As an alfalfa stand ages and thins, the primary question becomes: Is this stand good enough to keep? Now is a good time to evaluate stands as more time is allowed for planning crop rotations than when stands are evaluated in the spring and determined to be uneconomic. This assessment may be especially important in regions that had excessive rain, flooding or waterlogged soils in the spring or early summer since the waterlogged soils may have enhanced phytophthora, which will increase winter injury, and plants will suffer from droughty conditions next year.
Phytophthora is a fungal disease that flourishes in wet soils. Figure 1 shows a range from healthy plants at the left to severely diseased at the right. Black lesions develop on the roots. If the disease is sufficiently severe, the root may actually rot off. These plants with reduced root mass will look fine above ground until water is limited and then suffer yield and possibly die.
Fall evaluations can allow you to identify less profitable fields and those likely to suffer winter injury. Fall assessment allows time for fall tillage and alternative cropping strategies before fall fertilizations and spring herbicides are applied.
A two-step process is recommended to evaluate stands:
- Use stem count to estimate current yield potential of the field.
- Assess root and crown health to determine future yield potential.
The old method of estimating alfalfa yield potential was to count plants. This really didn’t work, as one can see in Figure 2. Five healthy plants may provide adequate yield with over 55 stems per square foot, whereas five spindly plants will have fewer stems per square foot and definitely not produce as much yield.
The relationship between stem density and yield is shown in Figure 3. This relationship is not dependent on stand age. Biologically, the relationship makes sense since you are harvesting stems, not plants.
Stem counts are easy to take in the fall after the last or second-to-last cutting when one can count the cut ends of harvested stems. After counting a couple square-foot samples, one can visually estimate stem density and determine portions of the field with good and less good stem density. One can assume that yield will be similar next year if the stand is healthy and winter injury/kill does not occur.
Recognize that stem density estimates yield potential, not actual yield, which will vary due to the growing environment, including soil fertility, water availability and disease or insect pressure.
Stand yield potential based on stem densities per square foot can be assessed in the following manner:
- Greater than 55 stems per square foot: Indicates stem density will not be a limiting factor.
- Between 45 and 55 stems per square foot: Indicates some yield reduction, but fields may be adequate in years of low inventories and high value.
- Fewer than 40 stems per square foot: Indicates a poor stand and should be considered for termination. Note that 40 stems per square foot density would have only 75% yield potential of a healthy stand. Would you plant any other crop for 75% of yield potential?
The second step in fall field assessment is to dig plants from three or four representative locations in the field. Dig to include the top 6 inches of the plant root.
Examine each plant for size, symmetry and the number of shoots present. A healthy plant should be large and have shoots coming from all sides of the plant in symmetry. Then cut the taproot lengthwise and check for rot and discoloration in the crown and root. Healthy roots are firm and off-white. Some discoloration is often present in older plants but should never exceed 50% of the root diameter.
If roots are generally healthy and stem count is within acceptable yield potential, one can assume that the stand will have the indicated yield potential next year. Stands will generally not increase in yield potential.
Determine the percentage of fields with each health category and yield potential and manage to meet your goals.
If a small percentage of the field has reduced stem density and you want to go over the affected areas with a drill, seed with a 50/50 mix of Italian (annual) ryegrass and perennial ryegrass (10 pounds per acre) to a depth of 1/2-inch in the spring. Italian ryegrass will contribute forage the fastest, and perennial ryegrass will carry more into summer. Other grasses will establish much slower and not contribute significantly in the growing season. Plan on terminating the stand at the end of the next growing season.
If a large percentage of the field is affected, consider terminating the stand immediately and replanting alfalfa elsewhere. Corn and sorghum-sudangrass can use the nitrogen credits from alfalfa and show yield advantage from rotation.
Remember that turned-down alfalfa is an economic benefit due to legume credits and yield enhancement of the next crop.
