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Alfalfa-Grass NDF Assessment

Timing of spring forage harvest is critical to obtain optimal quality for animal production. The NDF concentration is the principal forage quality variable of concern for forage that serves as the primary fiber source in the diet of dairy cattle. The target NDF at harvest is approximately 38% for pure alfalfa and 50% NDF for pure grass, and the acceptable range is relatively small. These goals assume a 10-15% decline in forage quality due to harvest, storage and feedout. There is a need for quick and accurate methods for determining NDF of grass-legume mixtures [GIS-22]. A number of methods have been developed to estimate alfalfa NDF, including models based on weather, chronological age, and plant morphology. Maximum alfalfa height works as well as anything for describing both alfalfa and grass development in a mixed stand.

We have equations for estimating mixed stand NDF, based on alfalfa height, sampling date, and an estimate of the grass:legume ratio. Prediction equations for NDF of mixed alfalfa-grass stands were developed based on sampling and separating mixtures from fields across NYS in 2004 and 2005. It will be possible in the future to predict forage composition of a field using NIRS from orbiting satellites, but until then our equations are the next best thing.