RESUMO
A 2-yr study was conducted to determine effects of N fertilization level on fiber digestion kinetics and protein degradability characteristics of stockpiled Tifton 85 bermudagrass (T85). Six 0.76-ha pastures of stockpiled T85 were cut to a 10-cm stubble height on August 1 of each yr and fertilized with 56 (56N), 112 (112N), or 168 (168N) kg N/ha (2 pastures/treatment). Fiber digestion kinetics included the 72-hr potential extent of NDF digestion (PED), rate of NDF digestion, and lag time. In yr 1 and 2, PED decreased over the stockpile season. Rates of NDF digestion did not differ ( > 0.05) among N fertilization treatments in either yr. In yr 1, rate of NDF digestion was greatest ( < 0.05) in October/November, and decreased beginning in December. In yr 2, rate of NDF digestion decreased ( < 0.05) in January compared with November, but digestion rates were similar for November and January 21 sampling dates. Lag time was greater ( < 0.05) for the 112N than 56N and 168N treatments, and increased ( < 0.05) across sampling dates in yr 1. In yr 2, lag time increased ( < 0.05) from 9.0 to 17.7 h across the season. In yr 1 and 2, a negative correlation ( < 0.05) between forage lignin concentration and both PED ( = -0.91 and -0.87 in yr 1 and 2, respectively) and rate of NDF digestion ( = -0.60 and -0.25 in yr 1 and 2, respectively) was observed. There was a trend ( = 0.06) for lignin concentration to be positively correlated with lag time ( = 0.39) in yr 1, and a strong relationship was observed in yr 2 ( = 0.91; < 0.05). The RDP fraction as a % of CP was ≥ 90% throughout both years. Concentration of RDP (% of total DM) decreased across the stockpiling season through January in yr 1 and 2. Results suggest that kinetic parameters of NDF digestion in stockpiled T85 were influenced more by temporal changes over the stockpile season than by N fertilization level. Supplement formulations based on kinetic parameters of fiber digestion may require periodic adjustment to insure that energy-yielding components of NDF are sufficient to meet animal requirements throughout the stockpile season. The CP fraction in stockpiled T85 contains sufficient RDP to support fibrolytic activity and growth of ruminal microorganisms throughout the stockpile season. Toward the latter end of the season, supplementation with sources of digestible fiber and RDP could be expected to increase MP supply to the host animal.
