May 23, 2006

Revised and updated May 24, 2006

 

CORN

 

Replant Decisions

 

For various reasons, some corn fields do not have the stand intended at planting time. 

 

The following table can help in making replant decisions:

Influence of planting date and plant population on corn grain yields (percent)

Stand X 1,000	April 20 - May 5	May 13 - May 19	May 26 - June 1	June 10 - June 16	June 24 - June 28
28 – 32	100	99	90	68	52
24	94	93	85	64	49
20	81	80	73	55	42
16	74	73	67	50	38
12	68	67	61	46	35

Numerous gaps of up to 4-6 feet can reduce yields by an additional 5-6%.

Assuming a cost of $10 per acre to destroy the existing stand, $10 per acre to replant, and $30 per acre for seed, the cost of a re-plant is about $50 per acre.  At $2 per bushel, that is 25 bushels of corn.  If the average yield is 180 bushels, then 25 bushels equals 14% of the yield.  Using the above chart, a re-plant in the May 26 – June 1 range has a yield of 90% of an earlier planting, so there is 10% yield loss due to lateness of planting.  Adding the loss from late planting to the loss due to the cost of replanting (10% + 14%), the total loss is 24%.  Looking at the above chart, you will note that a uniform stand of 16,000 has a loss of 26%, which is essentially a break-even with performing a replant.  And more recent research is suggesting the penalty for later planting is greater than the above chart suggests.  It is not likely that it will pay to replant stands of 14,000 – 16,000 or more, if the remaining stand is fairly uniform. Remember that the uniformity of the stand needs to be considered in making decisions.

For more information, see Pm-1885 "Corn Planting Guide" http://www.extension.iastate.edu/Publications/PM1885.pdf and NCR 344 "Uneven Emergence in Corn" http://www.extension.iastate.edu/Publications/PM1885.pdf.

 

 

Insects

 

I continue to receive scattered reports of black cutworm feeding at treatable levels and also an occasional report of corn flea beetles being observed.  Continue to monitor corn fields for these and other pests.  Black cutworm and corn flea beetle information can be found at http://www.extension.iastate.edu/Pages/eccrops/insect.html.

 

Along Highway 34 (Burlington – Mount Pleasant area) stalk borers are starting to move from grassy areas to adjacent corn plants.  Over the next few days, stalk borers farther north will begin their journey from grass to corn.  If you observe many dead grass heads, you may want to consider a management strategy.  See http://www.extension.iastate.edu/Pages/eccrops/insect.html for details.

 

 

SOYBEAN

 

Emergence Issues - Crusting

 

Crusting is causing some soybean emergence problems.  To rotary hoe or not to rotary hoe – that is the question.  Rotary hoeing may aid emergence, but it can also cause much damage.  The best way to decide about rotary hoeing is to run the implement for a few yards through the field and then go back and see how much damage is being done to the crop that is trying to emerge.  If little damage is being done to the crop, continue to rotary hoe.  If damage is severe, leave the field and see if the plants can make it on their own.

 

Hopefully the predicted rains will alleviate the crusting problems.

 

 

Emergence Issues – Slow Emergence

 

Many are concerned with the slowness of soybeans to emerge after planting.  The cold weather of last week is most likely the primary reason.  If the seeds / seedlings look basically healthy, most likely everything will be alright, but it would be best to continue to monitor as slow emergence gives more time for insects and diseases to attack.

 

 

Replant Decisions

 

 The following table can help in making replant decisions:

 

Plants/A	Yield (bu)
150,000	45.1
125,000	44.8
100,000	45.1
75,000	44.2
50,000	41.6
75,000 with 1 foot gaps	43.6
75,000 with 2 foot gaps	41.5

 

 

Note that a uniform stand of 50,000 and a stand of 75,000 with 2 foot gaps produced only 3.5 bushels less than the top yield.

 

The above chart and additional information on soybean replanting decisions can be found in PM – 1851”Soybean Replant Decisions” http://www.extension.iastate.edu/Publications/PM1851.pdf.

 

 

Bean Leaf Beetles

 

Bean Leaf Beetles can easily be found in most soybean fields.  While I have yet to be in a field that needed to be sprayed, fields should continue to be monitored.  Threshold information can be found at on pages 120 – 122 of the May 15, 2006 ICM Newsletter http://www.ipm.iastate.edu/ipm/icm/2006/5-15/blb.html.