Ecology and management of an exotic weed Phalaris minor - Invasive Plants: Ecological and Agricultural Aspects - page 166

may be controlled through tillage or herbicides, thereby reducing the P. minor
seed bank [29, 34]. However, this aspect needs further experimental evidence.
Herbicide efficacy and soil factors
Soil has many features that influence the behavior, persistence, and bioavail-
ability of soil-applied herbicides. Soil organic matter, clay content, pH, and
moisture generally vary from field to field, and thus influence bioactivity of
soil-applied herbicides [42]. Selected soil properties - organic matter and clay
content, pH, and cation exchange capacity - influence the fate and behavior of
herbicides in soil [42, 43]. Hydrolysis of sulfonylurea herbicides, for example,
is temperature- and pH-dependent [44, 45]. Triasulfuron (2-(2-chloroethoxy)-
N -[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfon-
amide) [46] and amidosulfuron ( N -[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]
carbonyl]amino]sulfonyl]- N -methylmethanesulfonamide) [47] become inac-
tive at low pH, because they rapidly degrade under those conditions. The
amount, distribution, and physical and chemical properties of various organic
and inorganic soil constituents along with the type of herbicide used, dose
applied, and prevailing climatic conditions influence herbicide movement in
soil, which affects herbicide uptake by target plants [48, 49].
Sprague et al. [50] reported that isoxaflutole degradation was pH-depend-
ent. Degradation was much faster at pH = 10 (86%) compared with pH = 4
(16%) or pH = 7 (20%). Isoxaflutole rapidly converts to diketonitrile in plants
and soil, which is an active component of isoxaflutole that inhibits HPPD (4-
hydroxyphenylpyruvate dioxygenase) enzyme [51-53]. Diketonitrile further is
converted into an inactive benzoic acid derivative and finally forms carbon
dioxide [53]. The term 'active herbicide' refers to the sum of the phytotoxici-
ty of isoxaflutole (parent compound) and its biologically active degraded
metabolite, diketonitrile.
Fields in northern India are irrigated with tube-well water and ground water
that is saline at some locations. When fields are irrigated with saline ground
water, soil pH and the concentration of soluble salts often are affected, which
can influence the fate of soil-applied herbicides. Additionally, several fields in
Haryana and Uttar Pradesh are infested with the perennial weed Pluchea
lanceolata [54]. Inderjit [55] reported that P. lanceolata has potential to alter
soil chemistry, particularly the concentration of soluble salts, chloride ions and
phenolics. P. lanceolata may affect the efficacy of soil-applied herbicides for
P. minor control, but this has yet to be documented.
Residual effects of herbicides employed to control P. minor
Herbicide persistence is an important consideration when choosing a herbicide
for P. minor control. Mung bean ( Phaseolus aureus ) is often grown after wheat