Written by Dan Wiersma, Alfalfa Business Manager

Summary of Findings

• Alfalfa forage harvesting equipment can damage alfalfa stems, crowns, and roots resulting in yield reduction, soil compaction, and reduced stand life.
• Alfalfa varieties with excellent disease resistance and high yield potential are best able to tolerate the stress of persistent wheel traffic.
• Plant damage and yield loss can be reduced with timely harvest after cutting, avoiding wet soil conditions, proper equipment sizing, and controlled traffic patterns.

Introduction

Every alfalfa forage harvest operation requires multiple trips over a field with tractors, mergers, choppers or balers, and wagons or trucks. This normal harvest-time traffic can injure plant crowns, crush newly emerged alfalfa buds and shoots, and easily cause soil compaction. Physical plant injury creates an opportunity for disease organisms to enter alfalfa crowns and roots. Damaged crowns and shoots can result in a reduction of root and crown carbohydrates, poor plant vigor, and shorter stand life. Soil compaction in the traffic lanes where harvesting equipment is driven reduces water infiltration rates and limits the amount of water available to the root system resulting in less than optimum plant growth. Compaction of soil layers can occur under a wide variety of soil types and field soil conditions with the most severe damage in saturated soils.

Many factors influence the severity of plant damage, yield loss, or stand reduction which occur with harvest-time wheel traffic activities. Understanding these alfalfa-damaging factors helps in developing management strategies to reduce the impact of wheel traffic on alfalfa.

Wheel Traffic Studies

Several universities have conducted alfalfa research trials to help understand the impact of wheel traffic on plant health, forage yield, and stand persistence.

A Univ. of Wisconsin study in 2000-2001 compared alfalfa yield with and without wheel traffic in eight states (Figure 1). Results show a 5-15% forage yield reduction for plots with wheel traffic for nearly all locations and years.

In a research study by D. Samac (USDA-ARS, Univ. of Minn.) comparing alfalfa wheel traffic versus no wheel traffic (Figure 2), yield reductions ranged from 5-26% averaged across locations and years.

By rating crown rot damage, researchers could measure the impact of wheel traffic on alfalfa plant health. At both locations, crown rot severity was 20% higher in plots that had wheel traffic as compared to no wheel traffic, contributing to a 15% reduction in final stand counts at the end of two years.

Factors Influencing Wheel Traffic Damage

Several factors affect severity of wheel traffic damage in alfalfa.

• Wheel traffic timing. By delaying chopping or baling beyond 1-2 days after cutting, traffic-timing studies show the largest yield reductions. Early post-cutting traffic treatments reduced forage yield by 5-8% while traffic treatments at 5-7 days post-cutting reduced yield by 16-26% on average. By 5-7 days post-cutting, new alfalfa buds and shoots can be visible and wheel traffic can break these off. When this happens, plants initiate the process of growing more shoots using up precious carbohydrate reserves and weakening the plants.
• Wheel traffic footprint. The type and size of harvest equipment changes the percentage of a field covered by wheel traffic. Larger equipment may reduce the total wheel traffic footprint, but may cause more severe damage due to higher weight density.
• Forage drying conditions. After cutting alfalfa forage, the drying process begins. This can be fast or slow depending wind, humidity, precipitation, and soil moisture conditions during drydown. When drydown is slow, growers make additional trips to invert or move alfalfa forage, helping it dry more quickly. These added trips add to the wheel traffic footprint. Frequently, these trips occur after more than 1-2 days of drying when crown buds and stems are more easily damaged.
• Soil moisture conditions. Excessively wet soils are easily compacted leading to poor alfalfa root growth and plant vigor in wheel tracks. Even a single incident of running harvest equipment in wet soil conditions can affect forage yield, plant health, and plant persistence. Soils types which are slowly-drained are more susceptible to soil compaction and plant damage.
• Alfalfa variety selection. Several companies have made alfalfa variety selections under wheel traffic conditions. The claims for “traffic-tolerant” varieties are that they yield better under intense wheel traffic conditions. In a 2003-2006 Pioneer study comparing commercial alfalfa varieties including “traffic-tolerant” varieties, the top-yielding alfalfa varieties without wheel traffic pressure were also the top-yielding with wheel traffic pressure (Figure 3). Rather than make specific selections for “traffic tolerance”, breeders who focus on high yield and excellent disease resistance will develop varieties which are better able to withstand wheel traffic stress.
  

Managing Wheel Traffic on Alfalfa Fields

The following alfalfa harvest management practices can help reduce yield and stand losses due to wheel traffic.

• Select elite alfalfa varieties with very high yield potential and excellent disease resistance since they are best able to resist disease and maintain better plant vigor under adverse traffic conditions
• Finish harvest operations as soon as possible after cutting to avoid the potential for damaging new crown buds or shoots. In silage operations, use “hay-in-a-day” strategies to decrease time between cutting and harvest.
• Avoid unnecessary wheel traffic in a field by managing field traffic patterns including how wagons and trucks move around a field.
• Use larger harvest equipment when possible to minimize total wheel traffic on the field. The downside of large equipment is increased plant damage and soil compaction when soil moisture content is high. Utilize small tractors for tasks like raking, tedding, or merging.
• Stay off wet fields whenever possible since the potential for soil compaction is high.