Assessing corn fields for replanting purposes when stands are marginal depends on lots of factors, so don’t just focus on plant population. Of course, the plant population gives a baseline of potential, but your actual corn productivity or outcome is also very dependent upon seedling spacing and emergence uniformity. Unfortunately, poor stands are always going to have some variability issues that we need to assess.
Our plant population research with irrigated corn shows about 6‐9% yield reduction for 24,000 uniform plants per acre. Dryland research shows similar expectations down to about 18,000‐20,000 uniform plants per acre. The degree of yield loss picks up considerably as populations drop off even further.
Corn plants are also very sensitive to both inconsistent plant spacing and emergence variability. Because cool temperatures were prevalent this spring, we often have much more emergence variability than normal. Corn plants are very intolerant of emergence variability because their growth habit is determinant, so late plants never “catch up,” and their plant size and compensation ability are rather fixed. We have a graduate student who has completed his first year of research evaluating corn response to variable emergence and those results confirm corn is quite intolerant. This research evaluated the effect of delayed emergence where treatments were up to about 1 ½ leaf stages later than the normal plants. The yield of individual late-plants fell in a linear trend to as little as 50% of the normal plants. The aggregate yield fell up to 11% when three adjacent late-emerging plants comprised 25% of the plot. Some of the fields I have seen have considerably more variability than this. Therefore, realistic yield reduction associated with the plant populations noted above are likely going to end up 12-20% lower than normal, when you factor in variability issues.
If you need to replant, the most important thing to remember is to resist the urge to “fill in” or “spot plant” a partial stand. Replanting into a partial corn stand will produce severe “weed” competition between the two ages of plants, severely reducing yield potential. Thus, it is absolutely imperative to destroy the remaining live corn plants with either tillage or herbicides, before you replant an unacceptable stand. We also suggest you spray any herbicide at least a day prior to replanting, so that herbicide activity is not hampered by tissue mangled by the planter.
Herbicide options for controlling failed stands of Roundup Ready corn include:
1. Select Max (6 oz/a) + 0.25% NIS + AMS: excellent control of RR or LL corn, can replant corn six days after application.
2. Gramoxone SL (40-48 oz/a) + 0.5% NIS + a photosystem II inhibitor (Direx, Linex, Atrazine, or Simazine @ 16 oz/a; or Metribuzin @ 4 oz/a): good control of RR and/or LL corn when applied as specified with adequate coverage, no plant back restriction for corn.
3. Liberty 280SL (32 oz/a) – is not effective on Liberty Link (LL) corn hybrids, moderate control of RR corn, no plant back restriction for corn.
Persistent rainfall restricting ground application of fertilizer and herbicides may also begin complicating corn production this spring. Cool temperatures have limited corn growth, so hopefully we will have and opportunities to get in the field soon to satisfy crop needs or issues. However, if you have not yet applied a bulk nitrogen application, then you may need to consider applying granular nitrogen fertilizer by air to satisfy crop need, until you can side-dress. Although corn’s early season nitrogen needs are low (less than 10% of seasonal N is needed before V6 growth stage), forgoing N application will reduce productivity. This figure gives guidelines for multiple applications of nitrogen fertilizer during the season.