Concerns about serious compaction caused by the use of heavy farm machinery weighed on the mind of James Andrews when he put together the accompanying page for Farmers Weekly magazine in the United Kingdom.

Looking strictly at equipment weights, he compared 10 of Great Britain’s heaviest farm machines with the Ford 5000 tractor that was a workhorse on British farms back in the early 1970s. This tractor pumped out 75 hp with its four-cylinder engine and 5,360-pound weight.

The accompanying chart shows the equivalent number of Ford 5000 tractors that it would take to equal the weight of some of today’s heavy farming machines. The “heavyweight” title goes to the Holmer T4-40 beet harvester (#10 on the list) with an empty weight of 32 tons and an earth-crushing 62 tons when fully loaded with beets. That huge weight works out to the same total weight as 23 Ford 5000 tractors.

Axle Loads Critical

Having done considerable research on compaction concerns, Randall Reeder says any compaction and weight comparisons between these heavy machines with a 1975 Ford tractor requires seeking answers to a couple of key questions.

The Ohio State University ag engineer says you need to know the number of axles on each machine, with an emphasis on the heaviest-weighted axle. You also need to determine the tire pressure or the average pressure under a rubber track.

Reeder says the older Ford 5000 tractor carried about 2 tons on the rear axle. This could be increased to almost 2.7 tons when pulling a heavy implement that takes most of the weight off the front axle.

“Today’s 4WD tractors have roughly 50% of their load on each axle,” Reeder says. “So a 4WD tractor weighing over 18 tons would have over 10 tons on the heaviest axle. A combine is usually carrying 80-85% of its weight on the main axle.”

Reeder points out that the self-propelled sprayer and the beet harvester shown in the accompanying chart each have three axles. This means the beet harvester would be carrying over 20 tons per axle and roughly 9 tons per axle for the sprayer.

Heavier Load Worries

Some of the earlier research done on soil compaction at Ohio State is being updated with a look at much higher axle loads that are seen with today’s larger combines and huge-capacity grain carts.

While growers have long felt they could correct 90% of compaction concerns with tillage, that’s not true with compaction going down 2 feet or more caused by heavier equipment, maintains Andrew Klopfenstein, an Ohio State ag engineer.

To demonstrate today’s more serious compaction concerns with heavier equipment, Klopfenstein studied the yield impact from running a 2,000-bushel grain cart in the field. The cart weighed around 200,000 pounds when fully loaded with corn, with an approximate weight of 30 tons per axle.

Running this fully loaded grain cart across a field resulted in compaction as deep as 34 inches under both normal and wet soil conditions. These serious compaction concerns could lead to yield losses of over 50% for the following year’s corn crop.

Klopfenstein says shallow traffic compaction with continuous no-till should be able to correct itself in about 5 years, providing there is no additional damage to the soil. On the other hand, deep subsoil compaction can take up to 10 years to eliminate, and normally can’t be done with deep tillage.

Rubber Track Options

When looking at heavy farm machinery on rubber tracks, Reeder says you need to calculate the pounds per square inch (psi) of surface contact. He reminds growers that this number is not equivalent to the amount of tire inflation pressure.

In a research project conducted in 1994 at Ohio State, Reeder says a rubber track machine with 10 psi resulted in similar compaction with machines running on tires inflated to 25-30 psi.

Based on this research, Reeder says that while an axle load of 10 tons can cause compaction, there likely won’t be any serious impact on corn or soybean yields after a year or two. However, a 20-ton axle load could cause deep compaction, which could trim yields by 5-10% for many years.

Tire Pressure Critical

Reeder says some of today’s bigger farm tires can be used in the field with pressures as low as 6 psi. However, some of the British machines shown in the accompanying chart may not have an inflation option due to overall equipment design restrictions.

While typical tire inflation pressures for these machines might be listed on the manufacturers’ websites, Reeder estimates the tire pressures would probably need to be at least 30 psi to carry the heavy weights of both the beet harvester and slurry tanker. With space and design limitations on some of these machines, he says tracks would have an advantage.