Weeds can reduce the vigor and stand density in seedling alfalfa. In established alfalfa, weeds reduce the forage quality and marketability of alfalfa. Many markets have near-zero tolerance for weeds without discounting the price of the hay significantly.

No one has a crystal ball to predict where we are headed when it comes to weed control and herbicide development in alfalfa, but it is often useful to look at where we have been to get a better sense of trends and where we may be headed.

Before 1960

The non-chemical weed control practices used before 1960 were the same as those used today. These included the use of weed-free seed; planting between peak periods for annual weed germination; properly leveled fields; keeping borders, ditches and ditch banks clean; timing cuttings to enhance crop vigor; summer fallow; cover crops; sheep grazing and other practices.

Some non-selective chemicals were also used at this time to control weeds around fields and within fields where particularly undesirable weeds, such as dodder, were present.

This included salts such as sodium chlorate and ammonium salts, oils and acids like boric, sulfuric and carbolic acids. Arsenicals including sodium arsenic trichloride and arsenic acids were also used.

1960 to 1970

Herbicides in the developmental stages in the ’50s began to be registered and used in the ’60s. Eptam (EPTC), Balan (benefin) and Chem Hoe (chlorpropham) were the first pre-emergent herbicides, and 2,4-DB and dinoseb were the first post-emergent herbicides used at this time.

Eptam was the first herbicide applied by chemigation, and it was used both preplant and on established alfalfa. Balan was the first dinitroanaline herbicide registered on alfalfa and is still on a limited scale today, while 2,4-DB was used in 1960 and is still widely used today.

1970 to 1980

New registrations continued this decade. Gramoxone, Tolban and Sencor were registered during this time period. Eptam, 2,4-D and dinoseb continued to be standards. Treflan EC was briefly registered for a Special Local Need, 24c for water-run applications in alfalfa for irrigation runs of 660 feet or less. This was prior to the development of TR-10, which worked much better; this registration was dropped in 1980.

1980 to 1990

This was a very active period of product testing and registration in alfalfa. Some of the registrations from this period continue to have a major impact on alfalfa weed control. Velpar (hexazinone), the soil-residual herbicide used on dormant alfalfa, was registered during this decade and remains a dominant herbicide in central and northern California and other Western states where the alfalfa has a winter dormant period.

Significant advances in selective grass control also occurred during this period. Treflan (trifluralin) granules were registered for pre- emergence control and the “over the top” products such as Poast (sethoxydim), Fusilade (fluazifop) and Select (clethodim) were developed and registered. Poast was first, followed by Select about 10 years later.

Fusillade was never registered in alfalfa. Treflan had been registered as an EC, and Treflan granules on a different granule formulation were tried earlier, but neither worked as well as this new formulation. Buctril (bromoxynil) and Kerb (pronamide) were also registered during this period, but their use was limited in some areas.

The problem with Buctril was a lack of crop safety in some areas (especially when applied under high temperature and sunlight intensity conditions) and was primarily only effective on small weeds. Reasons for the limited use of Kerb were leaching of the product below the germinating weed seed with flood irrigation, the cost of the product and its limited effectiveness on larger weeds, especially mustards.

1990 to 2000

The imidazolinone herbicides – Pursuit (imazethapyr), Scepter (imazaquin), Arsenal (imazapyr) – were registered during this decade. Pursuit, and later Raptor (imazamox), are still the standards for weed control in seedling alfalfa to this day. A broad spectrum of broadleaf weeds are controlled both pre-emergence and post-emergence with these herbicides.

Select (or Prism) was registered in this period and offered improved control of certain grasses such as annual bluegrass and sprangletop. Zorial (norflurazon) was also registered during this period.

2000 to present

The first part of this decade saw the registration of two new herbicides for alfalfa, Raptor and Chateau (flumioxazin), the expanded registration of two products used on other crops, Prowl H2O and Sandea (halosulfuron), and the deregulation and commercialization of Roundup Ready (RR) alfalfa. Raptor controls a broader spectrum of weeds than Pursuit with less soil residual and has become the most popular herbicide used on conventional seedling alfalfa.

Chateau offered pre-emergent control of winter annual weeds. Prowl H2O offered a “water run” option for grass control and could also be easily tank mixed with winter-dormant applied herbicides in central and northern California for year-round weed control in a single application.

Sandea is effective during the “summer slump” period for nutsedge control in areas that contend with that challenging weed. The old restriction on Velpar excluding its use in the low deserts was also dropped around 2010.

The future

What the future holds for herbicides available for use in alfalfa is unknown. It is clear that herbicide development for alfalfa has decreased dramatically. In the past (between 1960 and 2000), between four and eight new herbicides were developed and registered every 10 years for use in alfalfa.

However, new herbicide development and registration has dropped off significantly. In fact, there hasn’t been a new herbicide registration in this crop for more than 10 years (the longest drought in new registrations since alfalfa herbicides were developed) – and that registration (Prowl H2O) was just a new formulation and not a new active ingredient.

There are several plausible reasons for this lack of new herbicide active ingredients in alfalfa. One may be the advent of RR alfalfa and concerns about developing a product when the market share may be dramatically reduced.

The eventual long-term adoption rate of RR alfalfa is not known, but the acreage is increasing in most areas where it is permitted. Concerns over the export market, primarily Chinese acceptance, and the public’s perceptions about “GMO crops” has somewhat tempered the adoption rate of RR alfalfa.

The cost of bringing new products to market is probably a greater deterrent to the development of new herbicides for alfalfa than concerns over the market share and the prominence of RR alfalfa. New herbicide development often takes eight to 10 years and typically costs from $120 to $250 million.

One of the reasons for the cost is that a product intended for use on food crops must undergo approximately 120 health, safety and environmental fate tests to ensure their safety and effectiveness before being registered by the EPA. The success rate for products developed in the lab actually making it to the commercial market and being applied to a farmer’s field is low – it has been estimated that only one in 139,000 products ever make it successfully through the process.

Summary

Alfalfa weed control has advanced significantly over the past 50 years, largely due to the development of new and improved selective herbicides. Through industry and university collaborations, effective weed management programs have been developed that have enabled us to adequately control most of the weed problems encountered in alfalfa production systems today.

However, despite this effort, weed control challenges continue to arise and new approaches to effectively manage new weed issues (such as weed shifts and herbicide resistance in a safe and economical manner) are needed.

The herbicide development pipeline has slowed substantially due to the continually escalating costs of herbicide development and commercialization.

Moving forward we will likely need to continue finding techniques to fine-tune the use of time-honored standard alfalfa herbicides. Also finding potential new uses for old herbicides will be required, as it appears we have entered into an era of slower-paced herbicide development.  FG

This presentation was given at the 2014 California Alfalfa, Forage and Grain Symposium.

Steve Orloff is a farm advisor with University of California Extension, Siskiyou County. 

Barry Tickes
University of Arizona Extension