Declining Alfalfa Stand: Keep, Interseed, or Rotate?
A 40-acre alfalfa field in Wisconsin was seeded four years ago. On May 1, a stand assessment finds 7 plants/ft2 and 34 stems/ft2, with 25% grass and dandelion cover. The first cutting is scheduled for May 25. The field had potato leafhopper injury last July, and the last cutting was taken on September 10. The grower wants to renovate by no-tilling alfalfa into the existing stand after the first cutting. He has a no-till drill and wants to avoid losing a year. Soil pH is 6.4, and soil test P and K are in the medium range. He asks whether the stand is worth keeping, whether interseeding alfalfa will work, and what rotation alternative is better.
1. Assess the alfalfa stand and decide whether to keep it for another year.
Read the worked answer
The stand is marginal to poor. A density of 7 plants/ft2 is above the 6 plants/ft2 threshold for a poor stand, but 34 stems/ft2 is below the 40 stems/ft2 threshold, and 25% grass plus dandelion reduces forage quality and yield. The September 10 cutting is also late; alfalfa needs about 4-6 weeks of regrowth before frost to store carbohydrates. Given the marginal stem count and weed pressure, the field may produce one more cutting, but it is not a dependable long-term stand. If the first cutting is good, keep it through first cutting and then terminate; do not invest in fertilizer or herbicide for a stand that is already thinning. Plan replacement for the following year.
2. Evaluate the plan to no-till alfalfa into the existing stand after first cutting.
Read the worked answer
Interseeding alfalfa into an existing alfalfa stand is likely to fail. Alfalfa produces autotoxic compounds, and seedlings are sensitive to competition from the established plants. Killing the old stand and no-tilling within a few days does not give enough time for the toxins to break down; a 2-3 week wait after tillage is a minimum, and a full-year rotation is safer. The existing stand would also shade and outcompete the seedlings. A better approach is to terminate the stand, rotate to corn or a small grain for one season, and seed alfalfa in the following year. If the grower insists on no-till, use a different forage species, such as red clover or a grass-legume mix, not alfalfa.
3. Recommend a rotation and management plan for the field after termination.
Read the worked answer
Terminate the stand after first cutting, using glyphosate plus a residual herbicide if the label permits, and wait 2-3 weeks before tillage or planting. Rotate to corn for one year; alfalfa provides a significant N credit, roughly 40-80 lb N/ac in the first year after a good stand, so reduce corn N accordingly. If corn is not an option, use a small grain or a grass cover crop. Test soil pH and P/K before seeding; pH 6.4 is adequate for alfalfa but could be raised to 6.8 for the next stand. Control weeds and avoid a late-summer cutting in the establishment year. Seed alfalfa again only after a full year of a non-host crop and a firm seedbed.