13th October 2001

By Andy Doyle

With the maize harvesting season now under way, I grabbed the chance two weeks ago to take a look at the maize research work being conducted by Jim Crowley at Teagasc in Oak Park, Knockbeg.

Much of this work involved varieties, sowing date and plastic systems.

From these trials it was obvious that early sowing under complete plastic cover reduced plant height in early July and also final plant height compared to the punched plastic system for all varieties.

Normally the use of plastic with maize increases plant height and the later the sowing date, the taller the crop.

But in this trial the variety Loft sown early under complete plastic was shorter at maturity than the plot with no plastic at all.

The final plant population was also reduced in some varieties (Passat) where the complete cover plastic was used.

But the complete cover plots were more advanced and ready for harvest earlier. Whether or not the plant height and plant number effects depress crop yield remains to be seen.

This plant height depression has also occurred in occasional commercial crops this year.

Varieties, plastic and sowing date

Three sowing dates were used in this trial - April 23, May 15 and May 25. And three varieties were compared - Passat, Loft and Justina - using either no plastic, punched plastic (sown under complete cover and then punched to allow the plants through) and complete plastic cover.

The visible consequences of the complete plant cover with the 'early' April 23 sowing date were stark, especially with Loft and Passat.

Measurements on July 4 showed the complete cover to be shorter than the punched plastic treatment but still taller than the plants grown without plastic cover.

This held for all three varieties but Justina was taller in all treatments.

This height difference was also translated into plant dry matter weight on that date. Despite Justina's greater height, its plant dry matter weight was lower except in the complete cover system where Justina had a higher dry weight than Loft.

This early season plant height difference carried through to final plant height at maturity where the difference between Loft and Justina in the complete cover treatment was 0.35m.

With no plastic and punched plastic, Justina was only 20cm taller. And even with Justina, the height at harvest was the same as the control and lower than the punched plastic.

But with Loft the final crop height was much shorter than both the no-plastic and the punched plastic treatments. These findings indicated that Loft suffered in its efforts to burst through the complete cover treatment.

These findings showed differences in the ability of varieties to cope with complete cover plastic.

Justina seemed to be able to burst through quicker but Loft eventually got through to the same degree. Passat never reached full plant penetration through the plastic.

The effects on yield remain to be seen. The cob dry matter was higher in the case of Loft than Justina where plastic was used but then it is an earlier variety.

Complete cover treatment produced higher cob dry matter levels for the late April and mid May sowing dates.

The earlier the sowing date, the higher the cob dry matter content, irrespective of the system used.

In 2001 it took four weeks for the last of the plants to burst through the plastic and some did not make it at all. This compares with a two week period to complete the same process in 2000.

In the variety Passat almost 10 per cent of plants failed to make it through the plastic but all of the Loft and the Justina made it through.

With many crops sown under complete cover plastic now harvested, it is obvious that the plastic has proven more durable this year. Whether this is a plastic effect or something to do with the weather, no-one knows, but it is of concern.

On balance it would appear that the complete cover, especially in the early sown plots, had adversely affected plot height and appearance and may affect crop yield.

It would appear that a lot of energy was expanded in trying to break through the plastic and this may have affected plant height.

It may also be that the physical characteristics of young plants of Justina make it better able to burst through the plastic.

Herbicides

In another trial Bryan Mitchell is looking at the potential efficacy of herbicide combinations for spraying onto a punched plastic scenario.

No herbicide was applied to the soil before the plastic was applied and so any weed control found is a direct result of the over-spray post emergence with the plastic still intact.

Regardless of the specifics of each combination. a common thread was obvious - any combination that included atrazine worked while those that had no atrazine produced very poor or no control.

It would seem that the use of atrazine provided the means by which the herbicides penetrated the plastic to kill the weeds. And the addition of oil to any of the atrazine combinations resulted in reduced control.

Looking at the plots it was obvious from a distance what treatments controlled the weeds and what did not, without knowing the specific treatments.

Where the weeds were controlled, the plots were green and healthy. Where the weeds were not controlled, the plots were anaemic looking and yellow, and the plants were shorter as a result of competition from the weeds.

These early visual results seem to indicate that it is possible to kill weeds beneath the plastic in a punched plastic scenario.