Physiological and Morphological Responses of Barley to 2,4-Dichlorophenoxyacetic Acid.

Selective herbicides have been used for weed control since the turn of the century, but the use of 2,4-dichlorophenoxyacetic acid (2,4-D.) has developed during the past decade. This chemical is valuable because numierous broadleaved weeds are susceptible to low rates of application and mnany agronomic crops are tolerant to these small dosages. Barley has been considered to be one of the relatively tolerant crops, but the work of numerous investigators indicates that some stages of growth are less tolerant than others. The purpose of this study has been to correlate yield response with plhysiological activity and morplhological stage of development at the time of treatment with 2,4-D. KLINGMAN (9) reported more head abnormalities of barley, oats, and wheat wlhen 2,4-D treatments were applied before the jointing stage, wlhereas treatments made at the boot and heading stages caused greater reduction in yield. FOSTER (6), FRIESEN (7, 8), LEGGETT (10), OLsoN et al. (11), andl others lhave treated spring-sown barley at from 4 to 64 stages of growtlh, extending from pre-emergence to the dough stage. OLSON et al. (11) found that there were two critical periods in barley and wheat development when the yield was reduced sharply by treatments. One of these was an early seedling period when the plants were one to five inches tall. This period seemed to be somewhat slhorter in barlev than in wheat. The other period extended from the time that plants were well into the boot stage to a few days before heading. LEGGETT (10) designated the final stage of the early seedling period as the three-leaf stage for Olli barley, while DERSCHEID et al. (5). who treated several varieties at four stages of growth, showed that the five-leaf stage was most susceptible, follo'wed in order by heading, fully tillered, and milk stages. FOSTER (6), however, reported a reduction in the yield of barley when the shot-blade was formed, which apparently is a stage occurring between the two critical stages designated by OLSON et al. (11). SHAFER (12) showed that the yield of N\7emaha oats was reduced by treatment with 2,4-D at the jointing stage, and that the decrease was due to a reduction in number of florets per panicle, reduced seed weiglht and fewer flowering culms per plant. ANDERSEN and HERMANISEN (2) found that