17.1 Harvest Timing Determination & Field Grain Loss Assessment

Harvest Timing Determination

Determining the exact time to harvest is a complex process that forces producers to weigh various agronomic, economic, and meteorological factors. Harvest timing plays a critical role in preserving crop quality, maximizing yield, and minimizing post-harvest expenses. An earlier harvest typically results in higher grain moisture, which reduces field losses but increases drying costs. Conversely, a later harvest relies on natural field dry-down, saving energy costs but exposing the crop to weather events, stalk lodging, ear drop, and wildlife damage.

Understanding crop maturation is essential. For corn, physiological maturity is indicated by the formation of the "black layer" at the tip of the kernel. At this stage, dry matter accumulation is complete, and the moisture content is typically around 30% to 35%. Although the grain is mature, it is too wet for safe storage and requires either field drying or mechanical drying. Soybeans reach physiological maturity when the pods turn their mature color and seeds have fully filled the cavity; at this point, moisture is often near 50%, but it drops rapidly under favorable weather conditions. Wheat is physiologically mature at the hard dough stage, though it must dry down to about 13% to 15% for optimal harvest.

The decision of when to harvest involves calculating the trade-off between the cost of artificial drying and the potential cost of field losses. When corn is harvested at 25% moisture, mechanical damage is minimized, but drying costs are substantial. Harvesting at 15% moisture eliminates drying costs but dramatically increases the risk of stalk breakage and ear drop due to autumn storms or insect damage. Agronomists often recommend harvesting corn when moisture levels fall between 20% and 25% to optimize the balance between drying expenses and field losses.

Field Grain Loss Assessment

Regardless of the harvest timing, some degree of grain loss is inevitable during the harvesting process. These losses are categorized into pre-harvest losses and machine losses. Pre-harvest losses occur before the combine even enters the field and are typically caused by weather, insects, diseases, or wildlife. Machine losses occur during the gathering, threshing, and separating processes inside the combine.

Assessing field grain loss is a crucial skill for a Certified Crop Adviser, as it directly impacts a farmer's profitability. To accurately measure field losses, one must count the number of kernels or seeds left on the ground in a specific area and convert that count into a bushel-per-acre equivalent. The rule of thumb varies by crop, but for corn, the standard conversion is that two kernels per square foot equates to approximately one bushel per acre of loss.

To conduct a proper loss assessment for corn, the evaluator should construct a one-square-foot frame, toss it randomly in the harvested field, and count all the kernels within the frame. This process should be repeated in several locations to obtain a representative average. In addition to loose kernels, any unharvested ears must also be factored into the assessment. An average-sized corn ear weighs about 0.5 to 0.75 pounds; finding one ear in 1/100th of an acre (a 14.5-foot length of a 36-inch row) roughly equals a one bushel per acre loss.

For soybeans, the threshold is different due to seed size and weight. Approximately four soybean seeds per square foot equals a one bushel per acre loss. Wheat assessment uses a standard of about 18 to 20 kernels per square foot to equal one bushel per acre.

Machine losses are further divided into header losses (shattering, ear drop, or unsevered stalks) and threshing/separation losses (grain passing completely through the machine and exiting the rear). Most harvest losses—often up to 80%—occur at the header. Header losses can be mitigated by matching the ground speed of the combine to the speed of the gathering chains or reel, adjusting the header height, and ensuring cutter bars are sharp. Threshing losses require adjustments to the cylinder or rotor speed and the concave clearance. If grain is being cracked or damaged, the cylinder speed may be too high, or the concave clearance too tight. Conversely, if unthreshed grain is passing through, the cylinder speed might need increasing, or the concave clearance tightening.

Proper calibration and continuous monitoring of the combine's performance are essential. Modern combines are equipped with loss monitors, but these sensors must be ground-truthed manually using the square-foot counting method. A total harvest loss of 1% to 2% is generally considered acceptable and unavoidable; however, losses exceeding 3% indicate that combine adjustments or a change in harvest timing are urgently needed.

By mastering harvest timing and field loss assessment, agronomists can provide immense value, helping producers secure the maximum possible yield from their fields while maintaining grain quality.

To further elaborate on machine settings, the fan speed and chaffer/sieve openings dictate the cleanliness of the grain sample and the amount of grain blown out the back. If the fan speed is too high, lightweight, yet perfectly viable, grain is blown out with the chaff, increasing separation loss. If the fan speed is too low, excessive foreign material ends up in the grain tank, which can lead to dockage at the elevator and create localized hot spots during storage. Striking the perfect balance requires frequent checking behind the combine, especially as conditions change throughout the day. Crop moisture, temperature, and humidity fluctuate from morning to evening, necessitating corresponding adjustments to combine settings to maintain optimal performance and minimal loss.

Harvest Parameters & Grain Loss Assessment Summary