The influence of environment, season, plant growth stage and herbicide formulation the activity of glyphosate applied to Lolium perenne

Abstract

The behaviour and efficacy of glyphosate (as Roundup) applied to Lolium perenne L. cv. Grasslands Ruanui, and the influence of environment, season, plant growth stage and herbicide formulation has been studied.

A preliminary investigation was conducted in controlled environment conditions to establish the phytotoxicity, uptake, translocation and metabolism of glyphosate applied to L.perenne. Full control of vegetative plants was obtained with spray applications of 0.4 kg glyphosate ha⁻¹. Vegetative growth size, and contrasting growth temperatures of 6/12ºC or 12/18ºC, did not generally affect glyphosate performance, but significant tolerance to 0.4 kg ai ha⁻¹ was observed by plants grown in cool conditions that had initiated reproductive development. Plants absorbed 38-47% of ¹⁴C-glyphosate applied to the youngest fully expanded leaf after 10h and 63% after 123h. This was distributed in a typical phloem translocated manner from source to sink, and after 123h, 23-28% of applied ¹⁴C-glyphosate was recovered from the roots, 13-16% from the daughter tillers, 8-10% from the upper leaves plus stem of the treated tiller and 0.5% from the lower leaves. Chromatographic separation revealed no major ¹⁴C-metabolites although 24% of applied ¹⁴C-glyphosate was not recovered after 123h.

A controlled environment experiment was conducted to examine the influence of relative humidity (rh) and temperature close to herbicide treatment and during plant growth on glyphosate behaviour. Over twice as much ¹⁴C-glyphosate was absorbed by plants treated at 80/91% rh than at 48/52% rh. Preconditioning plants in high rh condltions during growth also improved their potential to absorb ¹⁴C-glyphosate. The relative partitioning of ¹⁴C-glyphosate translocated from the treated leaf was not affected by rh. Relative humidity treatments which most favoured uptake also gave the best control of plants when conventionally sprayed. Thus treatment and/or growth in high rh increased plant susceptibility to glyphosate. Temperature during treatment had a less marked effect on uptake than rh. Preconditioning plants to temperature during growth had no effect on their ability to absorb ¹⁴C-glyphosate. Tiller regrowth after spraying was not affected by temperature close to treatment, but was significantly increased by growth and recovery in warm conditions.

Seasonal tolerance of pasture in the field was observed to conventionally sprayed glyphosate during spring in two trials. Tolerance lasted for approximately 2 months and coincided with reproductive development and the seasonal increase of pasture growth during spring. Mowing pasture 14 days before spray application and the addition of 0.5% Silwet L77 surfactant to the spray formulation both considerably improved herbicide performance in the field. Spring tolerance was not attributed to variations of environmental conditions close to spraying, including, plant available water, aerial moisture, temperature, and solar radiation; or variations of spray contact and retention; or ¹⁴C-glyphosate uptake and metabolism. Both vegetative and reproductive plants exhibited similar uptake and translocation of ¹⁴C-glyphosate from the treated leaf, when it was applied in controlled environment conditions. However, reproductive plants partitioned more translocated ¹⁴C-glyphosate to the stem plus apex than vegetative plants.

There were some major differences in partitioning of ¹⁴CO2- labelled assimilate and ¹⁴C-glyphosate during the transition from vegetative to reproductive development. Partitioning of ¹⁴CO2-assimilate to the developing reproductive stem plus apex increased from 7% to 59%, while that to the roots declined from 27% to 9%. Over the same period, partitioning of ¹⁴C-glyphosate to the stem plus apex rose from 6 to 26%, while the roots remained a major sink at all times, and partitioned 42- 45% ¹⁴C-glyphosate. The relative shoot:root partitioning of ¹⁴C-glyphosate was greater in summer and autumn when plants were vegetative, than during the transition from vegetative to reproductive growth in spring.

The mode of action of Silwet L77 in overcoming spring tolerance was investigated. Contact angle of glyphosate spray solution on the upper leaf surface was reduced from 118º to 0º by 0.1% Silwet L77 and this was correlated with rapid initial uptake of ¹⁴C-glyphosate in the presence of Silwet L77. However, 0.1% Silwet L77 failed to improve the total quantity of ¹⁴C-glyphosate absorbed or translocated from the treated leaf. Silwet L77 did not influence wetting or uptake by the lower leaf surface. Spontaneous penetration of spray solution through stomatal pores was observed in the presence of 0.5% Silwet L77. However, 0.5% Silwet L77 reduced the percentage of applied ¹⁴C-glyphosate absorbed after 75h from 42% to 25% and reduced the percentage of absorbed ¹⁴C-glyphosate that was translocated from the treated leaf from 66% to 39%. It was suggested that mass flow of spray solution into the leaf disrupted the physiological process involved in glyphosate movement from the treated site. Silwet L77 did not influence the relative partitioning of ¹⁴C-glyphosate translocated from the treated leaf. The addition of 0.1% Silwet L77 reduced the post spraying period for which glyphosate performance was susceptible to the detrimental effect of simulated rainfall from 10h to 2h.

It was concluded that spring tolerance of L. perenne to glyphosate was promoted by insufficient distribution of glyphosate to the tillering system. This was caused by the tendency for glyphosate to accumulate in the roots as opposed to the treated shoot, even during reproductive development when partitioning of photoassimilate to the shoot increased dramatically at the expense of that to the roots. Increased production, partitioning and investment of photoassimilate in the tillering system during spring was suggested to effectively 'dilute' glyphosate at its possible site of action. Other factors may have contributed to tolerance during reproductive development, including increased separation between intercepting leaves and target sinks during stem extension, competition between reproductive tillers, vegetative tillers and daughter tillers for assimilate (including glyphosate), and shading of daughter tillers from spray interception by the tall reproductive canopy.

Silwet L77's action was attributed to its effectiveness in overcoming the highly repellent nature of the crystalline wax on the upper leaf surface, enabling improved distribution of spray solution and rapid uptake of glyphosate. Rapid penetration was considered particularly beneficial when adverse environmental conditions, such as rainfall, occurred soon after spraying.