Investigation of the Interaction between Season and Forage Particle Size on Feed Intake Patterns, Nutrient Digestibility, and Fecal Parameters in High-Producing Holstein Cows

Background: Previous studies have shown that in dairy cow nutrition, besides having a balanced diet in terms of nutrient content, the physical form of diets, including particle size, is also an effective factor in proper rumen function and, consequently, animal production performance.On the other hand, climate change, global warming, and subsequent heat stress in various forms affect the performance and production efficiency of dairy cows.Although it has been shown that dry matter intake decreases under heat stress conditions compared to normal temperature conditions, it is not clear how feed particle size during heat stress can affect feeding behavior, performance, and blood and ruminal parameters in cows.In fact, no study has simultaneously examined the interaction between environmental temperature and forage particle size on nutrient consumption and digestibility in lactating cows.Therefore, the present study aimed to simultaneously investigate the interaction between season (spring and summer) and forage particle size (coarse and fine) on nutrient intake and digestibility in high-producing Holstein cows.Methods: A total of 100 high-producing Holstein cows were used in two seasons, spring and summer, with 50 cows per season divided into two treatments: one containing alfalfa forage with coarse particle size (geometric mean 7.7 mm) and one containing alfalfa forage with fine particle size (geometric mean 4.2 mm).To calculate dry matter intake, the feed offered and the amount remaining the next day before offering fresh feed were weighed throughout the sampling period.To examine the feed intake pattern, the amount of feed remaining in the trough was weighed 2, 4, 8, and 24 hours after the morning feeding and then returned to the trough.Apparent nutrient digestibility was measured using acid-insoluble ash as a marker in feed and fecal samples.Results: The results showed that dry matter intake was significantly higher in spring than in summer (p < 0.01).Additionally, there was a significant difference between seasons in terms of feed intake rate parameters, with the amount of feed consumed (in kg) during the first 2 hours after morning feeding being significantly higher in summer than in spring (P = 0.02), while feed intake during 2-4 hours and 4-8 hours after morning feeding was higher in spring.Overall, dairy cows during spring (normal temperature conditions) had a more uniform feed intake rate throughout the day than summer (heat stress conditions) in the present study.It should be noted that the results showed no significant differences in daily feed intake between treatments containing forage with different particle sizes (fine versus coarse).It is worth mentioning that forage particle size did not significantly affect nutrient digestibility while the digestibility of dry matter, organic matter, and crude protein were significantly higher in summer than in spring