13 - PGR Weed Control (Lect 13).ppt

HRT 4300 LECTURE 13  PGR & WEED CONTROL Weed Control : most widespread practical use of PGR many types of PGR most important - auxin types auxin types of weedicides / herbicides : - phenoxy-type compounds - benzoic acids type - heterogenous compounds (refer hand...

HRT 4300 LECTURE 13  PGR & WEED CONTROL Weed Control : most widespread practical use of PGR many types of PGR most important - auxin types auxin types of weedicides / herbicides : - phenoxy-type compounds - benzoic acids type - heterogenous compounds (refer handouts) Importance of Herbicides : weeds are costly compete for water, nutrients & light reduction in yield & quality  75% of total weed control achieved by application of the 3 groups of compounds 2,4-D : most widely used simazine diuron  2,4-D Chrystals - most common systemic herbicide used in the control of broadleaf weeds. It is the third most widely used herbicide in North America and the most widely used herbicide in the world. 2,4-D in 1946, it was the first commercially successful selective herbicide. It allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crops. 2,4-D is sold in various formulations under a wide variety of brand names. It continues to be used because of its low cost despite the availability of more selective, more effective, safer products. Commercial formulations used today use the trimethylamine salt or one of several esters. The amine salt formulations can cause irreversible eye damage. The product Agent Orange, used extensively throughout the Vietnam War, contained 2,4-D. However, the controversies associated with the use of Agent Orange were associated with a contaminant (dioxin) in the 2,4,5-T component. Types of Herbicides : Several basic types, each targeting certain weeds : pre-emergent herbicides to stop germination of plant seeds post-emergent herbicides can be : selective , non-selective - non-selective herbicides – kill most plants - selective herbicides – kill specific types of plants, usually grasses or broad-leaf weeds Roundup ® : total vegetative, non-selective, post-emergent herbicides  surfactant – when spraying herbicides on plants, a surfactant can be used to increase the efficiency of herbicides. Selectivity in PGR herbicides : auxin-type herbicides – used to control weed without injuring crop unique toxic properties – can “select” and kill weed applied to foliage as foliar spray or applied to soil as soil drench or either way when applied as foliar spray : herbicide moves from foliage to young shoot tips or from older leaves to roots - movement generally follows translocation of photosynthate in the phloem when applied to soil surface : stage of dev. of crop & weed is very important may be applied at pre-planting, pre-emergence, or post- emergence  auxin types of weedicides / herbicides : - phenoxy-type compounds - benzoic acids type - heterogenous compounds CHLOROPHENOXY HERBICIDES major phenoxy herbicides : 2,4,5-T, MCPA, silvex, 2,4-DB – have the same advantages as 2,4-D effects : ~ morphogenetic effect ~ biochemical effect Morphogenetic effect - different growth responses occur due to different sensitivity - when injured but not killed, new organs may be initiated - stop elongation, increase width of plant parts – stems, petioles, inflorescences, roots - cell division stimulated, lateral roots produced - within a few hours, stems & petioles bend & twist downward bending (epinasty) more frequently than upward bending (hyponasty) - lateral swelling occurs due to enhanced lateral cell extension - disorganization of tissues leading to death Biochemical effect - very complex - type of herbicide, type of plant, environmental conditions - determine changes in plant metabolic status - affect changes in : composition of carbohydrates, lipids, nitrogen, auxin, water, minerals, nucleic acids and enzymes - also affect changes in : photosynthesis & respiration Compounds and Formulations - usual formulations for phenoxy herbicides : acids, esters, salts, and amines - forms : dust form, solid forms (granular & pellet – pre-emergent & aquatic weed control) Application - apply with great care - differ in solubility, volatility, specific gravity, toxicity  other phenoxy herbicides : 2,4-DES, MCPES, 2,4-DEP, 2,4,5-TES CHLOROBENZOIC HERBICIDES derivatives of chlorinated benzoic acids used extensively for perennial weed control in uncultivated areas or areas that are unsuitable for cultivation effective on wide range of plants esp. resistant to chlorophenoxy acid soil application higher application rate than chlorophenoxy herbicides  2,3,6-TBA & 2,3,5,6-TBA - most toxic effect mild compared to 2,4-D abnormal venation in treated plants Mechanism of Action act fundamentally similar to 2,4-D differences in toxicity between benzoic acids & 2,4-D are caused by variations in chemical persistence & mobility within the plants AUXIN HERBICIDES other than Chlorophenoxy & Chlorobenzoic Acids most important : NPA, N-aryl phthalamic acids and picloram NPA : pre-emergence applications – kills both monocot- and dicotyledonous weeds geotropic response of roots upset – often grow out of soil excellent selective herbicide for Cucubits (pumpkin, cucumber, watermelons)  picloram : induces same physiological responses as benzoic acids absorbed by foliage & roots – translocate both up & down in plants accumulates in developing tissues soil applications in granular form most effective The End