Recent Research Work

New Zealand Journal of Agriculture, Volume 96, Issue 2, 15 February 1958, Page 132

Recent Research Work

RESIDUAL EFFECTS OF WEEDKILLERS

SOILS to which weedkillers have been applied may remain toxic to plants for periods varying from a few days to several years according to the type and quantity of weedkiller used. Soil type and rainfall also influence the period of toxicity. Sometimes the toxicity is selective; that is, some plants are affected and others are not. Knowing what effects weedkillers have in the soil is therefore important, and these notes describe results of trials at the Department of Agriculture's Rukuhia Soil Research Station to determine the residual toxicity of several weedkillers.

TRIALS WITH TCA AND CIPC

TCA and CIPC were each sprayed at two

rates on cultivated plots in trials on both a heavy soil (Te Kowhai clay loam) and

a light soil (Horotiu sandy loam). The rates were 201 b. and 801 b. per acre of TCA and 21b. and 81b. per acre of CIPC. Each treatment was applied to different plots in October, November, and December. Two days after the December applications all plots were sown with a mixture of 301 b. of perennial ryegrass and 31b. of white clover per acre. Rainfall in the 2 months from the October application to the sowing in December was sgin. and a further 4in. fell in the 7 days after sowing.

RESULTS WITH TCA

A normal ryegrass-white clover sward, similar to that on the untreated, control plots, became established on all plots to which TCA had

been applied except where 801 b. was applied 2 days before sowing. Even this treatment had little effect on the pasture sward; there was a slight reduction of ryegrass on the light soil and of clover on the heavier soil. TCA is soluble and presumably the heavy rain which fell several days after sowing leached it out of the top layers of soil before it had much effect on the seedling ryegrass and clover.

EFFECTS OF CIPC

The residual effects of CIPC were more spectacu-

lar, particularly on the heavy soil. A pure white clover sward without a trace of ryegrass became established on most of the treated plots. Appli-

cations of 81b. of CIPC at each of the three times and the 21b. application in December completely inhibited ryegrass on both soil types. On the heavy soil there was a slight trace of ryegrass when 21b. was applied in October, but none after the November treatment. Ryegrass became established on the light soil when 21b. of CIPC was applied in October and November, but compared with the results in untreated plots, it was slightly and severely reduced respectively.

Three weeks after sowing there was very much less seedling white clover on the CIPC plots than on the untreated or TCA plots, particularly where the CIPC was

applied immediately before sowing. Even when applied 2 months previously the 81b. treatment had severely reduced the clover. However, 3 months later there was luxuriant, dense growth of white clover on all the CIPC plots except where 81b. was applied immediately before sowing. On these plots there was some bare ground and slightly less clover. From these trials and recent experiments at the Department’s Seed-testing Station, Palmerston North, which showed that CIPC reduced the germination of white clover, it is assumed that the good clover swards largely resulted from the late germination of hard seeds and that because there was no competition from grasses, any clovers which survived the CIPC made vigorous growth.

PRE-EMERGENCE APPLICATIONS OF CIPC AND MONURON

Further information on the residual

toxicity of CIPC and the toxicity of low rates of monuron (formerly called CMU) was obtained from

trials in which various rates of

these materials were applied as pre-emergence weedkillers with different crops. The duration of the residual toxicity varied according to the weather. Some of the trials were in dry seasons and the stage at which the weeds infested the treated plots depended on when there was sufficient rain to germinate weed seeds after the toxic effect of the weedkiller had worn off.

CIPC at 21b. per acre or less did not give good weed control even for short periods. Applications of 41b. kept the ground free of weeds for about 5 or 6 weeks, but after 8 weeks there was some reinfestation of weeds. CIPC at 81b. per acre gave good weed control for 8 weeks or longer.

Rainfall also had a marked effect on the toxicity of CIPC to onions. In a dry season 61b. had no adverse effect, but under high-rainfall conditions lower rates killed most of the onion seedlings. A pre-emergence application of 21b. of CIPC in April 1956 had, within 3 weeks, reduced the number of onion seedlings to 65 per cent, of the untreated controls, and at 16 weeks the figure was 22 per cent. Higher rates of CIPC caused more damage. The rainfall that season was exceptionally high. In the first 3 weeks after the CIPC was applied it was Bin. and a further 24in. had fallen at the end of 16 weeks.

RESULTS WITH MONURON

The residual effect of monuron, like that

of CIPC, varied according to the rainfall and the season. When the two

materials were each applied at rates

normally tolerated by vegetable crops, the period of general toxicity to weed seedlings in the surface layers of the soil was much longer with monuron than with CIPC. However, the otherwise good weed control obtained with monuron was often spoilt by the tolerance of certain weeds, such as paspalum, plantain, ribgrass, and scarlet pimpernel, to monuron. If the experiments were conducted when and where these species normally germinated, the plants frequently appeared at an early stage on plots treated with monuron when the soil was still too toxic for the survival of most germinating weeds. These variable factors make it difficult to summarise results, but generally rates of Ijlb. to 41b. per acre of monuron have given good weed control for 10 to 16 weeks.

In the high rainfall of autumn 1956 monuron was also very toxic to onion seedlings. Pre-emergence applications of 1-glb. and 31b. both killed 75 per cent, of the onions in 16 weeks.

CONCLUSIONS

A high rate of TCA, 801 b. per acre, which

normally would be very toxic to seedling

grasses had very little effect when very heavy rain fell a few days after the TCA was applied. Therefore adverse weather could easily cause poor results with the smaller quantities recommended for specific weed problems, that is, up to 151 b. per acre for the control of barley grass or 201 b. per acre for weed control in brassica crops.

—F. B. THOMPSON