Alfalfa stand evaluation

Paul Craig, Crop Management Extension Group, Penn State University

All hay producers recognize the fact that as an alfalfa stand ages, it eventually thins out. There are many factors that cause stand thinning. These include diseases, insect and weed pressure, poor fertility and poor harvest management. The big question is not why the stand is failing but rather is the existing older stand thick enough to keep for another season?

How thick a stand is directly affects both the yield and quality of the cutting; however economics of existing forage supplies and costs of reseeding compound the decision on individual farms. Unfortunately, at some point, a decision must be made. The current alfalfa stand evaluation tool is based on work done at the University of Wisconsin by Dr. Dan Undersander and evaluated in Pennsylvania by Dr. Marvin Hall at Penn State.

These forage agronomists recommend looking at alfalfa stands in the fall for the best method for stand assessment. They note that a second appraisal is helpful in the spring after the stand breaks dormancy. Fall evaluations help to identify troubled stands that may be prone to winter injury. This can allow for tillage or fall-applied herbicides for optimum rotation affects. Spring evaluations reveal winter injury damage.

Wisconsin research notes that stem counts are more accurate for estimating yield potentials than crown counts. A stand may have density of 5 to 8 crowns but individual crowns may have few, poorly growing shoots. Research indicates that total stem counts per square foot is a better method. Their conclusions are that stands with more than 55 stems per square foot will have maximum yields; stands with fewer than 40 stems per square foot were not profitable and needed to be replaced; and densities between 40 and 55 will need additional considerations.

In addition to counting stems, agronomists recommend evaluating the condition of crowns and root stands at the same time. By considering the crown and root health, an estimate of the long-range yield potential can be part of the final decision. Healthy crowns and roots are large, symmetrical or balanced in shape, have many roots and shoots, are resistant to bark peeling and have creamy bright internal coloration. Crowns and roots with few roots, a soft feel or darkened spots are damaged and prone to decline. Healthy stands have less than 30 percent injured crowns.

To evaluate your alfalfa stands, make a simple 12 inch by 12 inch frame of wire or small PVC pipe. Select three or four areas of the field, toss the frame and then count alfalfa stems in this area. An alfalfa height of about 6 inches helps. Keep track of your results, and then average the counts across the field. The most reliable estimate will result from multiple tosses. Then dig a few crowns from the nearby area and look for signs of weak crowns.

Stands with stem densities of greater than 55 per square foot can still be high yielding with some crown damage. However, many stands with densities in the lower 40s per square foot may also have a high yield potential if those stems are growing on healthy, vigorous crowns. There is no magical number to make this decision error-free. Nevertheless, taking the time to walk over your stands may help you identify potentially poorer stands in time to modify cropping plans. HG

—Excerpts from Penn State Field Crop News, Vol. 7, No. 28