Diurnal Variation in Nonstructural Carbohydrates, In Vitro Digestibility, and Leaf to Stem Ratio of Alfalfa1

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Abstract
Alfalfa (Medicago sativa L.) was sampled four times daily during first and second growths and eight times daily for three days during third growth in order to detect diurnal changes in carbohydrate constituency, in vitro digestibility, and leaf to stem ratio. The implications of these changes with regard to forage yield and nutritional value were considered.Reducing sugars and sucrose were removed from freezedried, ground material by alcohol extraction. Starch in the residue was hydrolyzed by takadiastase. Ferricyanide, resorcinol, and phenol‐sulfuric acid procedures were used to quantitatively assay sugars in the extracts. Glucose and fructose percentages increased slightly but significantly from 6 AM to 12 N and declined to 6 PM in second and third growth alfalfa. About three times as much glucose as fructose was present. Sucrose comprised less than 3% of the dry matter, but varied considerably diurnally, generally increasing from 6 AM to 6 PM. Leaf starch increased from 10.2 to 20.3% of the dry matter during daylight hours, most of the increase occurring between 9 AM and 3 PM. Stem starch remained between 4 and 5% of the stem dry matter. In general, all nonstructural carbohydrates studied began to accumulate after 6 AM. Hexoses peaked first in leaves followed by sucrose and then starch. These were depleted in the same order in the afternoon and at night. Peak levels of glucose and sucrose occurred in stems later than in leaves. Leaf stem ratio increased from 1.1 to 1.5 between morning and evening and declined at night.In vitro dry matter digestibility averaged 1.6% higher at 6 PM than at 6 AM.

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