Managing your combine

Matthew Digman, Biological Systems Engineering, University of Wisconsin - Madison

If this is your first wheat harvest or if you are a veteran farmer, you’ll find it profitable to take a closer look at how you are managing your combine. There are five basic functions of your combine: cutting and feeding, threshing, separating, cleaning, and handling. Let’s see how to optimize settings for each of these systems to minimize grain loss and damage while maximizing productivity.

The first system, cutting and feeding, necessitates a look at your grain header. The goal should be to harvest just enough straw in each pass, cutting the plant a little below the head, so the grain feeds evenly into the combine. This will maximize capacity and, therefore, the harvesting efficiency of the combine, ensuring timely harvest of the crop. The primary reason for minimizing straw in the combine is to maximize the machine’s capacity to collect grain rather than straw. By cutting less straw, you allow your machine to cover more acres in less time. Your combine, however, may have plenty of capacity, and you might benefit from harvesting grain and windrowing straw in a single pass if you plan to bale straw afterwards. In this case, the suggestions here are still valid, but managing your grain losses will be more challenging.

Let’s get back to the header. The reel’s job is to position the head of the wheat just behind the cutterbar so that when the plant is cut it falls gently onto the platform. To obtain this goal, the outside tip of the reel must be moving slightly faster than the ground speed of the combine. Check in your operator’s manual for suggested ground and reel speeds. Dull sickle knives and improper clearance between the sickle and guard not only cause greater shatter losses but also reduce capacity. So, checking the condition of the sickle and replacing it, if necessary, will minimize shatter loss without limiting ground speed.

Now the grain and straw are moving along the feeder-house to either the cylinder or rotor and concave. The rotor/cylinder adjustment is the next line of defense against grain loss. Proper clearance and speed will ensure maximum threshing while minimizing grain damage. Your operator’s manual will outline specific settings for your machine for wheat. On conventional machines, be sure to check concave clearance on either side of the cylinder to ensure adjustment is even across the cylinder. It is also prudent to physically measure this clearance, as wear and maladjustment can lead to erroneous indicator readouts. In general, your goal will be to operate the machine with minimal rotor/cylinder speed and maximum concave clearance while still threshing the grain. Under-threshing will mean trouble for downstream separation and cleaning processes while over-threshing will lead to grain damage. A good indicator of over-threshing is excess size-reduction of the straw. Be sure to also look for damaged grain on the ground rather than in the grain tank as the grain’s small size will cause it to be blown over the cleaning shoe.

Now that the grain has been threshed, we need to separate it from the straw. In a conventional combine this is accomplished through the straw walker while rotary combines rely on the centrifugal force generated by the rotor. Over loading either machine limits its ability to separate the grain. Conventional combines are particularly sensitive to overloading as the straw walkers become overwhelmed with straw.

Next, we are on to cleaning. The cleaning system consists of the fan, the top chaffer sieve where the gross cleaning occurs, and the shoe sieve where final cleaning is done. Losses in the cleaning shoe can occur from several factors, one of which is the over-threshing mentioned in the previous section. Small pieces of over-threshed straw can fall onto the chaffer sieve and overload it, preventing grain from falling onto the shoe sieve and consequently blocking air flow.

Another loss factor involves the chaffer openings and the airflow that ensure straw is passed over while grain falls to the shoe sieve. Once again, your operator’s manual should specify both fan and opening settings for wheat. It is important to note that openings for both the chaffer and shoe sieve are measured perpendicular to the louvers. Airflow that is too high and/or openings that are too small can cause a lot of material to end up in tailings, which limits combine capacity, or can cause grain to be ejected from the rear of the combine and be lost. Airflow that is too low and/or chaffer openings that are too large can overwhelm the shoe sieve, resulting in grain loss. Airflow is affected by changing chaffer openings, so the two should be adjusted together.

Finally, we are ready to adjust the shoe sieve. Sieve openings should be such that grain can fall through, but the straw cannot. If you see straw in the grain tank, the sieve is open too far, or if you recall from above, you may have an over-threshing situation. On the other hand, it is important to make sure the openings are large enough so that grain can fall through and isn’t being recycled as tailings.

Crop conditions change throughout the day and from field to field, so it’s important to periodically check combine performance. Grain losses can occur from the shattering of standing grain and the shattering at the header, in addition to the losses associated with threshing, separating and cleaning. Loss assessments can easily be made before harvest, after cutting with the header, and after threshing, separating and cleaning.

The first two losses, pre-harvest and header, can be estimated using a quick rule of thumb: twenty wheat kernels in one square foot of ground means one bushel per acre loss. So, if you can fashion a one-foot square or if you can estimate a foot with your size 13 boots, you can get a pretty good idea of your grain loss. Also, be sure to take the measurements in a few places in case you unluckily set your square in an area that’s not representative of the rest of the field.

Estimating your threshing, separating, and cleaning losses is a bit more challenging. First, you have to subtract out your field and header losses (unless you are just looking for a total loss value). Second, you have to consider that your combine may be consolidating up to a 35-foot swath into a narrow area, depending on how you’re managing your straw. If your machine is spreading the straw the full width of the combine then you can use the previous rule of thumb. Otherwise, a group at Kansas State provides us a correction factor. They suggest that for machines without a spreader you’d look for 80 kernels per square foot; for a set up with a bat type spreader, 65 kernels; and for a chaff spreader, 25 kernels. This assumes a header to separator width ratio of 4:1. Rotary combines may push this ratio to 5:1 or higher. Under this scenario your loss counts would increase to 100, 80 and 30 for no spreader, bat spreader and chaff spreader, respectively. When using these adjustment factors be sure to take your loss measurements in the area where the straw was spread.

Harvesting losses (remember to subtract pre-harvest or field losses) of about two percent of total grain yield are acceptable, but all loss programs must be weighed against their timeliness costs. Continual machine adjustment and slowing harvest rate to ensure every last grain ends up in the grain tank can be detrimental if test weight is decreasing. Lost test weight is only the beginning of lost revenue. The crop becoming too dry can lead to shattering while standing or at the header. Furthermore, rehydration of grain can result in grain damage. For more information or resources, please visit the University of Wisconsin Cooperative Extension Team Grain website at http://www.uwex.edu/ces/ag/teams/grains.