A 2-yr study was conducted to determine effects of rate of N fertilization on kinetic parameters of fiber digestion and protein degradability in stockpiled Tifton 85 bermudagrass. Six 0.76-ha pastures of stockpiled Tifton 85 bermudagrass were cut to a 10-cm stubble height on Aug 1 and fertilized with either 56 (56N), 112 (112N), or 168 (168N) kg N/ha (2 pastures/treatment). Kinetic parameters of fiber digestion included 72-hr potential extent of NDF digestion (PED), rate of NDF digestion, and lag time. The PED did not differ among N fertilization treatments in Yr 1 or Yr 2. In Yr 1, PED was greatest (P 0.05) among N fertilization treatments in either Yr 1 or Yr 2. In Yr 1, rate of NDF digestion was greater (P 0.05) from Nov 28; and was greater (P 0.05) from Dec 13. In Yr 2, rate of NDF digestion was greater (P 0.05) from Nov 11; and was greater (P 0.05) from Jan 21. In Yr 1, lag time was greater (P 0.05) among N fertilization treatments; lag time was least (P 0.05) to be positively correlated with lag time (r = 0.39). In Yr 2, there was a negative correlation (P 0.05) to be negatively correlated (r = -0.25) with lignin concentration. In Yr 1, DIP was greatest (P < 0.05) on Oct 24 and Dec 13, intermediate on Nov 28, and least (P < 0.05) on Jan 16. In Yr 2, DIP was greatest (P < 0.05) for the 56N and 168N treatments, and least (P < 0.05) for 112N; DIP was greatest (P < 0.05) on Jan 21, intermediate on Nov 25 and Jan 7, and least (P < 0.05) on Nov 11. Mean monthly air temperature was correlated (P < 0.05) with all parameters of NDF digestion; PED (r = 0.69, r = 0.91) and rate (r = 0.56, r = 0.51) were positively correlated, whereas lag time (r = -0.54, r = -0.85) was negatively correlated in Yr 1 and 2, respectively. Results of this study suggest that kinetic parameters of NDF digestion in stockpiled Tifton 85 bermudagrass were influenced more by temporal changes over the stockpile season than by N fertilization level. Furthermore, changes in protein degradation characteristics were less pronounced than changes in kinetic parameters of NDF digestion. Supplementation formulations should utilize kinetic parameters of fiber digestion to insure that energy-yielding components of NDF are sufficient to meet requirements throughout the stockpile season. The highly degradable CP fraction in stockpiled bermudagrass produces sufficient degradable intake protein to support fibrolytic activity and growth of ruminal microorganisms throughout the stockpile season. Supplementation with sources of digestible fiber and undegradable intake protein could be expected to increase metabolizable protein supply to the host animal, depending on changing forage quality throughout the stockpiled grazing season
