Diurnal Variation of Non Structural Carbohydrate Concentrations in Alfalfa and Timothy.

Limited readily available energy combined with fast and extensive degradation of proteins in forages contributes to poor N use efficiency in dairy cows. Non structural carbohydrates (NSC), a major source of readily available energy for rumen bacteria, are known to accumulate in forages during the day. Few studies, however, have established the time of day at which maximum NSC concentration is reached. We studied the diurnal variation of NSC in alfalfa (Medicago sativa L.) and timothy (Phleum pratense L.) to determine the best cutting time to maximize NSC concentration. Field-grown alfalfa (cv. AC Caribou) and timothy (cv. AC Alliance) were sampled every two hours between 6h00 and 20h00 on six different days around the recommended stage of development for harvest in each of the spring and summer growth cycles of 2007 in Lévis (QC, Canada, 46°48’N). All forage samples were scanned using near infrared reflectance spectroscopy. Seventy-five samples per species were selected for the calibration and validation sets and chemically analyzed for NSC [soluble carbohydrates (SC) + pinitol + starch in alfalfa; SC + fructans + starch in timothy]. The NSC concentration of both species increased during the day on all sampling days. Averaged over six sampling days, alfalfa NSC concentration increased from 95 mg/g DM at 6h00 to a maximum value of 133 mg/g DM at 16h00 in spring and from 74 mg/g DM at 6h00 to 129 mg/g DM at 18h00 in summer, whereas timothy NSC concentration increased from 59 mg/g DM at 6h00 to 83 mg/g DM at 18h00 in spring and from 57 mg/g DM at 6h00 to 109 mg/g DM at 18h00 in summer. Our results confirm that NSC accumulate during the day in both alfalfa and timothy, reaching a maximum concentration between 16h00 and 18h00 under Québec conditions.