Safe use of Lontrel® Advanced in southeast Australian crop rotations

Take home messages

Soil degradation is fastest in warm, moist soil
Plant degradation is slow and relies on growth dilution to comply with maximum residue levels (MRL’s)
Degradation of stubble residues occurs when it is returned to the soil, under warm, moist conditions by microbial activity
Cereals and canola (or other brassicas) are very tolerant of Lontrel^TM residues
Dry planting is worst case for injury to sensitive crops due to residues
Stubble concentration from harvesters increases likelihood of injury
Soil very low in microbial activity or organic matter may result in carryover
Non-wetting soil may be more likely to result in carryover due to lack of microbial activity in top 5-10cm.

Background

Lontrel^® L (300 g ai/L clopyralid) was first submitted for registration for use in cereals and canola in 1985 in Australia. After the initial registration, further research was conducted to determine if low rates, tank mixed with MCPA or 2, 4-D could be used for control of thistles and volunteer legumes, which were problematic in the crop rotations that were used at that time.

This led to further registrations for this use at less than the initial 90 g ai/ha clopyralid rate in 1995. At this time, plant back work was being conducted by Dow Elanco Australia to determine the safe plant back for various herbicides including Lontrel (Wells, 1999). This work showed that Lontrel persisted for less than a season provided there was significant rain in the warm months of the year (work was conducted on black cracking clays in almost all trials).

In the early 2000’s a series of dry years resulted in carryover of Lontrel residues. In most cases less than 30% of average annual rainfall occurred. Further, stubble concentration in header rows had occurred in many places. Investigation of many sites in 2002-3, showed that there were very low rates used of 45 g ai/ha clopyralid or less. This led to a further label update, with statements to suggest to only plant cereals or canola in the year following a very dry season.

Dow AgroSciences Australia continued to conduct research in the early 2000’s to address these issues. This work showed that higher rates were likely to persist for longer, stubble incorporation or removal reduced or eliminated the potential for Lontrel carryover and significant rain of more than 25mm, or enough to keep surface soil wet for 2 weeks or more, reduced the likelihood for carryover injury. This work resulted in a further label update and results were published in 2003 (Wells, 2003).

Soil residues

Lontrel is degraded most rapidly in aerobic, warm, moist soil by microbes. As a general guide soil should be more than 17^oC and wet down to at least 10cm for two weeks or more. Most herbicide residues are found in the top 5cm of soil, unless there is very heavy rain on very coarse textured soil, in which case, Lontrel may move to more than 10cm depth, as it is quite water soluble.

Photolysis is not an important method of breakdown, so Lontrel applied to soil will not be degraded whilst it sits on the soil surface under dry conditions.

Lontrel is not degraded by hydrolysis and it is stable in a range of soil pH’s.

Lontrel has very low volatility and this is not a pathway for degradation.

In soil Lontrel is either free in soil solution or adsorbed to soil particles (e.g. clay). It is anionic (negatively charged) in soil solution and therefore somewhat mobile due to fewer binding sites. This is the opposite of both glyphosate and paraquat, which bind readily to soil (Congreve and Cameron, 2018). Adsorption of Lontrel to soil may increase over time.

Higher rates applied will likely result in longer persistence in soil. Typically, 90 g ai/ha clopyralid will persist longer than rates of 45 g ai/ha or less. Application late in the season means Lontrel is more likely to persist in the soil to the next crop due to the shorter time available for degradation. Low rainfall (less than 30% of average annual rain) will result in carryover. Application in the cold months of the year increases soil persistence due to lack of microbial activity. Grazing heavily with sheep or cattle may lead to soil surface sealing which can limit water infiltration and therefore microbial breakdown. Legumes are all sensitive to Lontrel residues in soil, whilst cereals and canola are tolerant.

Plant residues

Lontrel degrades slowly in plants and may persist as either clopyralid or its conjugates. MRL’s are met by growth dilution, therefore the earlier the application in the season the more likely to have low residues by the end of the season. Crop type, spray timing, period after treatment and seasonal conditions all impact how much Lontrel residue may remain in the soil and crop stubble. For example, early post-emergence application in cereal will result in treatment that is applied with low soil cover percentage and therefore lower levels of plant interception. Early leaves and some tillers may be shed as the cereal crop matures. Soil temperature in the early part of the season may be warm enough to allow for some degradation of Lontrel soil residues. Early season application also allows maximum time until a sensitive crop may be planted in the following season. Warm, moist soil conditions in the autumn, spring and summer will result in significant degradation. Our investigations have shown that rain of 25mm or more that keeps soil moist for at least 2 weeks to a depth of 10cm is enough to result in significant residue degradation.

Standing stubble in reduced or zero tillage systems is a potential reservoir of Lontrel residue. Cereal stubble may persist longer than canola and is therefore more likely to cause issues in the following season. Where possible burn, bale, slash, incorporate or remove stubble prior to planting a sensitive crop the following season. Wet summer or autumn conditions may help with breakdown of stubble residues. Harvesters that leave a thick trail of trash should be modified to spread stubble evenly on fields. Only plant canola or cereals if conditions and plant back requirements for sensitive crops have not been met

There are many new treatments for use in cereals (e.g. Paradigm® Arylex® active herbicide, Talinor® herbicide, Velocity® selective herbicide) that have good efficacy on volunteer legumes and thistles, which do not have the stubble persistence of Lontrel. Always check the plant back times and rainfall required for these new products, as they are sometimes longer than 9 months and talk with your local agronomist or product supplier.

Conclusions

Lontrel may persist in both soil or crop stubble. Things to avoid are late season use, with heavy crop canopy, stubble concentration in harvester rows followed by dry planting. Plan rotations to include canola or cereals if unsure about Lontrel carryover or not. Check organic matter levels, encourage microbial activity, and avoid use on non-wetting soils. Manage grazing in systems to avoid surface sealing that limits water movement to depth. If in doubt, plant a test crop and get soil or stubble analysed for Lontrel residues.

References

Wells, G. (1999). Herbicide plantbacks are shorter in northern Australia. In Proceedings 12^th Australian Weeds Conference, Hobart, pp. 312-315. Tasmanian Weeds Society, Hobart.

Wells, G. (2003). Safe recropping after Lontrel in winter cropping systems in southern Australia. In Proceedings 13^th Australian Weeds Conference, Perth, pp. 357-360. Plant Protection Society of WA, Perth.

Congreve, M. and Cameron, J. (2018). Understanding post-emergent weed control in Australian farming systems – a national reference manual for agronomic advisers. (GRDC publication Australia – ISBN 978-1-921779-59-6).

Contact details

Greg Wells
Corteva agriscience
PO Box 838, Sunbury, Victoria, 3429
Email: wells1@dow.com

® Lontrel, Paradigm, and Arylex are trademarks of Dow AgroSciences, DuPont or Pioneer and their affiliated companies or respective owners.
® Talinor is a trademark of the Syngenta Australia Pty Ltd.
® Velocity is a trademark of the Bayer Group.

Was this page helpful?

YOUR FEEDBACK

To protect your privacy, please do not include contact information in your feedback. If you would like a response, please contact us.