Uneven stands of corn are associated with difficult field planting conditions and reduced tillage. Field symptoms include tall and short corn, large within-row gaps, and groups of crowded plants. Variable seedbed soil moisture is a major factor causing uneven corn emergence, whereas high planting speeds (in excess of 6 mph) and poor planter maintenance/adjustment are primarily responsible for uneven within-row plant spacing.

Uneven plant spacing characterized by large within-row gaps of 4 to 6 feet can reduce grain yields up to 5%. Potential yield can be lost from every ear in a tight grouping of two or more plants. Uneven plant emergence can also have a major impact on potential yield even if within-row spacing is relatively uniform (Nafziger et al., 1991). If half or more of the plants in a stand are delayed 1.5 weeks in emerging, yield losses of 5 to 8% can be expected. If half or more of the plants in a stand are delayed in emerging three weeks or later, yield losses of 20% or more can occur.

Doubles, triples or worse and large gaps can result in lost yield potential. A study at the University of Illinois concluded that skips contribute to plant spacing variability slightly more than doubles but both increase variability (Nafziger, 1996). Skips reduce yield in fields where the intended population is at or below the optimum, while doubles increase yield when populations are less than optimum. Conclusions from this study are that:

1. Population is the most important management practice that must be considered during planting time decision-making,

2. Standard deviation in plant spacing alone does not predict yield loss, and

3. Skips are much more limiting to yield than doubles.

Corn sometimes emerges unevenly because of environmental conditions beyond the control of growers. However, timely planter servicing and adjustment, as well as appropriate management practices, can help prevent many stand uniformity problems. The following are some tips for improving the uniformity of seed placement during planting:

Avoid excessive tillage trips and tilling wet soils.
Check seed depth and seed soil contact periodically during planting.
Plant at speeds between 4.5 to 5 mph.
With plate-type planters, match the seed grade with the correct planter plate.
Planters with finger pick-ups should be checked for wear on the back plate and brush, use a feeler gauge to check tension on the fingers, and then tighten them correctly.
Check for wear on double-disc openers and seed tubes.
Make sure the sprocket settings on the planter transmission are correct.
Check for worn chains, stiff chain links, and improper tire pressure.
Lubricate all chains and grease fittings.
Make sure seed drop tubes are clean and clear of any obstructions.
Clean seed tube sensors if you have a planter monitor.
Make sure coulters and disc openers are aligned properly.
With air planters, match the air pressure to the weight of the seed being planted.
Make sure press wheels are adjusted to close the seed slot.

Literature Cited

Nafziger, E. D. 1996. Effects of missing and two-plant hills on corn grain yield. Journal of Production Agriculture 9:238-240.

Nafziger, E. D., P. R. Carter, and E. E. Graham. 1991. Response of corn to uneven emergence. Crop Science 31:811-815.