# Wet-Bulb Temperature as a Superior Predictor of Milk Yield and Reproductive Performance in Holstein Cows in a Continental Climate

## TL;DR

Wet-bulb temperature better predicts milk yield and fertility in Holstein cows than traditional heat stress measures in continental climates.

## Contribution

Wet-bulb temperature is shown to be a superior predictor of heat stress effects on dairy cows compared to the standard THI.

## Key findings

- Cows produced significantly more milk in dry years with lower wet-bulb temperatures.
- Fertility problems in winter were linked to summer heat stress, indicating a carry-over effect.
- Wet-bulb temperature outperformed THI in predicting thermal stress impacts on milk yield.

## Abstract

This study compared heat stress indicators for Holstein cows in a continental climate. We analyzed milk production over two years. One year was “humid” and the other was “dry”. Although air temperatures were similar, cows produced significantly more milk during the dry year. This is because dry air allows cows to cool down more effectively. The standard Temperature–Humidity Index (THI) failed to explain this difference. However, the Wet-Bulb Temperature (Twb) accurately predicted the higher milk yield. We also found that summer heat stress caused delayed fertility problems in the winter. Therefore, we recommend using Twb instead of THI to manage heat stress in dry regions.

This study analyzed how climate variability affects lactation yield and reproduction in Holstein cows in a continental climate. It specifically compared Wet-Bulb Temperature (Twb) with the standard Temperature–Humidity Index (THI). We conducted a retrospective study on a dairy farm in Konya, Türkiye, analyzing a total of 144 complete lactation records from a herd of 90 cows calving between 2022 and 2023. To rule out nutritional factors, a consistent TMR diet without pasture access was maintained in both years. Average Daily Milk Yield (ADMY) was calculated to adjust for lactation length. Climatic data showed a distinct contrast. Ambient temperatures and THI were similar between years (p > 0.05). However, 2022 was characterized by “humid heat” (high Twb), while 2023 presented a “dry heat” profile with significantly lower Twb (p < 0.001). This difference significantly impacted performance. Cows in the 2023 group produced much higher milk yields (50.55 ± 3.01 kg) than the 2022 group (30.74 ± 0.81 kg) (p < 0.001). Unexpectedly, milk yield peaked during the Autumn and Summer seasons of the low-humidity year. In contrast, fertility declined with thermal load. Poor winter fertility suggested a “carry-over” effect of previous heat stress. These findings show that Twb is a better indicator of thermal comfort than THI in continental climates. Furthermore, low humidity can significantly reduce the negative impact of heat on milk production.