Affiliation
- 1 Department of Animal Science, Northfield University
- 2 Department of Surgery, Riverside Veterinary School
- 3 Institute of Animal Physiology, Eastbrook College
Abstract
Background: Rising ambient temperatures threaten the productivity and welfare of high-yielding dairy cows.
Methods: Sixteen multiparous Holstein cows were enrolled in a randomized crossover design with two 21-day periods: thermoneutral (temperature–humidity index, THI < 68) and heat stress (THI 78–82). Rumen fluid, milk yield and composition, and physiological variables were measured.
Results: Heat stress reduced dry matter intake by 12% and milk yield by 3.9 kg/day (P < 0.01). Total volatile fatty acid concentration decreased while ruminal pH and the acetate-to-propionate ratio increased.
Conclusions: Heat stress impairs rumen fermentation independently of intake, supporting nutritional strategies that target rumen function during hot seasons.
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1. Introduction
Heat stress is one of the most important environmental constraints on dairy production worldwide. When the temperature–humidity index (THI) exceeds approximately 68, high-yielding cows begin to reduce feed intake and milk yield, and the effects are expected to intensify as climate change raises summer temperatures. Robust experimental designs and transparent reporting are essential for interpreting animal studies in this field [1, 2].
Most previous work has attributed production losses to reduced dry matter intake. However, direct effects of heat load on rumen fermentation have been less well characterised. The aim of this study was to quantify changes in rumen fermentation, milk yield and milk composition under controlled heat stress, while accounting for differences in intake [2, 3].
2. Materials and methods
2.1. Animals and design
Sixteen multiparous Holstein cows (days in milk 95 ± 12; body weight 642 ± 38 kg) were enrolled in a randomized crossover design with two 21-day periods separated by a 14-day washout. The sample size was calculated a priori to detect a 2.5 kg/day difference in milk yield with 80% power [3]. The study adhered to the principles of the 3Rs [8].
2.2. Measurements
Rumen fluid was collected by oro-ruminal tube on days 19–21 of each period. Volatile fatty acids were measured by gas chromatography and rumen papillae were imaged for morphometric analysis using ImageJ [6].
2.3. Statistical analysis
Data were analysed with linear mixed-effects models including treatment, period and sequence as fixed effects and cow as a random effect [4], using R [7]. P values for multiple outcomes were adjusted with the Benjamini–Hochberg procedure [5].
3. Results
Heat stress increased rectal temperature and respiration rate (both P < 0.001) and reduced dry matter intake by 12%. Milk yield decreased by 3.9 kg/day, and milk protein concentration decreased slightly (Table 1).
| Variable | TN | HS | SEM | P |
|---|---|---|---|---|
| Dry matter intake (kg/d) | 24.1 | 21.2 | 0.6 | <0.01 |
| Milk yield (kg/d) | 38.6 | 34.7 | 0.9 | <0.01 |
| Milk protein (%) | 3.21 | 3.08 | 0.04 | 0.03 |
| Total VFA (mmol/L) | 118.4 | 104.9 | 3.1 | <0.01 |
| Ruminal pH | 6.12 | 6.31 | 0.05 | 0.02 |
Reduced volatile fatty acid production under heat stress was evident even after adjusting for dry matter intake.
4. Discussion
The decline in total volatile fatty acids accompanied by a higher ruminal pH indicates reduced fermentative activity, consistent with lower intake but also with direct effects of heat load on the rumen microbiota [2, 4–6]. These findings support nutritional strategies that target rumen function during hot seasons, such as buffers and yeast cultures.
Limitations include the controlled-environment setting, which may not capture diurnal temperature variation on commercial farms. Future studies should include larger herds and longer exposure periods [1].
5. Conclusions
Heat stress impairs rumen fermentation and milk production in Holstein cows. Mitigation should combine environmental cooling with dietary strategies that support rumen function.
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