Threshing

Italic numbers in parentheses refer to Literature Cited.

Modern agronomic threshing machines are called combines because they are a combination of a feeding section (consisting of reel, divider, cutterbar, and feeding mechanism) and a threshing section. In harvesting seeds of wildland species, we rarely have the opportunity to use a combine. The processes inherent in the threshing section of a combine are worthwhile to review because these same processes are necessary in hand or mechanical threshing.

Mechanical threshers consist of a threshing cylinder and fixed concave bars. The turning cylinder bars rub the herbage (straw) and seeds against the fixed concave bars. This action breaks the seeds or fruit bases from the inflorescences and often removes the glumes, bracts, pods, or fruits covering the seeds. The adjustment of the clearance between cylinder and concave bars is of the utmost importance in mechanical threshing. A general rule is to have a clearance of one and one-half times the thickness of long seeds or one and one-half times the diameter of round seeds (9).

There are versatile small plot threshers that are adapted to threshing small lots of seeds of wildland species (8). Special rubber-coated concave bars are available for these small threshers to reduce damage to fragile seeds.

Hand Threshing

Hand threshing duplicates the action of concave bars in the mechanical threshing cylinders. The object is to rub the seeds to break loose the inflorescences or fruit covering.

A simple threshing method is to rub the collected material against a coarse screen. A more efficient method is to cover two wooden paddles with rough rubber matting (figure 1). Rubbing the collected material between the paddles or on top of a coarse screen threshes the seeds. If a large volume of material is to be threshed, this is a very time-and energy-consuming operation. A simple threshing cylinder can be made by cutting a tire innertube. The collected material is poured into the cut end of the tube, and the tube is rubbed by hand or foot until the seed is threshed clean. Another simple threshing device involves the use of two clay bricks or the halves of a single brick. This technique is especially applicable to seeds borne in capsules or nonsplitting pods. PI-ace the fruits, capsules, and pods between the bricks and press with a grinding action. Leaving a few stems in the sample with the fruits insures ample spacing so the seeds are not crushed.

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