Winter
annual weeds and early spring insects are common pests of alfalfa, Medicago
sativa, grown along the Front Range of Colorado. Winter annual weeds
of alfalfa include downy brome, Bromus tectorum; flixweed, Descurainia
sophia; and blue mustard, Chorispora tenella. These weeds
compete well with newly growing alfalfa and contaminate first-cutting
hay, thereby reducing the quality and economic value of the hay. The
alfalfa weevil, Hypera postica, typically is encountered in northeastern
Colorado and also can significantly reduce the quality and quantity
of first and second cutting alfalfa. A trend towards sustainable agriculture
and organic products has encouraged efforts towards identifying effective,
economical alternatives to herbicides and insecticides. The purpose
of this study is to examine whether propane flaming treatments of alfalfa
fields in the early spring can reduce the winter annual weeds and/or
insect pests that reduce alfalfa quality. The alfalfa was flamed using
the Red Dragon TD-12 LPS Alfalfa Flamer. Pursuit herbicide and Baythroid
2 insecticide treatments were used as conventional comparison plots.
Flaming dormant alfalfa when blue mustard was a quarter to half-dollar
size reduced weed density significantly when compared to the untreated
check. Increasing the burn time (3 miles per hour tractor speed versus
4 miles per hour) increased blue mustard weed control levels. Flamed
alfalfa treatments significantly shortened alfalfa stem length when
compared to any treatments that contained an insecticide treatment early
in the season. Early season weevil larvae assessments (7 days after
insecticide treatment and 54 days after flaming) showed good activity
from flaming on weevil larvae populations, as compared to any insecticide
treatment. However, flamed alfalfa treatments revealed a significant
decline in alfalfa weevil control at 23 days after insecticide treatment
and 70 days after flaming. Alfalfa yield evaluations revealed the flamed
alfalfa treatments were similar to that of any other chemical treatment
and the untreated check in both experiments. No significant differences
were observed between any treatments when relative feed values were
compared. Alfalfa quality as measured by leaf defoliation showed significantly
greater leaf loss in the flamed treatment when compared to the insecticide
treatments. The flamed treatment had significantly less defoliation
than the untreated check. The addition of a phosphate fertilizer (10-34-0)
with the Pursuit herbicide treatment increased soil phosphorus 25% compared
to the untreated check. Leaf necrosis (phyto-toxicity) of up to 6.7%
occurred 9 days after the Pursuit + (10-34-0) treatment. The weevil
larvae had greater impact on alfalfa yield and quality than the weed
populations encountered in this study.
