Wheel traffic is a necessity for the production of alfalfa. Regardless of the harvest method (green chopped or dry bales) producers must make decisions of when and how to drive equipment on alfalfa fields. Early studies demonstrated that as much as 70% of the field area could be driven upon for each cutting/harvest performed. Over time, the size of equipment to cut, rake, bale and remove hay from the field has change dramatically. As well, the green chop equipment for baleage, silage or alfalfa mills has also changed. Generally, equipment is larger today and thus carrying a greater gross axle weight both while empty and at harvest load capacities.
Soil compaction is a common concern for alfalfa producers and can be linked to increased disease incidence contributing to plant damage and yield reduction. However, the wheel traffic itself can also cause physical damage to the alfalfa crown or regrowth shoot. Traffic during cutting does not have the same harmful effect of later traffic from the baling and bale removal. Compaction due to the multiple trips is a factor, but breaking shoots that may have started to regrow after cutting, has a depleting effect on stored root carbohydrates (Sheesley et. al., 1982).
The reduction in alfalfa plant populations and the resulting decrease in yield and hay quality is a major reason for growers to decide to remove an alfalfa field from production. Heavy wheel traffic in which plants are trafficked multiple times per cutting can increase crown damage (leading to entry points for pathogens) and decrease shoot and root growth. Repeated damage to new shoots from multiple traffic passes can be attributed to a change in carbohydrate partitioning in which greater root reserves are needed for plant recovery (Rachel, 1984).
The amount of wheel traffic yield reduction is likely going to vary from field to field and cutting to cutting depending on plant and soil conditions. Yield loss to the next cutting is greater as the traffic occurs later after mowing. The yield loss has generally been 4 to 6% per day after mowing (e.g. traffic 5 days after mowing creates a yield loss of about 22%) (Undersander, 2010).
Dan Undersander, University of Wisconsin, offers ways to reduce alfalfa yield and stand losses.
1. Plant alfalfa varieties more tolerant to wheel traffic
2. Use small tractors when possible (to reduce soil compaction).
3. Drive over the field as soon after cutting as possible
a. Raking at 24 hours causes less damage than raking at 48 hours after cutting
b. Merge swaths into large windrows so harvesting equipment has less driving on the field
c. Making haylage at 1 to 2 days after cutting causes less yield loss than making hay at 3 to 5 days after cutting.
4. Avoid unnecessary trips across the field when harvesting
a. Get full wagons/trucks off the field with as little driving as possible
b. If bales are dropped and collected – can this be done with less driving?
c. Do not drive on alfalfa field when harvesting crop of adjacent field.
5. Consider using larger harvesting equipment (there is some question about this because while less area is affected by wheel traffic, the affected area has greater weight applied to it). This could be another benefit of contract harvesting
6. Using duals on harvesting machinery is not recommended unless necessary to avoid ruts.
Sheesley, W.R., D.W. Grimes, W.D. McClellan, C.G. Summers, V.Marble. 1982. The Influence on Wheel Traffic on Yield and Stand Longevity in Alfalfa. P. 42-46. 1982 California Alfalfa Symposium Proceedings, UC Cooperative Extension, university of California, Davis CA 95616.
Undersander, D. 2010. Effect of Wheel Traffic on Alfalfa Yield. University of Wisconsin-Extension Cooperative Extension. http://www.uwex.edu/ces/forage/articles.htm Madison, WI 53706.