Nonstructural carbohydrate concentrations in Timothy as affected by N fertilization, stage of development, and time of cutting

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Forages with increased total nonstructural carbohydrate (TNC) concentrations improve the N-use efficiency of dairy cows. This study determined the effect of time of cutting (0700 vs. 1500 h), stage of development (heading and anthesis), and N fertilization (30, 50, 70, 90, and 110 kg N ha–1 as NH4NO3) on the fermentable carbohydrate concentration of timothy (Phleum pratense L.) grown in northern Ontario, Canada. Forage dry matter (DM) yield and concentrations of N, starch, sucrose, glucose, fructose, and fructans were determined. Concentration of soluble carbohydrates (SC) was estimated by the sum of sucrose, glucose, and fructose, while TNC was obtained by adding SC and starch. Nitrogen fertilization did not affect forage carbohydrate concentrations, but slightly increased DM yield. Forage had higher TNC, starch, sucrose, and fructose concentrations (+9 to 63%) but lower glucose concentration (–27%) when harvested at anthesis compared with heading. Concentration of high degree of polymerization (HDP) fructans was close to 0 at heading and increased to 64.3 mg g–1 DM at anthesis. The afternoon-cut forage had higher TNC (+53%), SC (+60%), and sucrose (+87%) concentrations than the morning-cut forage; this positive effect was greater when timothy was harvested at heading compared with anthesis. Starch and HDP fructan concentrations were similar for both times of cutting, whereas results for glucose and fructose were inconsistent. Delayed cutting during the day and an extended growth period increased timothy TNC and HDP fructan concentrations to an extent likely to improve the N use efficiency of dairy cows.

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