Summary

Demand for pulse-based protein is one of the drivers of increased pulse production with field pea and faba bean being used to produce high protein flours and food ingredients. Herbicide tolerant (HT) pulse varieties are now widely available in Australia and as pulse production increases, so does the potential risk of herbicide residues impacting future international commodity market access. This project examined differences in herbicide metabolism between pulse genotypes and identified circumstances under which protein fractionation concentrated herbicide residues

Background

Different pulse crops break down herbicides at different rates so residue levels in grain may vary. Plant-based protein markets are using HT pulse varieties for protein isolate production. This is the process of extracting protein from plants to create a highly concentrated powder used in products such as plant-based meat or protein shakes. The impact of these processes on imidazolinone (imi) residues is not well understood.

Although herbicides applied to HT pulses must meet approved maximum residue limits (MRL) in grain, if the process used to extract proteins concentrates herbicide residues in the final protein product, there is a risk levels will be too high.

The MRL of a herbicide varies between countries and is subject to change. Identifying genetics and processing methods that reduce herbicide residues in grain will help reduce market access risks and give SA farmers the confidence to adopt weed management strategies that incorporate HT pulse varieties.

Research Aims

The core objectives of the project were to:

• Identify natural genetic variation in herbicide metabolism dynamics to identify cultivars with reduced herbicide residues in harvested grain.
• Investigate whether herbicide residues are retained or reduced in plant-based protein extraction processes.

In The Field

To examine residue levels, three lentil lines PBA Flash (herbicide sensitive), GIA Metro (metribuzin-tolerant) and SP1333 (metribuzin-tolerant) were grown in pots in a controlled environment room and exposed to metribuzin.

Three metribuzin tolerant faba bean lines (11HKMI12, AF03109 and AF14092) and one sensitive line (1952/1) were grown in pots in a controlled environment room and exposed to metribuzin.
Additional funding from the Yitpi Foundation enabled the inclusion of field pea genotypes from a previous field trial run by the SARDI Agronomy group at Clare. Seed from three PBA Wharton derived imi-tolerant lines (17KAHI002, 17KAHSU007 and 17KAHSU008) and three commercial varieties GIA Kastar (imi-tolerant), GIA Ourstar (imi-tolerant) and PBA Wharton (herbicide sensitive) that had been sprayed with Intercept® (imazamox + imazapyr) herbicide were tested for imi residues.

Fractionation is a process that splits flour into protein, starch and fibre components and can be done with dry or wet methods. To investigate the impact of protein fractionation on imi herbicide residues, faba bean and field pea grain from commercial crops that had been sprayed with Intercept® was cleaned, de-hulled, milled and subjected to dry fractionation processes. Faba bean samples were also subjected to a wet fractionation process that mimicked commercial protein purification.

Results

No metribuzin residue above the limit of detection (0.005mg/kg) was recorded in any of the lentil varieties despite the application rate being six times greater than the permitted label rate for post-emergent use in tolerant lentils.

No detectable metribuzin levels were recorded in the grain of any faba bean genotype.

Results indicate that the risk of metribuzin MRL infringements is low when the herbicide is used according to label direction in the field. However, the inability to detect any residues means genetic variation in the lentil and faba bean genotypes tested could not be identified.

No imazamox residue was detected in any field pea samples above the limit of detection (0.001mg/kg). Imazapyr residues were detected but were all below the proposed maximum residue limit (MRL) of 0.2mg/kg at both application rates. Significantly lower imazapyr levels were detected in 17KAHI002 and GIA Ourstar compared to PBA Wharton and GIA Kastar. Differences may be based partly on genetics as all tolerant genotypes recorded similar tolerance levels and no differences in residue levels were recorded between GIA Kastar and herbicide sensitive variety PBA Wharton.

For faba bean grain subjected to dry fractionation, no detectable levels of imazamox and imazapyr residues were recorded in flour, coarse or fine fractions, fibre, starch or protein.

For field pea grain subjected to dry fractionation, no detectable level of imazamox was recorded in any sample. However, imazapyr residues in the high protein fraction were detected at a concentration of 4.5 times that detected in the low protein fraction, which may warrant further investigation. All detected levels were well below the 0.2mg/kg MRL.

No detectable imi residues were found in any of the faba bean samples that underwent wet extraction.