Top-dress applications of manure onto established alfalfa are sometimes made because these are the only fields available during the growing season. Although several studies have shown that top- dressed manure can be successfully applied to established alfalfa, this practice is considered risky due to possible plant injury, stand reduction, or nutrient runoff. The potential for alfalfa injury arises from the salts contained in the manure, including free ammonia, soil compaction, and from the physical damage to the crowns during application.

Crop Responses
A Wisconsin study with liquid dairy manure (Table 2) shows that although pre-plant manure application improved yields, subsequent top-dressed manure application decreased yields. The apparent recovery of yields for the top-dressed plots in 1983 was due to increased weed growth. The wheel track areas from repeated applications were particularly affected. Crown counts confirmed that there was less alfalfa in the top-dressed plots.

In other situations, topdressing manure has provided some yield benefits. For example, data from Minnesota showed yield increases up to 30% over the control from top-dress applications, but bare spots were apparent in the high rate top-dressed plots. Timing of the top-dress applications may influence the subsequent crop performance. In two separate studies where manure was top-dressed in the winter on frozen soil, there was a tendency for yield improvements where manure was added compared to the untreated control.

Nutrient Runoff
The effect of alfalfa’s vegetative cover on runoff losses from fields that receive manure can be significant. As shown by a west central Minnesota study, topdressing manure on alfalfa fields may constitute more of a pollution hazard than spreading manure on plowed corn ground (Table 3). Some variation existed between years, but total N, total P and soluble P losses averaged about 10 times higher from the manured alfalfa than from the manured corn.

A southwest Wisconsin study used small runoff plots to examine the N and P losses from winter- applied fertilizer and manure in runoff water from fallow soils or alfalfa sod. The concentration of nutrients in the runoff water was lower for fallow areas compared to vegetated areas. This suggested that although runoff volume is usually much less for vegetated soil compared to bare soil, vegetation prevents waste components from encountering the soil, thereby increasing the likelihood of then being lost in the spring runoff.

On unfrozen soil, however, simulated rainfall studies show that surface-applied manure does not appear to greatly increase the pollution potential of runoff from alfalfa fields. Clearly, the major risk is associated with surface-applied manure to frozen soils. Several studies have observed much larger runoff nutrient loads where winter versus fall manure was applied to established alfalfa.

These experiments emphasize that under some conditions, especially where manure is top-dressed on frozen soil, runoff losses of manure nutrients may be unacceptably large. The magnitude of this environmental risk must, however, be determined on a site-specific basis.

Management Considerations
Because of possible alfalfa injury or environmental problems from top-dress manure applications, the management suggestions provided below should be followed to minimize potential problems.

1. Apply to older stands. With a younger stand of alfalfa, any injury caused by manure applications will more severely affect subsequent cuttings. Since the production level and the quality of younger stands are also generally higher than for older stands, application to older stands reduces the risk.
2. Apply to poorer stands. Less dense stands generally have more grasses and/or weeds contributing to yields. Grasses/weeds are more tolerant to top-dressed manure applications than alfalfa. Grasses/weeds will also directly benefit from the N in manure. Thus, with thin alfalfa stands, manure applications may increase forage production, but it may be at further loss of the alfalfa stand.
3. Apply where nutrients are needed. On low testing soils, although top-dressed manure applications may cause some alfalfa injury, the overall effect is for better production because the response to the nutrients more than offsets the injury caused.
4. Limit rates to not more than 3000-5000 gallons of liquid or about 10 tons of solid dairy manure per acre in a single application. Higher salt manures, i.e., swine manure from finishing houses should be reduced proportionally. The primary issue for most producers is the maximum application rate that does not cause stand injury or environmental problems. The specific characteristics of the manure in question needs to be considered since the burn potential of the manure is a function of ammonium N and salt content.
5. Apply manure as soon as possible after harvest because as top-dressed manure applications are delayed, more alfalfa regrowth will be present and the burn potential increases. Adjust manure equipment to provide a uniform application. The distribution from the outlet port must be uniform for liquid manure and clumps of solid manure must be eliminated from solid manure equipment.
6. Apply to fit soils. It is important to pay attention to the condition of the soil when applying manure. Due to the compaction injury on crowns, driving over fields with moist or wet soil increases the injury risk. Where possible make applications when cooler temperatures and post- application rainfall will reduce burn potential.