Modern combines can maintain grain quality achieved in the field when set properly and checked frequently. However, poorly adjusted combines can have a devastating effect on grain quality, especially if the grain is wet or light. Cracked or broken kernels are the usual outcome of a poorly adjusted machine. These damaged kernels often incur further breakage with subsequent handling. By storage time, an abundance of broken kernels and “fines” can restrict air flow and provide a ready substrate for insects and diseases.
Grain drying is critical to maintaining quality. High-temperature drying results in stress cracks in the kernel, especially if corn is cooled rapidly after heating in the dryer. With subsequent handling, stress-fractured corn begins to break, resulting in storage problems in the bin and lower value for end uses such as dry milling or wet milling.
The goal of proper combine settings is to achieve a smooth, even flow of crop material moving through the machine. The combine should run nearly full to minimize impact on the grain. A near-empty machine, on the other hand, leads to multiple contacts of the machine and the grain, which increases breakage.
Ground and Snapping Roll Speed: The ground speed depends on the condition of the crop, but should generally be as fast as possible without plugging the head or threshing mechanism. Snapping rolls should be set relative to ground speed. When set too fast, snapping rolls increase the impact of the ear on the stripper plates. This causes kernels to be shelled and lost, increases breakage of ear butt kernels and results in ear bounce.
Cylinder/Rotor and Concave: The cylinder or rotor is designed to thresh corn from the cob. It is no surprise then, that cylinder/rotor speed is the leading cause of grain damage by the combine. In 1 study, increasing the cylinder speed from 300 to 600 rpm increased kernel damage from below 5% to over 30%. However, if threshing is too gentle, unshelled kernels can be lost with the cobs.
Growers should use the lowest possible cylinder/rotor speed that will shell the grain within acceptable loss levels (1% in good-standing fields). To reduce unthreshed losses without increasing grain damage, try decreasing the concave clearance before increasing cylinder/rotor speed. If this does not achieve satisfactory threshing, then begin to increase cylinder/rotor speed as required.
Cylinder rasp bars can damage kernels, especially when new. Manufacturers suggest to break in the rasp bars for 50 hours in a crop where quality is not as critical. This break-in period will allow sharp, square edges to be rounded and smoothed. Avoid using the chrome-plated rasp bars that are much harder, retain their edges longer, and tend to chip and develop sharp edges that damage grain.
Augers: Several augers convey the grain through the combine, including the cross auger, the clean grain auger, the tailings augur and the unloading auger. Badly worn augers become sharp and can shear grain. Augers running at less than capacity can break and damage grain. To reduce grain damage from augers, replace badly worn auger flighting. Idle engine until unloading auger is filled, then increase throttle. Keep internal augers as full as possible by running the combine at capacity.
Separation and Cleaning: After threshing, the grain is separated from non-grain crop material by the chaffer and shoe sieves and the cleaning fan. Lighter chaff is blown out the back of the combine, while heavier unthreshed cob segments are returned to the thresher by the tailings system. Screens allow fine grain particles and foreign matter to be removed in the cleaning process.
The goal of separation and cleaning is to achieve a clean, high-quality end product while minimizing grain losses. To accomplish this, sieve and fan settings are critical. Begin with manufacturer suggested settings and check and adjust frequently. Crop conditions, including non-grain crop moisture, can change rapidly during autumn days. Monitor losses behind the combine and grain quality in the grain tank throughout the day.