Seed dressing on maize — cultivars and the interaction with pre-emergence herbicides

ELBÉ HUGO, ARC-GRAIN CROPS INSTITUTE

Most maize hybrids are delivered to producers, coated with an insecticide or fungicide, or both, as a seed dressing.

Seed treatments are added to seed to protect seedlings against soil and seedborne diseases and pests, to enhance germination and to promote seedling emergence during the critical first few weeks after planting.

As a standard, maize seed is treated with a fungicide (i.e. Captan® or Celest®) and frequently also with an insecticide (i.e. Cruiser®, Gaucho® or Poncho®), or with both, before planting, to protect emerging seedlings from soilborne fungal diseases and insect pests. Insecticidal seed dressings can also protect stored maize seed from pest infestations, such as grain borers, weevils and grain beetles.

Some seed companies also add nutritional products such as boron and molybdenum to their seed dressings to enhance seedling germination, vigour and emergence. Unregistered or self-mixed seed dressings added to seed on-farm are highly unreliable and is not recommended, but are still being practiced by some maize producers.

Furthermore, the use of registered pre-emergence herbicides for the effective control of grasses and early season broadleaved weeds, are still practiced by most maize producers. Due to these practices, maize producers requested via Grain SA that the interaction between different seed dressings and pre-emergence herbicides be investigated to determine the effects on emergence and crop growth. The participating seed companies have done their respective standard maize seed dressings and the study was done with their consent.

The interaction effect was determined in glasshouse trials on the treated and untreated maize seed of five Pannar and six Pioneer Hi-Bred cultivars. Seed was stored at 15°C in a seed store and trials were done within three months after seed was received.

It is, however, important to note that the aim was not to compare hybrids from the seed companies, but to look at the interaction of herbicides and seed treatments. The seed treatments from the two companies were also not the same, which make hybrid comparisons irrelevant. Seed from Pannar was treated with Celest®, Sodium Molybdate and Cooperfos, and seed from Pioneer was treated with Captan and Poncho®.

Herbicides containing the following active ingredients: acetochlor 700 g ai (Wenner®), acetochlor 840 g ai (Guardian®) and s-metolachlor 915 g ai (Dual Gold®) were applied at the registered label rate and double the label rate. Various growth parameters such as the total number of emerged seedlings, plant height dry mass and visual symptoms of phytotoxicity, were recorded throughout the observation period.

Visual phytotoxicity was observed as skew growth of the coleoptile, curling of leaves, whiplashing of leaves, chlorosis, stunting and malformation of plant parts and was recorded by comparing plants in seed and herbicide treatments with those at the untreated control.

Pannar hybrids

Hybrids with seed treatment (treated) emerged within four days after planting, while those with no seed treatment (untreated) took an average of six days to emerge. Total seedlings emerged was higher for treated cultivars and an average of six plants emerged per cultivar, while only four plants per untreated cultivar emerged.

Phytotoxicity was mostly observed as a tight folded coleoptile, usually dark green in colour and twisting or curling of the whirl. These symptoms were observed only for up to two weeks after maize emergence. Only one hybrid showed significantly more symptoms where seed was treated, while the rest showed no significant interaction between treated and untreated seed. Seedlings, both treated and untreated seed, outgrew these phytotoxicity symptoms, however, at four weeks after emergence of maize and no visual symptoms, other than stunting, was observed ten weeks after emergence of maize.

The percentage visual phytotoxicity was higher for untreated seed (between 17% and 30%) where herbicides were applied and were more sensitive to double herbicide dosage rates. Seedlings from treated seed showed significantly more stunting compared to those with no seed treatment, but stunting was less than 10% where herbicides were applied at registered label rates.

Pioneer hybrids

Hybrid seed treated with Captan and Poncho emerged within five days after planting and faster when compared to seed without treatment. The untreated seed emerged significantly later (approximately six days) where Wenner were applied at the label rate and double the label rate compared to treated seed and the other herbicide treatments.

Visual phytotoxicity was observed as tight folding of the coleoptile (dark green in colour), twisting of the whirl and stunting, but was only observed for two weeks after emergence of maize. Untreated seed showed more phytotoxicity for all hybrids tested (between 7% and 55%).

Pioneer hybrids were less sensitive to double dosage rates and only Wenner, at both dosage rates, showed phytotoxicity that was commercially unacceptable. All these hybrids that were tested, outgrew these symptoms four weeks after maize emergence, and growth was comparable to control treatments at ten weeks after maize emergence when no visual symptoms could be observed.

Significant stunting was only observed for the first four to six weeks after maize emergence where Wenner was applied at double the label rate. However, ten weeks after maize emergence, all seedlings in all the herbicide treatments compared well with control plants and outgrew the stunting effect.

Although this study has tested only a few hybrids from two maize seed companies, the interaction between seed dressings and herbicide applications could be established for these hybrids evaluated. Seedling emergence was positively affected by the various seed treatments of the relevant cultivars tested and the total number of seedlings emerged were higher where seed was treated. Furthermore, all treated seed emerged faster compared to seed that received no seed dressing.

Stunting of seedlings was observed for both Pannar and Pioneer hybrids, but only for the first few weeks after emergence. It is, however, very important to note that all hybrids outgrew these stunting effects, as well as visual phytotoxicity symptoms ten weeks after maize emergence.

Seed treatment with Celest, sodium molybdate and Cooperfos gave severe phytotoxic symptoms as well as reduced dry mass. In a previous study done on seed dressing and storage conditions, it was also reported that Celest in combination with Cruiser, significantly reduced germination of smaller maize seed. Seeds treated with Captan and Poncho showed less phytotoxic symptoms and plant height and dry mass was not adversely affected by the seed dressing and herbicide interaction.

Producers should take note that when any other chemical substance(s), other than what was added by the seed company itself, are placed on seed on-farm, it will negate any claims according to SANSOR regulations. Unfavourable storage conditions (too hot or cold) and rough handling of seed can also influence the vigour and subsequent germination and emergence of seed.

Herbicide damage to crops is rarely only due to the herbicide(s) alone and detrimental environmental conditions (cold and wet weather during application), choice of hybrid (sensitive or tolerant to certain herbicides) as well as inaccurate dosage rates, are important factors playing a role in the severity of crop damage.

Hybrid reactions differ greatly from each other under various environmental conditions and by adding any chemical substance(s) to the equation, crop growth are influenced in most cases by the interactions between all the factors and actions involved in the crop growth cycle.

The interactions between seed dressings, hybrids, pre-emergence herbicides and environmental conditions remain complex and further research, in collaboration with seed companies, is necessary and will contribute to clarify certain aspects perceived in maize production.