Effect of fiber digestibility and conservation method on feed intake and ruminal ecosystem of growing steers

Abstract

Fiber digestibility is an important factor regulating DMI in ruminants. Additionally, the ensiling process can also affect digestibility and chemical composition of the forage. The objective of this study was to investigate effects of sugarcane NDF digestibility (NDFD) and conservation method on intake, rumen kinetics, and ruminal ecosystem of steers. Eight ruminally cannulated Nellore steers (275 ± 22 kg BW) were used in a replicated 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. Two sugarcane genotypes divergent for stalk NDFD were used: IAC86-2480 with high NDFD, and SP91-1049 with low NDFD. Experimental diets were formulated with 40% sugarcane, either freshly cut or as silage, and 60% concentrate on a DM basis. Each experimental period lasted for 14 d, with the last 4 d used for determination of intake, rumen evacuation, and rumen fluid collection. The effect of fiber digestibility on DM and NDF intake was dependent on the forage conservation method (P = 0.01). High NDFD increased (P < 0.01) DMI only when sugarcane was offered as silage, having no effect (P = 0.41) on DMI when offered as freshly cut. Conservation method had no effect on total rumen mass, with only a tendency (P < 0.10) for greater NDF and indigestible NDF rumen mass in animals fed the low NDFD genotype. The NDF turnover and passage rates were greater (P < 0.05) for the genotype with high NDFD, but only when offered as silage. Liquid turnover rate in the rumen was greater (P = 0.02) for diets containing silage, with no effect of genotype (P = 0.87). There was no effect of NDFD genotype on rumen pH (P = 0.77); however, diets containing sugarcane as silage increased (P < 0.01) rumen pH. Total concentration of short chain fatty acids was greater (P = 0.05) for diets containing fresh sugarcane and proportion of propionate in the rumen was also greater (P = 0.01) for diets with fresh sugarcane. Diets with fresh sugarcane increased rumen population of Streptococcus bovis (P < 0.01) and Ruminococcus albus (P = 0.03). The relative population of R. albus was also greater (P = 0.04) for diets containing the genotype with high NDFD. Feeding diets containing the genotype with high NDFD increased Fibrobacter succinogenes population, but only when sugarcane was fed as freshly cut (P = 0.02). Using sugarcane genotypes with high NDFD can increase intake and benefit fiber degrading bacteria in the rumen. (AU)