Nutritive Value of AM and PM-Cut Alfalfa, Red Clover, and White Clover.

Delayed cutting during the day has been shown to increase the non structural carbohydrate (NSC) concentration in alfalfa (Medicago sativa L.), resulting in improved N use efficiency by dairy cows. This increase in NSC concentration may vary with forage species and growth period, and it is unclear how it affects fibre concentration. Our objective was to compare the effect of a delayed cutting during the day on forage concentrations of NSC, neutral detergent fiber (NDF), and acid detergent fibre (ADF) of alfalfa, red clover (Trifolium pratense L.), and white clover (Trifolium repens L.) in spring growth and summer regrowth. The three species were field-grown at Normandin, QC, Canada (48°51’N, 72°32’W), and either cut in the morning (8h45) or in the afternoon (15h45) at flowering for both growth periods in 2007 and 2008, except for the spring growth of alfalfa in 2008 that was cut at early bud. White clover was only harvested in 2008. Forage sample were pre-treated in a microwave oven for 1 min, dried at 55°C, ground at 1 mm, and analyzed for soluble carbohydrates by HPLC and starch by colorimetry. Forage NSC concentrations in all species, estimated by the sum of sucrose, glucose, fructose, pinitol, and starch, were increased with PM-cutting compared to AM-cutting in both growth periods and production years. Averaged across growth periods and production years, forage NSC concentration increased from 62.2 to 82.2 mg/g DM in alfalfa, from 84.6 to 112.1 mg/g DM in red clover, and from 90.9 to 123.4 mg/g DM in white clover with PM-cutting vs. AM-cutting. Fibre concentration was not affected by a delayed cutting during the day. Delayed cutting during the day increased NSC concentration of all three forage legume species without affecting fibre concentration.