Cover Crop Termination Before No-Till Soybean
A grower is in the third year of no-till and planted a cereal rye cover crop in late September after corn harvest. The rye has grown well and now stands about 60 cm tall in mid-May. The grower wants to plant soybean in 10 days and plans a single burndown of glyphosate at the label rate, then plant directly into the standing residue. Night temperatures have been near 8 degrees Celsius, and daytime highs are forecast near 15 degrees Celsius for the coming week. The grower has had patchy control of rye in prior years and blames the product. A neighboring field planted into a terminated rye stand showed yellowing soybean seedlings in the first two weeks.
1. Diagnose the most likely reason for the patchy rye control and assess whether the planned burndown timing is sound.
Read the worked answer
The most likely cause is environmental and application conditions, not the product. Glyphosate is slow-acting in cool weather; with nights near 8 degrees Celsius and highs near 15 degrees Celsius, translocation is poor and control will be slow and uneven. Rye at 60 cm is also large and nearing the stage where a single glyphosate pass is less reliable. Applying 10 days before planting leaves little margin, because glyphosate injury to soybean is possible if the rye is not fully dead and the interval is too short. The judgement is to apply earlier, add a second mode of action where resistance is a concern, and use a higher carrier volume with a quality adjuvant so coverage of the dense canopy is adequate.
2. Explain the yellowing soybean in the neighboring field and state what it implies for this grower's plan.
Read the worked answer
Yellowing soybean seedlings planted into a recently terminated rye stand is most consistent with nitrogen immobilisation and allelopathic effects from decomposing rye residue, compounded by cool soil slowing mineralisation and root growth. It is not necessarily a herbicide failure. The implication is that planting directly into a large, freshly killed rye mat with cool soils raises the risk of a slow start. The grower should terminate earlier so the residue begins breaking down before planting, ensure good seed slot closure and adequate seeding depth, and consider a starter nitrogen band near the seed to offset immobilisation. Waiting for warmer soils before planting would further reduce the risk.