Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
The indicator most often used is live plants per square foot. For a highly productive stand a minimum of 4-5 plants should occupy each square foot. The correlation between plants per square foot and yield is actually rather low since individual alfalfa plants respond to decreasing stand density by producing more stems. An increase in stems per plant compensates for fewer plants and will often maintain yield.
Therefore, another indicator which should also be used is the number of stems per square foot. In many cases this can be a better indicator of a productive field, because it reflects the health and vigor of the plants. Fields with 55 or more stems per square foot produce maximum yields. As the stem number declines below 55 per square foot yields begin to decline. Once stem numbers fall below 40 per square foot alfalfa fields begin to loose profitability and should be rotated out of alfalfa.
Other factors to consider include the uniformity of the stand and the weed situation. Are there areas within the field where the population is below the desired number? Is weed competition affecting the number of stems per square foot or will it reduce forage quality and reduce future yields? Has there been any frost heaving? If an alfalfa crown is more than an inch out of the soil most likely the plant will die or will be cut off at first mowing.
Table 1.2-6 in the 2015-16 Penn State Agronomy guide states that a field with at least 50% alfalfa, on highly productive soils, will provide 120 units of nitrogen for the following corn crop. This too can be a consideration when assessing that aging field of alfalfa.