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 60days 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 110g 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 4–11days 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 6days before to 20days 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 3–9days 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.