Vol. 51—1997 - NorthEastern Weed Science Society

112
E FECTS OF REPEATEDLATE-WINTERHERBICIDE APPLICATIONS
ON SEMI-DORMANTALFALFA
H. Menbere and R. L. Ritter*
ABSTRACT
Th s study was initiated on a two-year-old stand of 'Legacy'
alfalfa (Medicago sativa L.) located at the University of
Marylan Hayden Farm in Beltsville, MD. Herbicide applications
were ma e on March 22, 1995 and March 27, 1996 when the alfalfa
had app oximately 2 to 4 inches of growth. Treatments included
imazeth pyr (Pursuit) at 0.063 and 0.094 lb ai/Ai paraquat
(Gramox ne Extra) at 0.23, 0.313, and 0.469 lb ai/Ai metribuzin
(Sencor Lexone) at 0.375 lb ai/Ai terbacil (Sinbar) at 0.375 lb
ai/Ai b omoxynil (Buctril) at 0.375 lb ai/Ai and 2,4-DB (Butyrac)
at 1.5 lb ai/A. These same treatments were applied to the same
plots th years. Visual crop tolerance (phytotoxicity) ratings
were ma e in April. Four cuttings were made each year to check
for pot ntial yield effects from the herbicide applications.
1995, crop tolerance ratings indicated that the highest
f foliar injury occurred from all three rates of
t, ranging from 23 (low rate) to 35% (high rate) on April
ignificant amount of foliar injury was also observed from
metrib zin and terbacil which averaged 18% and 13%, respectively,
on Apr'l 5. All of the other treatments averaged less than 10%
injury on the April 5 rating. Visual ratings made on April 20
found 11 rates of paraquat and metribuzin still causing a
signif'cant amount of crop injury. similar trends occurred in
1996, hereby the highest levels of foliar injury were obtained
from a 1 three rates of paraquat, ranging from 32 to 40% on April
11. F liar injury was also observed from metribuzin and terbacil
(13 an 15%, respectively). Ratings made on April 26 showed
contin ed injury to the alfalfa from these five treatments. All
other reatments averaged ratings of 10% or less on both dates.
I
from a
the si
no yie
August
paraqu
lowest
showed
metrib
yield
1995, yield data obtained May 6 indicated lower weights
falfa treated with the two highest rates of paraquat and
gle rate of metribuzin. With the June and July harvests,
d differences were observed between treatments. Yet, the
harvest found the plots treated with the high rate of
t and the single rate of metribuzin were yielding the
among all treatments. In 1996, first cutting yield data
a decrease in yield where the high rate of gramoxone or
zin were applied. Subsequent cuttings showed a decline in
here terbacil had been used.
T ese data indicate that repeated late-winter herbicide
applic tions on semi-dormant alfalfa may not result in a loss in
alfalf yield over time.
*Agric Res. Tech. and Assoc. Prof., Agric. Exp. Stn., Agron.
Dept., Univ. of MD, College Park, MD 20742.