Alfalfa response to grazing, cultivar evaluation and visual modelling

Abstract

Interest in grazing alfalfa ('Medicago sativa' L.) is increasing among beef and dairy producers of the Northern Great Plains. Concerns over cultivar persistence and bloat have deterred more widespread acceptance of alfalfa as a pasture crop. New grazing tolerant cultivars have recently been developed, but their persistence is unknown under the climatic and edaphic conditions of this region. Deficiencies in understanding the plant-animal interface introduce a further constraint to the adoption of grazing alfalfa. The objective of this research was to characterize alfalfa response to grazing by: (1) evaluating a method to assess alfalfa persistence under grazing over diverse environments of the Northern Great Plains and to select germplasm tolerant to grazing; and (2) constructing a model of alfalfa growth and development using the morphogenetic responses to rotational and continuous stocking by cattle. A three-year mufti-location grazing trial evaluated persistence of 23 cultivars and/or experimental lines under rotational and continuous stocking in pure stands and in mixture with grass. Visual estimates of percent alfalfa within a row provided a rapid assessment technique with similar accuracy as more resource intensive quantitative measurements. Evaluation under intensive grazing with continuous stocking of pure stands was more effective in demonstrating differences in cultivar persistence than rotational stocking and the use of mixed stands. However, severe winter stand losses in some grazing-tolerant cultivars suggested that tolerance to grazing does not ensure winter survival in the Northern Great Plains region. Three-dimensional architectural models were created using the mathematical formalism of L-systems. Differences in grazing intensity and frequency (i.e., rotational vs. continuous stocking) regulated the final size and number of internodes and leaves, stem thickness, branching angle, branching delay, as well as the number of shoots arising from the crown. Regression functions quantifying these parameters were derived from data obtained from original plant measurements. Computer simulations produced realistic images of alfalfa regrowth in the different grazing systems. Architectural models, essentially "virtual plants", represent a new and powerful technology providing an indepth understanding of plant morphology as affected by environment.