Issue No. 11, Article 3/June 5, 2009
Replanting Soybean Is No Easy Decision
Corn planting was 82% complete and soybean planting 34% complete as of May 31, as reported by the USDA NASS Illinois Weather and Crops Report. That was up from 62% and 12%, respectively, the previous week. Last week I wrote about scouting for potential soybean emergence issues and seedling blights, what to expect, and what to look for within the first two weeks of planting. I didn't elaborate on information that may affect a replant decision. A decision to replant takes into account many factors, including the availability of seed with equal quality and yield potential for a given field, the time you have to replant in relation to planting and/or spraying other fields, and keeping and managing the stand you have vs. the stand you hope you'll get the second time. And there is some emotion involved in having a "failed" stand establishment as well, so replanting is not an easy decision.
There are no clear-cut rules on balancing all of these factors, but before you consider terminating a partial stand, make certain you are sure how many plants per acre you have and consider how uniformly distributed they are. The data I presented in issues 6 and 7 of the Bulletin (May 1 and 8, 2009) could serve as a guide to help you assess what to expect from your population and needed final stand. For instance, the planting date data in issue 6 would indicate you are losing roughly half a bushel of soybean yield potential per day of delay in June. The seeding rate data in issue 7 would indicate that an evenly spaced seeding rate of 50,000 plants per acre will produce 91% and 90% of expected yield from seeding 100,000 and 150,000 seeds per acre, respectively. So with all other factors considered equal, if you seeded 150,000 seeds per acre on June 1 and expected 60 bushels per acre yield but achieved a stand of 50,000 plants per acre, you might still expect 90% yield potential, or 54 bushels per acre. However, if you eliminated the stand and replanted 150,000 on June 12 and achieved a full stand the second time, your yield potential would still likely be reduced to 54 bushels per acre due to the delay in planting. Looking at the numbers, in many cases I find it hard to justify terminating an existing stand.
If you can identify and rectify a low-stand situation early, particularly if the low stand is not uniform, one idea is to plant more seeds into an existing stand. There is no good guideline for how far apart soybean plants can be in their growth and development before the later ones may compete with the earlier plants like weeds, but done within a couple of weeks of the original planting date, this practice could prove successful. If your decision is to simply maintain a low soybean population, one of the biggest considerations is increasing your efforts to scout for weeds and eliminate weed competition. The lower the soybean plant stand, the longer it will take to achieve canopy closure, so there will be a longer period for weeds to take advantage of light, water, and nutrients to establish and compete with your soybeans. An extra application of postemergence herbicide to protect the yield in a low stand may be more economically viable than the cost and time to replant for a "fuller" stand.
Hopefully these comments on incorporating yield expectations to seeding rate, yield expectation to planting delay, and increasing weed management efforts in low stands will offer some guidance to those making this difficult decision.--Vince M. Davis