Deciding to replant a soybean crop is difficult and involves many interacting factors: plant response to reduced plant populations, calendar date, variability of existing plant stand, cost/price relationships, and the potential yield of the existing stand versus the potential yield of the replant stand.
When making a replant decision, follow a set procedure that includes these factors:
- Identify the cause of the stand reduction and whether it will be a problem when the field is replanted. Take steps to correct the problem before replanting.
- Estimate the yield of a full stand at the original planting date.
- Determine the population and distribution of the existing stand.
- Estimate the yield potential of the reduced stand. Often, the stand is not uniform and the impact of patches or gaps in the rows would likely be greater than the population effect itself. Based on Midwest data, yields will not decline appreciably until stands drop below 60,000 plants per acre (Table 1.6-3). In one study, the impact of gaps lowered the yield potential of a 70,000-plants-per-acre stand from 95 to 73 percent of normal (Table 1.6-4). One technique suggests estimating the number of gaps (at least one-half-pace long) encountered in several row sections consisting of 20 typical paces each. Add up the number of paces in half- or full-pace gaps and estimate the percentage of the total number of paces in your sample. Then refer to Table 1.6-4 for your yield-loss estimate.
- Estimate any additional costs associated with keeping the stand. A thin stand may require an additional postemergent herbicide application.
- Estimate the yield potential of a replanted full stand (Table 1.6-5) later in the season.
- Estimate the cost of replanting.
- Compare the value of reduced stand to replanted stand. For example, assume the following:
- Estimated yield of full stand at original planting date is 50 bushels/acre.
- Average percent of row lost to gaps is 50 percent.
- The average plant population is 70,000 plant per acre.
From Table 1.6-4, the estimated yield from the reduced stand would be 39 (50 × 0.78) bushels/acre.
The estimated yield from planting can be obtained from Table 1.6-5. Assume the original planting was on May 12, and the stand was evaluated on June 4 and could be replanted on June 5. The estimated yield would be approximately 45 bushels per acre, or 6 bushels per acre more than not replanting. Remember that the cost of replanting must be considered, and there is no guarantee that replanting will give a full stand.
Another alternative is to fill in an existing stand to bring it up to an ideal population. This would require less seed. Be sure to include some consideration of the plants lost in the replanting process. Soybeans planted into an existing stand have far fewer negative effects than corn planted into and existing stand. Repairing a planting with a planter, if possible, rather than a drill may cause less damage to the stand.