Evaluation of the length of adaptation period for changeover and crossover nutritional experiments with cattle fed tropical forage-based diets

It was evaluated the impact of the diet switch on nutritional characteristics and bacterial community composition in cattle fed tropical forage-based diet aiming to establish the minimal length of the adaptation period that could be applied to changeover or crossover design protocols. Twelve rumen-cannulated crossbreed (B. taurus × B. indicus) bovines were used in this experiment (four young bulls, four steers, and four heifers). The experiment lasted 60 days and two diet switches were done at d 21 and d 41. Four diet sequences were evaluated (one animal of each category per sequence): sugarcane-corn silage-sugarcane, corn silage-sugarcane-corn silage, high-low-high concentrate diets, and low-high-low concentrate diets. All diets were adjusted to 110 g crude protein/kg dry matter by using urea. Four groups of variables were evaluated along experimental days: voluntary intake characteristics, fecal excretion and composition, ruminal digesta compostion, and ruminal fermentation profile. The pattern of the groups along time was interpreted through a non-hierarchical clustering procedure and the stabilization of each group of characteristics was achieved within 9-13, 9-14, 6-13, and 4-11 days after diet switch, respectively. The diversity of the liquid-associated rumen bacterial community was also evaluated by using PCR-DGGE in the young bulls from 6 days before to 20 days after the second diet switch. The pattern of microbial diversity was evaluated through a clustering procedure using the unweight pair group method with arithmetic mean. The bacterial community in the liquid phase stabilized within 3-9 days after diet switch. From the results of this experiment, among all obtained times to adapt intake, fecal and digesta composition, and ruminal fermentation and microbial diversity, a 14-days adaptation period is recommended for changeover and crossover experiments with cattle fed tropical forage-based diets.