Time of Cutting and Genetic Selection Affect Non Structural Carbohydrates and Some Attributes of Nutritive Value in Alfalfa.

High non structural carbohydrate (NSC) concentration in forages improves ruminant N utilization and performance. Forage NSC concentration increases during the day and may be improved by genetic selection but the extent of these effects is not well known.  We determined the effect of time of cutting on NSC concentration and other nutritive value attributes of two field-grown alfalfa (Medicago sativa L.) populations obtained by inter-crossing 10 genotypes selected for high (NSC+) or low (NSC-) NSC concentration from 500 AC Caribou genotypes. The two populations were established near Quebec City, Canada (46°48´N; 71°23´W) in 2006 and 2008, and harvested in AM (9h00) and PM (15h00) at early flowering once during the establishment year and three times during the production years. A split plot design with eight replications was used with time of cutting as main plots and populations as subplots. Forage samples were evaluated for NSC (soluble sugars + starch), crude protein (CP), acid detergent fibre (ADF), neutral detergent fibre (NDF), in vitro true digestibility (IVTD), and in vitro NDF digestibility (dNDF) using NIR spectroscopy. In the establishment year, NSC concentration increased by 46% with PM-cutting, and by 13% with NSC+ population. In the production year, NSC concentration increased by 30% with PM-cutting, and by 6% with NSC+ population. Greater NSC concentration with either PM-cutting or the NSC+ population was mainly due to higher starch concentration. In general, PM-cutting decreased ADF, NDF, and CP concentrations, and improved IVTD and dNDF. The NSC+ and NSC- populations did not differ for ADF, NDF, IVTD, and dNDF. Alfalfa NSC concentration can be increased by cutting in the afternoon and via genetic selection.