cows showed a tendency for an increased BHB concentration with elevated THI, suggesting a greater risk of a negative energy balance in the presence of heat stress. Furthermore, the concentration of the fatty acids C14:0 and C16:0 was reduced at higher THI, reflecting an altered udder activity. Milk SCS tended to increase with higher average daily THIs, maximum daily THIs, and average daily THI at 12:00 to 16:00.

The results showed that milk traits, including fat, protein, casein, and urea content, start to deteriorate at lower THI values than milk yield and SCS. Because summer months are more prone to extreme THI values, the timespan included in the indexes should be considered according to the traits to be investigated, so as not to hide certain trends. The yearly indexes are optimal for fat, protein, casein, and urea content, whereas the summer indexes are optimal for milk yield and SCS, that are affected at more extreme THI. The threshold at which milk quality starts to deteriorate varies according to lactation number, breed, and lactation stage, implying that dedicated trials and validated reference heat stress markers for breakpoint identification are advisable. The results imply that the effects of acute and chronic heat load on milk composition and biomarkers are of great importance for implementing mitigation strategies against climate change, especially in hotspot areas for global warming such as South Africa.