Effect of maturity stage, nitrogen fertilization and seasonal variation on ruminal degradation characteristics of neutral detergent fibre in timothy (Phleum pratense L.)

The effect of maturity stage, seasonal variation and N fertilization on ruminal in situ neutral detergent fibre (aNDF) degradation characteristics was investigated. Total indigestible aNDF (IaNDF), potential degradable aNDF (PaNDF) and fractional degradation rate (kdPaNDF) of aNDF in timothy (Phleum pratense L.) c.v. Grindstad grown at Vollebekk, Ås (59°N), and c.v. Engmo grown at Holt, Tromsø (69°N) were evaluated. Grass materials were obtained from the same experimental plots for 3 (Vollebekk) or 2 (Holt) years. Two levels of N fertilizer were used. The low level, at first and second cut, was 80+50kg N/ha at Vollebekk, and 70+40kg/ha at Holt. High levels were twice the amount of fertilizer at low levels maturity stage was expressed as days from heading during at spring growth. To measure aNDF degradation characteristics, feed samples were incubated in rumen for 0-144h. The 144h incubation was used as a measurement of IaNDF. A positive curvilinear relationship was observed between aNDF content and maturity stage. The IaNDF content (g/kg aNDF, g/kg DM) increased (P<0.05) with maturity stage. At Vollebekk, the daily increase during spring growth was 2.2, 6.1 and 3.4g IaNDF/kg DM in 1991, 1992 and 1993, respectively, and corresponding values during summer growth was 3.0, 1.2 and 2.6g IaNDF/kg DM. At Holt, the content of IaNDF in summer growth was low and not affected (P<0.05) by maturity stage. The kdPaNDF decreased (P<0.05) with maturity stage. Increased N fertilization resulted in a higher (P<0.05) crude protein (CP) content, but did not affect (P>0.05) aNDF degradation characteristics. At Vollebekk the accumulated temperatures, measured in degree-days from the start of growth until heading, were 496, 486 and 504 degree-days (°Cd) over the 3 years. Despite similar maturity stage and °Cd, the IaNDF content differed (P<0.05) considerably between years, indicating interactions between temperature and climatic factors like precipitation. Multiple regressions were performed to predict kdPaNDF from content of aNDF, CP and PaNDF, and equation based on the total dataset explained 0.66 of the variation in the dataset. Splitting the dataset with regard to location and spring growth/summer growth, R2 ranged from 0.53 to 0.90. It is concluded that the ruminal aNDF degradation characteristics are affected by maturity stage, and interactions with climatic factors such as temperature and water availability probably interact with maturity stage. Precautions should be made when prediction feed degradation characteristics from multiple regression analyses.