With mechanization, the equipment size has increased. However, the need to drive over the field multiple times for each harvest continues.

Early studies demonstrated that as much as 70 percent of the field area could be driven upon for each cutting. Equipment traffic occurs at various times during the season.

Traffic during cutting does not have the same harmful effect of later traffic from baling and bale removal. The resulting reduction in yield from the traffic areas has been attributed mainly to new shoot and regrowth damage. Soil compaction and increased disease incidence are also contributing factors in plant damage and yield reduction.

In the late 1970s research at the University of Nevada, Reno was conducted to measure the effects of wheel traffic damage. The test was done on irrigated alfalfa in the arid growing region of western Nevada.

In that four-year study, 10 varieties were treated with no traffic or a traffic treatment of a fully loaded bale wagon driven over the entire plot area at either one or seven days after cutting.

Yield reduction was similar for all 10 varieties, but there was a significant difference in yield among the varieties. Plant density was not decreased nor was crown rot increased by the traffic treatments.

“Because there was no reduction in yield of the first harvest in 1980, it was concluded that soil compaction from wheel traffic of the previous three years was not the cause of the reduction in yield for subsequent harvests.” The cause of the yield reduction was damage to new shoots or regrowth, the damage being greater when traffic was imposed at seven days after cutting.

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 and decrease shoot and root growth.

In tests conducted in the San Joaquin Valley of California on irrigated sandy loam soils, heavy traffic increased soil density and decreased water infiltration to a depth of 90 to 120 centimeters.

Repeated damage to new shoots from multiple traffic passes attributed to a change in carbohydrate partitioning in which greater root reserves are needed for plant recovery. These factors combined in the result of reduced alfalfa plant growth and vigor.

Stand decline under warm, humid growing conditions is often associated with the crown rot complex, the most common causal organisms being Fusarium, Phoma and Rhizoctonia species. Damage from these diseases is most severe after some sort of damage to the crown such as mechanical injury from harvest equipment.

Research has shown there are varietal differences in yield and persistence when alfalfa is subjected to normal grower harvest traffic conditions.

Advanced “grazing and traffic resistant” breeding lines have displayed increased Phoma crown rot resistance due to the selection of these lines under traffic conditions. It is also believed the germplasms selected under traffic conditions display increased food reserves in the roots.

This germplasm selection procedure was not used in the development of older alfalfa varieties, such as those evaluated by Jensen in the late 1970s.

Dealing with the problem

Reduce traffic
It only stands to reason that reducing the amount of traffic will reduce the amount of damage done by traffic. Traffic occurring at cutting or within a few days is less damaging to shoots than traffic during the later harvest operations. Wheel traffic may overlap, either from the same operation or from subsequent operations. There is a normal field pattern with several different levels of traffic damage occurring.

Wheel traffic yield reduction can be light, normally caused by cutting equipment. Moderate and heavy traffic patterns occur from operations done later in the harvest operation.

Moderate traffic can be from the tractor and rake, and heavy traffic can be from the tractor, baler and bale wagon. When any of these tracks overlap, the total area is decreased, but the yield reduction is increased.

Compaction due to multiple trips is a factor, but breaking shoots that may have started to grow for the second time after cutting has a depleting effect on stored root carbohydrates.

Modifying the way tractors and implements track can effectively reduce the total amount of traffic area in an alfalfa field. These harvest equipment modifications were tested in grower fields alongside the conventional equipment configuration.

Two separate studies were not able to measure a significant increase in yield due to the equipment modifications. However, similar type modifications are commonly used in the Imperial Valley of California where alfalfa is grown on 40-inch beds and the wheel traffic is concentrated in the furrows.

Bedded alfalfa is used to drain fields better, eliminating standing water and reducing scald. Limiting traffic to furrow in this bedded alfalfa system did not adversely affect soil compaction and permeability in the shrinking-swelling fine-textured soil common to the Imperial Valley.

Green chopping instead of conventional baling does have the potential to reduce damage and increase yield. A field-scale experiment in the San Joaquin Valley of California measured a 12 percent increase in yield from green chopping over conventional hay making. This corresponds with the lower level of yield reduction in traffic trials where the traffic treatment is applied at two days compared to that at five days.

Stay off wet soils
Harvesting forage grown under non-irrigated, rainfall conditions can be difficult due to untimely rain at or near harvest time. Growers are often forced to drive on soil not as dry as desired in order to get the hay off the field.

Soil compaction can be a problem under these conditions, especially in heavy soils. When soil moisture in the top 3 to 6 inches is near field capacity, the potential for soil compaction increases as clay content increases and soil organic matter decreases. These heavy soils hold more water than the sandy, lighter soils and do not drain excess water as quickly.

This problem is not as prevalent in the West where summer rain is unusual and soil moisture is provided by irrigation. With these controlled conditions, heavy soils are allowed to dry and may even crack due to the shrinking and swelling characteristic. In the West, it is the sandy loam soils that have a higher tendency to compact.

Use traffic-resistant varieties
Alfalfa variety wheel traffic trials with a number of varieties selected for grazing and wheel traffic tolerance have been conducted in seven midwestern states and California.

Traffic treatments (100 hp tractor driven over 100 percent of the traffic plots) were applied at two and five days after cutting and the yields were compared to plots of the same variety with no wheel traffic. The data show there is a wide variation in variety tolerance to wheel traffic.

Don’t tissue sample in traffic lanes
Alfalfa tissue sampling is used to evaluate the nutrient status of the crop. The standard procedure for sampling has been to sample at 1/10th bloom. Although this procedure is under revision, it is important samples are uniform in maturity. Wheel traffic affects the maturity of the crop.

Samples of mixed maturity plants will result in inaccurate tissue tests. The best situation for tissue sampling is to not sample the obvious wheel traffic areas delayed in maturity by three to five days. Time lag can make a tremendous difference in the sample results.  FG

References omitted due to space but are available upon request by emailing editor@progressiveforage.com.

—Excerpts from National Alfalfa Symposium Proceedings

Jerry Schmierer
Farm Advisor
University of California