Back

On May 30, 2018

To Assess Corn and Soybean Populations and Replanting Decisions

Heavy rains and excessively wet soil conditions have taken a toll on recently planted corn and soybean fields. Extension Agronomist Andrew Frankenfield suggests it is the time to take a look at those fields.

 

About two weeks after corn and soybeans have been planted the seeds that are viable would have emerged if they were going to emerge. Now is the time to go out and assess your stands.

Corn fields will likely have some stand variability between the well-drained areas and the poorly drained areas. For corn on 30” rows measure 17 feet 5 inches, this represents 1/1000 of an acre, count the plants in that distance. The number you get multiply by 1,000 to get your plants per acre. For example you count 21 plants that equals 21,000 plants per acre. Do this in multiple areas of the field to determine your average for the field.

Know the key planting times and relation to yield. This will come into play as stands are deemed inadequate and a replant is in question. Using Table 1.4-8A  from Penn State Agronomy Guide,  https://extension.psu.edu/the-penn-state-agronomy-guide we know that planting corn on April 30th gives us 100% of our yield potential with a final stand of 30,000 plants per acre. However, if our average stand count is 17,500 plants per acre, the same chart for corn planted April 30th still has 87% of its optimum yield potential. If we replant the same field on May 29th and achieve a stand of 30,000 plants per acre, our yield potential is now down to 81%. Nearly a 20% loss just by delaying the planting date by a month. Knowing this allows us to say, “Well if I want to replant today May 29th I will lose 20% of my yield and if I don’t replant I will only lose 13%, so it probably will not pay.” It is likely that there are areas in fields that will have less than half a stand and would benefit from replanting. If replanting is necessary, it would be better to plant a shorter season hybrid to aid in a more uniform field dry down.

 

When evaluating soybeans remember the soybean plant has the ability to branch and fill in. However, there are limits to the lowest population establishment without losing top end yield. The other consideration is that the yield penalty for planting soybeans in late May and early June isn’t as severe compared to corn. However, do not be too quick to replant a field with reduced emergence. Using Table 1.6-3 from the Penn State Agronomy Guide,  https://extension.psu.edu/the-penn-state-agronomy-guide  a 60,000 plants per acre soybean stand has the potential to still yield 92% of its full yield potential. Those soybeans would have been planted at the ideal time compared to the field being replanted on May 30th and having 95% of full yield potential or June 10th at only 88% of its full yield potential.

 

So how do you assess soybean stands? Soybeans are more difficult than corn since there are multiple row width options. To determine stand count on 30” rows it is just like the example above for corn. For soybeans planted on 15” rows, you double that distance and measure off 34 feet 10 inches and count the plants. The number of plants x 1000 = plant population in plants per acre. Take multiple counts of adjacent rows in different areas of the field to get an overall stand for a field. For drilled soybeans, one can use the hula-hoop method, randomly toss the hoop and count the plants inside the circle. Convert plants per hoop to plants per acre by multiplying the number of plants by the appropriate factor. A 28 inch hoop is the easiest to calculate since the multiplication factor is 10,000, so 13 plants in the circle x 10,000 = 130,000 plants per acre. If you don’t have a hulu-hoop, you can make your own. Cut a 1/2 inch piece of pex pipe to a length of 88 inches and attach the two ends with a ½ inch hose mender.

 

Next 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 (Agronomy Guide 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 (Agronomy Guide Table 1.6-4)

 

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 an existing stand. Repairing a planting with a planter, if possible, rather than a drill may cause less damage to the stand.

For more information go to:

https://extension.psu.edu/assessing-corn-and-soybean-populations-and-replanting-decisions

 

 

Quote Of The Week: “Do not be angry with the rain; it simply does not know how to fall upwards.” Vladimir Nabokov