Forager weighing systems – the options

In many ways, the development of the yield monitoring system for forage harvesters is a logical development – for the first time it allows the contractor and his customers to know just how much crop is being harvested and which part of a field is producing it.

Deciding just how the crop could be measured accurately has clearly taxed the minds of engineers for some time, but a system has now been developed that is able to provide reasonably accurate results.

No surprise to learn that the system is used by most self-propelled forage harvester manufacturers, emphasising that, in many ways, the general design of such machines is very similar – a pick-up, feed rollers, chopping unit, fan and chute.

That said, it is worth noting there are only two areas of a forage harvester that change when the volume of crop entering the unit increases or decreases. One is the engine loading and the other is the distance between the rollers that feed the crop to the chopper unit.

While it would be convenient to be able to assess the volume of crop entering a forager by monitoring the demands made on the engine, it is a fact that the engine is also asked to power the vehicle across a field – uphill, down hill, faster, slower – and it would be difficult, if not impossible to provide an accurate assessment of incoming volume.









On-board weighing systems are appearing on fertiliser spreaders, balers, muckspreaders, loaders and foragers. 

Instead, the feed rollers and the gap created between them by the incoming crop provide the information from which the volume can be derived. The Mass-Flow sensor on a John Deere forage harvester, for example, is a plunger rod connected to the upper front feed roll. As the thickness of grass flowing between the feed rolls changes, the plunger lengthens or shortens – a movement that is converted into electronic pulses. These are then used by the control system to denote changes in volume intake.

To be really useful information though, crop volume/hectare, needs to be converted to tonnes/hectare and, to be more precise, to dry matter weight/hectare. For this, the system first needs to be calibrated by weighing a sample trailer load and then adjusting the system so that the volume figure can be interpreted as a weight.

To help ensure accuracy, the trailer being used needs to be empty and sample loads should not be taken when starting a field when throughputs can be low and swath widths not representative of the field as a whole.

The moisture content of the grass is gleaned from a monitor in the chute where sensors on either side of the spout measure electrical conductivity and temperature of the chopped crop as it passes through or, alternatively, it is assessed by using a system based on light beam reflection analysis.

Knowing the moisture content then allows the dry weight of the crop to be calculated in respect of the volume passing though the feed rollers.

All required calculations are performed by an on-board computer that also provides displays for operators regarding tonnages, fields harvested and ultimately, with the aid of GPS, yield maps.


Claas has four foragers using yield monitors this year to assess the effectiviness of the system.

Producing accurate results with the yield monitor also requires some help from the operator – it is recommended that throughput is as high as possible, grass windrows are evenly formed and ground speed maintained at a constant rate. For the system to work there needs to be access to a weighbridge or a set of weigh pads.

Although the technology is still in its infancy, forage harvester manufacturers are keeping a close eye on the performance of the yield monitoring systems being used by contractors. Claas, for example, reports it has just four units working this year and says it intends to assess their performance before making them available country-wide.

Depending on the degree of information required – basic tonnage or yield mapping – such systems can cost between £10,000 and £20,000 and it will be interesting to see to what extent they are adopted by contractors.