How to Determine When to Harvest Field Corn

Deciding when to start harvest depends on several factors, including total acres, relative maturities (RMs) planted, access to grain drying and harvest capacity. If you faced wet conditions in the spring, you may be concerned about grain moisture content ― especially when choosing to let the field air dry. Allowing corn acres to field dry can be costly, depending on the weather. Harvesting at lower moistures can reduce drying costs, but often increases harvest losses from dropped ears, stalk lodging and header loss. A good balance between minimizing harvest losses and keeping grain-drying costs down is to start harvesting at higher levels, such as 23–25% grain moisture.

Below are 3 concerns Golden Harvest agronomists advise farmers to consider ahead of harvest:​​

• Stalk lodging: As your stalks dry out, they become weaker and may topple over or break below the ear. According to Purdue University, annual yield losses due to stalk lodging in the U.S. range between 5% and 25%. Stalk lodging not only affects your yield quantity, but your grain quality too. Consider harvesting your fields affected by stalk issues first to minimize the impact on your yields.
• Ear drop: In addition to stalk lodging, ear drop is a common problem farmers face prior to harvest. As the stalk loses moisture, it becomes brittle. During this time, there is a high potential for ears to break off the plant and fall to the ground. If your crop is at risk, consider harvesting quickly and operating the corn head higher than normal to reduce ear loss during harvest.
• Ear mold: As husks open for the plant to dry, wet conditions can sneak in. Heavy rain during drydown may cause corn kernels to re-sprout, which can be disastrous for your grain quality. Ear molds can continue to cause damage until harvest. Be sure to carefully consider drydown time to reduce your exposure to wet conditions and the potential for ear mold in your corn.
  
  

Forecasting days to harvest

Predict days to harvest in 3 steps:

1. Calculate how many days remain to kernel maturity.
2. Determine days required to reach desirable moisture after maturity. 
3. Add days to maturity and days to desired moisture for total days to harvest.

Based on which stage the crop is in, the number of days to maturity can be estimated (see table below for reference).

A common question that growers ask is when their corn will be ready for harvest. As a rule of thumb, it takes roughly 10 days to advance to the next reproductive stage. From silking, it typically takes 55–60 days to reach maturity. At physiological maturity, a black layer forms at the base of the kernel, indicating it has achieved maximum dry matter accumulation. The kernel black layer moisture content can range from 25–40%, but often averages around 30%.

         
    The image above shows a mature black layer, found at the kernel base.

Once a maturity timeline is calculated, the second step is to estimate days to desired grain moisture. The corn drydown rate is highly dependent on temperature and growing degree units (GDUs) available after maturity. Warm temperatures in late August and September favor earlier maturing hybrids with faster drydown rates, because the black layer forms sooner. Drydown rates can range from ½–1% per day in September. Corn maturing later due to delayed-planting or full-season RMs will typically dry slower, due to fewer GDUs available after reaching maturity. October drydown can easily reduce to ¼–½ point per day or less, although higher rates can be seen in years with favorable weather.

To help illustrate the year-over-year variability in drydown rates by geography, the following charts show historical weather data combined with an estimated 20 GDUs required per point moisture drop:   

     

Drydown rate influencers

Product drydown rates are influenced by many product characteristics, such as how tight husk leaves are and pericarp, or thickness, of the seed. With higher temperatures, it’s easy to underestimate grain drying rates. Environmental stresses, such as fall frost or severe drought, can cause plants to die early and form a premature black layer, leading to excessively slow grain drydown. Cool weather or delayed planting during the growing season resulting in reduced GDUs can also delay the normal maturation, causing higher moisture content.  

Harvest order

Monitor fields for issues that could affect your harvest order. Potential stalk and root lodging, disease pressure and moisture content could adjust your harvest plan. Stalk cannibalization and related lodging issues can be due to nitrogen loss from excessive, early season rainfall. Closely monitor fields to help minimize lodging and harvest loss.

The pinch and push tests are 2 methods to determine stalk integrity. For the pinch test, squeeze the second or third internode above the ground. If it collapses, stalk quality is weak. Alternatively, push a corn stalk at the ear to approximately a 45° angle. If the stalk returns to its upright position, the quality is strong. Use either method on 10 consecutive corn plants in several locations to see if the entire field, or just an isolated area, is affected. If more than 10% of the stalks tested show poor stalk quality or lodge at the root, plan to harvest the field early.

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