Effect of heat stress on feed intake and milk production in Holstein-Friesian dairy cows

Heat stress is detrimental to dairy production since it affects numerous variables. It is known that heat stress has a negative impact on feed intake and milk production, but there is incoherence in the literature data related to the degree and mechanism of this negative impact. The aim of this study was to investigate the feed intake and milk production in lactating cows exposed to different ambient temperatures. The experiment was carried out on 20 cows during the summer and 20 cows during the spring season. All cows were observed 2 months i.e. From day 30 to day 90 of their lactation. During the experiment, ambient temperature and humidity, as well as dry-bulb and wet-bulb temperatures were determined hourly. Based on those data, the temperature humidity index (THI) was calculated. THI ranged from 61.04 to 81.00 during the summer, and from 54.94 to 75.99 during the spring season. This data indicates that cows were exposed to multiple heat stress during the summer period. Feed intake was measured as a difference between offered and left-over food for each cow in the experiment. Milk production was monitored on daily base. Results from this study showed that dry matter consumption in cows exposed to heat stress was for 2.36 kg (8.99 MJ NEL) lower than in cows exposed to optimal ambient temperature. In both groups of cows, the highest milk production was performed from day 76 to day 85 of lactation, when milk production ranged from 44.3 +/- 7.0 to 46.3 +/- 5.2 L (spring season) and from 36.4 +/- 4.1 to 38.4 +/- 4.5 L (summer season). Differences in milk production were significant for all days mentioned above (p is less than 0.001, respectively). Total average milk production per cow was higher, but not significantly, in spring compared to summer season from day 30 to day 60 of lactation (1360.75 +/- 170.89 : 1284.35 +/- 140.5 L; p is greater than 0.001), and significantly higher from day 61 to day 90 of lactation (1388.15 +/- 177.86 : 1183.2 +/- 105.5 L; p is less than 0.001). It was one of the reasons why lactation curve had a decreasing trend starting from day 60 of lactation during the summer season, while the lactation curve maintained within high levels from day 30 to day 90 of lactation during the spring season. These results indicate that feed intake and milk production is lower in cows exposed to heat stress than in cows kept under optimal ambient conditions. Since it has an enormous negative economic impact on the dairy industry, further research should focus on measures that can protect cows from heat stress during the summer season.