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Effects of 2,4-dr (amine salt) on the growth, nodulation and nitrogen fixation of annual medics

P. Gianquitto, A. Fajri, C. Oti-Boateng and E.D. Carter

Departments of Plant Science and Agricultural Technology, The University of Adelaide, SA

Annual medics (Medicago spp.) provide high-quality feed and raise soil nitrogen (N) levels in the lower rainfall regions of southern Australia especially on neutral to alkaline soils. In South Australia 2,4-DB is used to control broad-leaved weeds in medic pastures particularly in medic seed crops. However, farmers report that when the herbicide is used, nodulation of young medic plants is reduced. There have also been other reports of herbicides adversely affecting nodulation and N2 fixation of legumes (1, 2). This paper reports the effects of the herbicide 2,4-DB on the growth, nodulation and N2 fixation of 10 medic cultivars.


Seeds of 10 medic cultivars were inoculated with commercial inoculant and sown in washed river sand in 2L pots. At the first trifoliate leaf stage, plants were sprayed with either deionised water (control) or 2,4- DB at 1.4 L/ha and 2.8 L/ha (half and recommended rates respectively) and 24 days later assayed in situ for acetylene reduction (AR) activity (3). Medic plants were washed out, nodulation assessed and total dry weights recorded.

Results and discussion

Overall 2,4-DB significantly reduced total dry weight, nodule numbers and N2 fixation (Table I) but there were marked differences between cultivars and species. On well-nodulated, herbicide-treated plants, there was a large number of small nodules and an absence of leghaemoglobin in the larger nodules indicating a loss of activity. Nodule distribution on the roots showed that with increasing rate of application, the herbicide reduced nodule numbers on the top half of the root while those of the lower half increased. This suggests that the plants may have the ability to recover from the herbicide with time. Generally AR decreased with herbicide concentration.

Table 1. Changes in total plant weight, the distribution of nodules on roots and AR of ten medic cultivars sprayed with the herbicide 2,4-DB at the first trifoliate leaf stage.


1. Eberbach, P.L. and Douglas, L.A. 1989 Plant and Soil 119, 15-23.

2. Garcia, M.M. and Jordan, D.C. 1969 Plant and Soil 30. 317-334.

3. Hardy, R.W.F.. Holsten, R.D., Jackson. E.K. and Burns, R.C. 1968. Plant Physiol. 43, 1185-1207.

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