Over my many years as an agronomist and consultant I have seen many instances of unintended herbicide damage to crops or new pasture establishments. A lot of these instances can be avoided by studying herbicide labels prior to sowing.
A recently released manual "Rotational Crop Constraints For Herbicides Used In Australian Farming Systems" is an outstanding publication that will allow farmers and agronomists to readily reference if there is likely to be a residual issue with a given herbicide. It is published by the Grains Research and Development Corporation (GRDC) and authored by Mark Congreve and John Cameron (ICAN).
With few exceptions, most herbicides are degraded in soil by microbial processes. Thus, sustained warm and moist conditions that favour the development of microbes also favour the breakdown of many herbicides. However, sustained cold and dry conditions prolong soil life of many herbicides.
Widely used for over 70 years, 2,4-D, considered relatively safe from a soil residual perspective, can adversely affect some broadleaf crops and pastures sown too close to its application, like in a fallow prior to sowing. Residual damage is possible for up to 28 days for susceptible crops like canola at higher rates. Certain plant-back periods are advocated to ensure that 2,4-D has degraded sufficiently to allow safe sowing of intended crops or pastures. Most newer 2,4-D labels also require at least 15mm of rainfall before commencement of plant back periods.
A common residual problem occurs when plant back times are not adhered to for Clopyralid (Lontrol Advanced). Lontrol is often used as part of fallow or in-crop treatments. While relatively safe for cereals and canola (a few days plant back restriction), residual to badly impact on some species can be high. For example clover, chickpea, faba bean, field pea, lupins, medics and vetch plant back can be long depending on factors like soil and rainfall. For example when applied at 40ml/ha it can be nine months for lucerne and at higher rates, plus low rainfall, up to 24 months.
An issue with pyridine herbicides such as aminopyralid and clopyralid, is that stubble from a cereal crop treated with these may still contain stubble residues post-harvest, depending on when it was treated. Breakdown of crop stubble and residues of that stubble does not start until it is in soil contact. Effective safe re-crop interval can therefore be far longer than the interval between application and sowing the next crop.
Even a relatively safe product like dicamba, also often used as part of fallow as well as in-crop, has significant plant-back restrictions. Plant-back for fallow inclusion specifies, if applied to a dry soil, at least 15mm of rainfall before plant-back period begins. When applied at 375 ml/ha it is 14 days for wheat, and 28 days for safflower.
Newer herbicide Paradigm (Florasulam+halauxifen) has significant re-cropping intervals for susceptible species like grain legumes, millets, lucerne, pasture legumes, cotton, safflower and sunflower. For some of these species minimum plant back period is eight months with a minimum rainfall amount of 100mm.
For many herbicides detailed in the publication, many issues are important for assessing safe plant back periods. More than one or two issues influence length of herbicide persistence and some of these vary significantly from year to year.
Factors that impact on herbicide soil persistence include time of year of application, temperature, moisture, soil microbial activity, soil type and soil properties such as pH, soil structure, stubble type and amount, weed and crop presence, chemical properties of the herbicide, application rate and crop agronomy.
Many herbicides used in agriculture can persist in soil for extended periods. Herbicides designed to be used for residual weed control need to provide weeks or months of soil persistence to achieve their weed control objectives. Additionally, some herbicides that typically are used for post-emergent weed control and which are not typically considered 'residual' may also persist for some time after application.
"Rotational Crop Constraints For Herbicides Used In Australian Farming Systems" can be obtained via the GRDC web site.
Next week: More on grazing management.
- Bob Freebairn agricultural consultant at Coonabarabran. Email robert.freebairn@bigpond.com phone 0428 752 149.