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Knowing when and where fungicides fit in alfalfa management

Damon L. Smith for Progressive Forage Published on 27 April 2018

Many diseases caused by fungi affect alfalfa. Common leaf spot, leptosphaerulina leaf spot, spring black stem and leaf spot, and stemphylium leaf spot are among the more important foliar diseases often encountered in the Upper Midwest.

Management of these foliar diseases includes the use of resistant cultivars, timely cutting in order to limit the damage they cause to the foliage and application of a fungicide.

The fungicide pyraclostrobin (Headline) has been widely used in recent years on alfalfa. Pyraclostrobin is a quinone outside inhibitor (QoI) fungicide that affects fungal respiration (energy production). In addition to their fungicidal effects, the QoI fungicides have been attributed to also affecting plant metabolism, which could also increase yield and quality of a crop.

Together, the combination of antifungal activity and increase in plant health offered by the QoI fungicides has resulted in increased interest to spray fungicides to reduce plant stress and increase yield of field crops even in the absence of disease.

Recently, interest in capitalizing on the plant health activity by QoI fungicides has resulted in applying pyraclostrobin and other QoI fungicides such as azoxystrobin (Quadris) on alfalfa. This interest has also resulted in recent labeling of other fungicides in the succinate dehydrogenase inhibitor class; these products include boscalid (Endura) and penthiopyrad (Fontelis). More recently a pre-mix product containing fluxapyroxad (SDHI) and pyraclostrobin (Priaxor) was labeled for use on alfalfa.

To better understand how these new fungicide formulations might fit in alfalfa for dairy production, field research plots were established in Wisconsin during the 2011-14 field seasons. Plots were in commercial production fields located on working dairy farms in Wisconsin or on the Arlington Agricultural Research Station located in Columbia and Dane counties.

All trials were established on stands of alfalfa in at least the second year of production. All alfalfa used was targeted for dairy production. Therefore, frequent cutting (28- to 30-day intervals) was used with optimal harvest targeted at the early bud stage in order to obtain high-quality hay. In Wisconsin, four harvests (cuttings) per season are typical under this system.

All replicated treatments were applied at 6 to 8 inches of growth after each cutting in order to limit damage to the alfalfa stand and allow enough time for the pre-harvest interval (14 days for all fungicides used) to expire. The labels of all fungicides used in the studies restrict total number of applications per season to three. Therefore, fungicide applications were targeted for the first, second and third or fourth per season. Nontreated controls were also included at all locations.

When using Headline fungicide in this system, results can be highly variable. Sometimes yield is high enough to offset cost, but the overall probability of offsetting the cost of the fungicide application is generally below 50 percent, even when prices are favorable (Figure 1).

Probablility of offsetting fungicide costs

In 2015, a new alfalfa research trial was established at the Arlington Agricultural Research Station in Arlington, Wisconsin. Two cultivars of alfalfa (DKA44-16RR – Conventional Roundup Ready; HarvXtra – Reduced-lignin, Roundup Ready) were sprayed with seven fungicide treatments and compared to a nontreated control. Yield, quality and return on investment (ROI) of the treatments were evaluated under two cutting duration schemes (30-day versus 40-day) for both cultivars.

In the 2015 study (seeding year), both cultivars responded to fungicide in a similar way. In the 30-day cutting duration, fungicide application resulted in little discernible difference in disease level, defoliation or quality compared to not treating with fungicide. ROI calculations indicated no positive return was achieved if the hay was sold or was kept on the farm and fed to dairy cows for the 30-day duration of cut.

For the 40-day duration, significant differences in fungicide treatments were identified for disease levels, defoliation and quality compared to the nontreated controls. These differences resulted in positive ROI (using the Milk 2006 model) for the second crop where the fungicides Headline and Quadris were used, under the scenario where hay would be kept on the farm and fed to dairy cows. If hay was sold, no positive ROI was identified for either treatment for this crop.

Considering these results, we continued this study in a second year using the same established stand of alfalfa. We investigated the first, second and third crops in 2016. We considered these three crops together in this analysis to examine success of using fungicide in a 30-day cutting interval system or a 40-day cutting interval system.

Using hay pricing to calculate ROI, Headline, Priaxor and Quadris fungicide used on either cultivar subjected to the 30-day cutting interval generally resulted in negative ROI (Table 1).

Hay return on investment when applying three application of Headline

Two exceptions were identified where Priaxor provided a slight positive ROI for DKA44-16RR subjected to the 40-day cutting interval, while Quadris provided a positive ROI for the HarvXtra cultivar subjected to the 40-day cutting interval.

Using milk pricing resulted in a larger number of positive ROI cases. For DKA44-16RR subjected to the 30-day cutting interval, both Priaxor and Quadris provided positive ROI estimates (Table 2).

Milk return on investment when appliying three applicatoin of Headline

No positive ROI estimates were observed for HarvXtra subjected to the 30-day cutting interval. Using milk pricing to calculate ROI for the 40-day cutting interval resulted in positive ROI for both cultivars and all fungicides except for Headline applied to DKA44-16RR (Table 2).

Fungicide applied to alfalfa in Wisconsin under our typical dairy production system has resulted in infrequent cases where the application resulted in a significant increase in yield or a positive ROI; subjecting alfalfa to timely cutting (e.g., 30-day cutting intervals) usually results in plants with low foliar disease, undetectable defoliation and extremely high quality.

Subjecting alfalfa stands to longer cutting intervals (e.g., 40-day cutting interval) results in more disease pressure, detectable defoliation and an inherent reduction in overall quality. Applying fungicide to alfalfa stands subjected to these longer cutting intervals appears to result in a higher likelihood of positive ROI.

If you desire to extend your cutting interval past 30 days, then the likelihood of positive ROI increases with the application of fungicide. If you desire to cut on an interval shorter than 30 days, then the likelihood of positive ROI is much lower when fungicide is applied.  end mark

References omitted but are available upon request. Click here to email an editor.

Damon L. Smith is an associate professor and extension specialist with the Department of Plant Pathology at the University of Wisconsin – Madison. Email Damon L. Smith.

 

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