25 July 1997

Going for yield map precision…

COMBINE operators can do much to influence yield readings and so improve the accuracy of the yield maps they produce, says Massey Fergusons Mark Moore.

"A good operator can improve it enormously," he says. "A few minutes a day checking can make all the difference."

A key point is to keep the cutterbar as full as possible, because the computer assumes a constant width of cut to calculate area. "Good combine drivers can cut 95-100% of header width, so most set it to this."

However, this can be a problem in laid oilseed rape. Unless an accurate estimate of average width of cut can be made, it is better not to yield map such crops, advises Mr Moore.

Cutting in lands can also result in narrow strips being left. Smoothing effects when the software produces contour maps can reduce the impact of that to about 1.2% across a field, although spot errors can climb to 6.8%.

Reprogramming width for the narrow cut can help, but cutting up and down stops the problem.

When turning at headlands the cutterbar must be lifted above 50cm or the computer will continue recording, producing lower yields in that area.

Combines also take time to fill when setting into the crop. That, plus the time taken from lowering the cutterbar below 50cm to entering the crop, is called lead time. It must be allowed for – record too soon and a low yield reading will be assumed, since grain flow will not have reached true field value.

Lag time – the time between drop off in grain flow and lifting the cutterbar when leaving work must also be allowed for. Errors of 1% across a field are common, and spot errors can reach 10%.

To overcome both problems drivers should time crop entries and exits. On MF combines a graph of yield against time simplifies that – operators of other makes may need a stopwatch. Each driver should set his own lead and lag times. "No two drivers operate in the same way," says Mr Moore.

Time taken for the grains to travel from cutterbar to meter – delay for yield – must be measured if spot yield readings are to relate to the correct part of the field. Wheat may take 15 seconds, pulses longer, oilseeds shorter. Extremes of moisture can also alter the time.

Wrong settings can give big errors. For example, if the setting is too fast and the combine speeds up, so covering a bigger area, the resulting higher grain flow will be recorded from the part of the field which was cut slowly. The result is spot yield readings which are too high.

When changing crops, or at extremes of moisture, drive slowly, then quickly, advises Mr Moore. "The yield reading should stay constant. Check three or four times to allow for natural variations in yield."

Also check the clean grains elevator, where MF yield meters emit radiation to assess the flow of grain. Trash build up or wear can alter the zero point value, producing a false reading. Leaving the combine ignition switched on for at least five minutes is all that is needed for a reset.

"Tea breaks, or when fuelling up, are ideal," says Mr Moore. Once a day is enough, unless cutting sticky crops like damp rape.

Keeping the cutter-bar full is important if accurate yield maps are to be achieved.Inset:Mark Moore.


&#8226 Reset yield meter daily.

&#8226 Keep cutterbar above 50cm when not harvesting.

&#8226 Keep cutterbar full.

&#8226 Set lead, lag and delay for yield times for different crops and operators.

&#8226 Lower and lift cutterbar at same point on headland.

&#8226 Avoid stop/starts when cutting. Cut round obstacles and keep going – clear up later with equipment switched off.