Quantifying Morphological Development of Alfalfa for Studies of Herbage Quality1

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Abstract
A more precise and quantitative method of specifying the morphological development of alfalfa (Medicago sativa L.) is needed for communication among scientists working with the crop and for associating alfalfa phenology and forage quality. A 10‐stage numerical system for individual stems (Stages to 9) was defined, and two procedures for calculating the mean stage of a herbage sample were compared on alfalfa samples grown in the spring, summer, or autumn and representing weekly age increments up to 10 weeks from all seasons. Samples were collected during one growing season from a 3‐year‐old stand of ‘Iroquois’ alfalfa grown in Tompkins County, N.Y. Mean stage by count (MSC) was the average of the individual stages present in the herbage sample, weighted for number of stems in each stage. Mean stage by weight (MSW) was the average of individual stages present, weighted for dry weight of herbage in each stage. Both methods quantified morphological development of alfalfa canopies with the anticipated effects of seasonal temperature, but the MSW procedure was superior in showing greater numerical differentiation and in distinguishing samples of diverse age and stage structure.Linear regression in vitro true digestibility (IVTD) on MSW across seasons and ages of herbage samples was highly significant:IVTD = 92.93−3.98·(MSW), r2 = 0.984, n = 11. Crude protein (CP) was characterized by a highly significant quadratic regression: CP = 40.89−7.38·(MSW) + 0.57 (MSW)2, R2 = 0.995, n = 11. The association of alfalfa quality and MSW has not been sufficiently tested to represent a general prediction model. However, the observed relationship indicates the potential usefulness of MSW in developing such a model.