Effects of maize grain and lucerne particle size on ruminal fermentation, digestibility and performance of cows in midlactation.

This study evaluated the effects of, and interactions between, maize grain particle size (MPS) and lucerne particle size (LPS) on dry matter intake, milk production, milk composition, ruminal fermentation, microbial yield, chewing activity and nutrient digestibility in midlactation cows. Four multiparous Holstein cows with ruminal cannulas were assigned randomly to a 4 x 4 Latin square design, averaged 595 kg (SD = 52) of body weight and 121 days in milk (SD = 21) at the start of the experiment. Experimental periods were 21 days in length (14 days of treatment adaptation and 7 days of data collection). All diets were fed as total mixed ration and were formulated to meet or exceed the requirements of a 600 kg multiparous cow producing 20 kg milk/day with 4.0% fat. The ratio of concentrate to forage was 39:61 (dry matter basis). Treatments were arranged in a 2 x 2 factorial design; two levels of LPS (2.54 and 6.22 mm) were combined with concentrates based on either ground maize grain (711 mum) or cracked maize grain (1755 mum). Maize grain and LPS did not affect milk production and milk fat percentage. Milk protein percentage increased when MPS was decreased (p = 0.04). Milk urea nitrogen was lower for cows fed ground maize grain compared with cracked maize grain (118-134 mg/l, p = 0.05). Estimated microbial N supply increased 41.9 g/day for ground maize grain compared with cracked maize grain. Cows fed long lucerne (LL) hay spent more time ruminating compared with cows fed short lucerne (SL) hay ranging from 293 to 336 min/day (p < 0.001). Total time spent chewing by cows increased from 505 to 574 min/day (p = 0.002) for SL and LL respectively. Based on the results from this study, midlactation cows can be fed diets that contain ground maize grain and SL hay without leading to negative effects on ruminal pH and nutrient digestibility.