Despite the development and registration of highly effective herbicides in alfalfa, weeds are as much of a problem today as they have ever been. Alfalfa is non-dormant in the low deserts and subject to winter annual, summer annual and perennial weeds. No herbicide controls all of the potential weed problems in this environment. Growers have continually selected for the weeds that escape their control practices. Highly effective and selective herbicides have been extremely valuable tools for alfalfa growers. However, they have not eliminated all weeds and have maintained the continual selection for those weeds that are difficult to control.

Table 1 illustrates how weeds have been selected with the use of herbicides from the 1950s to present. This table contains six common broadleaf weeds and six common grasses.

1950-1960
Various chemicals were used to control weeds in the 1950s. These were mostly toxic salts, acids, and oils and were largely nonselective vegetation killers used around crop fields rather than in them. They were occasionally used for spot weed control in fields where particularly undesirable weeds were present. The best weed control in alfalfa has always been a thick healthy crop and cultural practices were heavily relied on to control weeds during this period. Nonetheless, a variety of weeds could be found in most fields. Those that predominated were those that were most competitive and prolific. These included the mustards, mostly London rocket and wild mustard and the summer annual grasses such as watergrass and barnyardgrass.

1960-1970
New selective herbicides were registered during this period and became widely used. The use of these herbicides, especially EPTAM and 2,4-DB, caused some major shifts in the weeds present in alfalfa fields. Chem hoe, furloe and balan were used for preemergence weed control and DNBP or Dinoseb was used as a postemergence contact herbicides for control of broadleaf weeds, although the most commonly used products at that time were eptam and 2,4-DB. Eptam was the first herbicide registered for water-run applications and it was the most commonly used herbicide in alfalfa for the 20-year span from the mid 1960s to mid 1980s. Eptam was inconsistent on grasses because of its volatility, short residual activity and the continuous emergence of summer annual grasses. Many summer grasses could be found in alfalfa fields during this period. Eptam did, however, suppress perennial weeds such as nutsedge and bermudagrass, which became more serious problems once Eptam use declined.

2,4-DB was the other herbicide that was widely used in the 1960s. This herbicide is effective on many broadleaf weeds with the exception of shepardspurse and malva. Consequently, many weeds such as London Rocket, goosefoot and lambsquarters declined in prevalence during this period while malva and shepardspurse proliferated.

1970-1980
Although new herbicides were registered during this period, the weeds that they controlled were not significantly different than those controlled with compounds that were already being used. Treflan EC and tolban were available for only a short time and controlled many of the same weeds as balan and eptam. Sencor and gramoxone, which are still registered today, have had limited acceptance because of the lack of crop safety. No major weed shifts occurred during the period as a result of herbicide use practices.

1980-1990
This period saw the introduction of new grass herbicides that caused significant changes in the type of weeds present in alfalfa fields. Trifluralin 10G (treflan) was registered and quickly accepted by growers for the control of annual grasses and some broadleaf weeds. Much of this replaced eptam for preemergence weed control. As eptam drums disappeared from the ditch banks, nutsedge and bermudagrass appeared in the fields. Trifluralin 10G is highly effective on most annual grasses, which were removed from competition, while perennial weeds (Bermudagrass and Nutsedge) which had been suppressed by eptam proliferated. Poast was also registered during this period and had an equal impact in changing the relative predominance of weeds in the field. As a highly selective postemergence treatment for grasses, poast, along with the preemergent use of trifluralin 10G, greatly reduced the presence of many annual grasses like watergrass and barnyardgrass. Both trifluralin 10G and poast were ineffective or weak on grasses that survive through the winter and come back from established crowns rather than seed. These included sprangletop and sandbur. Annual bluegrass is also not controlled by poast. As a result, sprangletop, sandbur and bluegrass became more prevalent during this period.

1990-2000
The registration of new herbicides began to slow down in this period although two new registrations occurred that had significant impacts on the distribution of weeds in alfalfa fields. Pursuit was registered by American Cyanamide and was very effective on a large spectrum of broadleaf weeds including malva and shepardspurse. It was weak, however, on the composites that included sowthistle and prickly lettuce and lambsquarters. As a result, malva and shepardspurse became scarcer while sowthistle, prickly lettuce and lambsquarters began to appear in fields that did not seem to previously contain them. Select/prism was also registered during this period. This postemergence grass herbicide was similar to poast but more effective on bluegrass and sprangletop, which became slightly less prevalent as a result.

2000 to Present
Two things happened in the 2000-2002 period that had significant impacts on the prevalence of weeds in alfalfa. One was the registration of raptor, a new imadazolinone herbicide similar to pursuit. Unlike pursuit, however, raptor is more effective on sowthistle, lambsquarters and grasses. This is expected to reduce the prevalence of these weeds. The other major occurrence was the development of herbicide resistance in littleseed canarygrass to the grass herbicides, poast and select/prism. This resistance was documented in the Imperial Valley, California, where canarygrass has started to again become more widespread.

Table 1. Selection of Weeds with Herbicides in Alfalfa

Herbicides

Date Introduced

Infestation Key:

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Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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