Rapid drydown rates and drought stress can affect crop quality at harvest. Once the crop reaches black layer (or R6 growth stage) scouting for variables (including: grain moisture, stalk quality and ear retention) can help schedule harvest. Proper harvest order can help minimize mechanical loss and grain storage problems.
Corn at maturity is considered to have 30 to 35% moisture. Depending on the hybrid, approximately 2,800 crop heat units (CHU), would be accumulated at maturity.6 In a typical year with a May 1st planting date, these levels of moisture and CHU are estimated to be reached near the end of September. However it is important to be mindful of the fact that CHU accumulation can be ahead of the average with early planting dates and hot temperatures during the growing season.
Grain is commonly harvested when moistures are between 20 and 25%. Field drydown rates could range from 0.5 to 0.66 percentage point of moisture per day between late August through October.4 During cool, wet days drydown rates could be as low as 0.3% moisture loss per day.
Harvesting at lower moistures can increase mechanical losses due to ear drop, stalk lodging, and kernel shattering. Mechanical losses are expected, but keeping them to a minimum of 1% for ear loss, 0.3% threshing loss, and 0.5% loose kernel loss should be the goal.5
Ear loss can be minimized by setting snapping rolls to fit stalk width, and running snapping rolls at the same speed as ground speed. Cylinder or rotor speed can be adjusted to minimize threshing losses and kernel damage. Loose kernel losses can be affected by fan and shoe settings, and combines should be adjusted where stressed plants produced lighter kernels.
Lodged corn can occur when stalks are weakened by diseases such as charcoal rot, Fusarium, Gibberella, and anthracnose. Drought and other stresses can reduce root system growth in corn. Consequently, nutrients and water uptake can be reduced and carbohydrates production may not be adequate for the plant. If carbohydrate production is below normal, stalks can be depleted even under average conditions because ears draw on stalks for carbohydrates during grain fill. Lack of water increases competition for carbohydrates between the ear and stalk and leads to stalk weakening and potential stalk lodging.
Physiological stalk lodging occurs when not enough lignin is deposited in rind of the stalk during grain fill. Lignin helps cement corn tissues together, and periods of heat or moisture stress can increase the likelihood of physiological stalk lodging.
Nutrient Imbalance. Drought conditions could also lead to nutrient imbalances and deficiencies that negatively affect stalk quality. Stalk strength and resistance to stalk rot can be reduced by potassium (K) and chloride deficiencies.2 Limited soil solution throughout the root profile can be associated with K deficiency as the nutrient moves to the roots by diffusion through soil solution.
Potassium deficiency combined with high nitrogen levels can increase premature stalk death, stalk rot, and lodging. Nitrate accumulation in the lower stalk is a concern for drought-stricken corn. In normal years, nitrate is taken up by corn plants, and if soil availability is in excess, the plant will store nitrate in the lower stalk as a reserve for future use. In late-season drought conditions when leaf and stalk growth is below normal and/or ear development is reduced, the stored nitrates are not needed and remain in higher concentrations in the lower stalk. Two methods of scouting for stalk rots can help determine if early harvest is necessary: The Push Test and/or the Pinch or Squeeze Test. For either method, select 20 plants from five different locations in the field for a total of 100 randomly selected plants1.
- Push Test. The top portion of plants are pushed 15 to 20 cm (6 to 8 inches) from the vertical. Numbers of lodged plants are noted.
- Pinch or Squeeze Test. The area above the brace roots is squeezed (lower leaves removed if necessary). Numbers of rotted stalks are noted.
When 10 to 15 percent of plants are found to lodge or are rotted, consider slating the field for earlier harvest. Lodged plants will prematurely reach black layer, if they have not already reached that stage. Ears on lodged stalks dry slower than ears on standing stalks. If the corn was relatively green prior to the lodging, the lodged plants will remain green for a few days but mature or die before plants that remained upright.
Disease. Ear moulds and mycotoxins can diminish kernel quality. Aspergillus flavus and Fusarium verticillioides are ear moulds favoured by hot weather during the growing season. Mycotoxins that are harmful to humans and animals may be produced by these moulds. Therefore, fields with ears infected by A. flavus and F. verticillioides may need additional management and early harvest. The presence or severity of A. flavus fungus on the ear is not necessarily a good indication of how much of the mycotoxin Aflatoxin will be present in the grain. For example, the fungus can be present with little to no Aflatoxin produced, or very little fungus may be visible with significant Aflatoxin produced. F. verticillioides is favoured by warm dry weather at silking and high moisture at harvest.3
Ear Retention. Ear shanks may not develop properly in stressed conditions. Additionally, rapid drydown can result in brittle tissue where the stem attaches the cob to the stalk. Shanks may also be “pinched” or have constriction on one side, and kernels may not develop at the base of the ear on the constricted side. Dropped ears can result if Fusarium infects ear shanks, and deteriorates tissues. Fusarium stalk rot can be more common during drought and in poor fertility conditions.2
Reasons to Slate Fields for Early Harvest
1) Grain Moisture
2) Stalk Quality
3) Ear Retention
Monitor fields closely to schedule harvest while there is still enough stalk strength remaining to facilitate harvest. A well-adjusted combine could be needed soon. Growers may need to prepare combines fitted to handle dropped ears, compromised stalks, and possibly low test weight corn.
Contact your local DEKALB® brand seed representative for more information about determining corn harvest order.
Physiological stalk rot is caused by stalk tissue weakening and not disease. However, weakened stalks become prone to disease.
1 OMAFRA. 2011. Publication 811. Diseases of field crops: corn diseases.
2 Watson, S. 2011. High temperatures may reduce irrigated corn yields. K-State Research and Extension News. http://www.ksre.ksu.edu (verified 9/6/2012).
3 USDA Grain Inspection, Packers & Stockyards Administration. 2006. Grain fungal diseases & mycotoxin reference.
4 Elmore, R. 2009. Evaluate corn drydown rates. WallacesFarmer.com (verified 9/7/2012).
5 McNeill, S. and M. Montross. Corn Harvesting, Handling, Drying, and Storage. www.ca.uky.edu (date verified 9/4/2012).
6 OMAFRA Staff. 2009. Corn: development. http://www.omafra.gov.on.ca (verified 10/2/2012).
Hurburgh, C.R. 2005. Grain quality and grain handling issues in drought areas. Iowa State University Integrated Crop Management. IC-494(23).
Nielsen, R. 2008. Stress during grain fill. A harbinger of stalk health problem. Purdue University.
Watson, S. 2011. Drought-stressed corn needs timely harvest to avoid stalk lodging, ear drop. Kansas State University. www.ksre.ksu.edu (verified 10/1/2012).