FARMING NOTES

Bay of Plenty Beacon, Volume 12, Issue 76, 2 August 1948, Page 3

FARMING NOTES

LATEST INFORMATION ABOUT HORMONE WEED KILLER ON THISTLES The following notes have been supplied by the Fields Instructor, Mr E. R. Marryatt, from reports on the cohtrol of weeds by chemicals during the past year:— California Thistle (Cirsium arvense): In nearly. 40 trials severe damage was caused by hormones either sprayed or The exceptions were a trial where showery weather was thought to have washed off the spray and dust, a trial where application was made to mature thistles which were dying down and a trial in a crop where treatment was applied before emergence of the thistles. Large thistles were resistant to DNOC compounds. Weedone spray was the most effective in 5 trials, but in three trials Agroxone gave |he best results, while in 12 trials out of 20 where both treatments were included (and effective) little difference was seen between th two typs. Effectiveness of treatment was approximately as follows: No effect in 20 per -cent of trials; 50 per cent effective in 25 per cent of trials; effect considerable in 25 per cent of trials; 75-90 per cent effective in 20 per cent of trials; and 100 per cent effective in 10 per cent ' of trials. In all trials the best results were obtained from the heaviest rates of application. Heavy dust applications were as efficient as sprays but generally per unit of active principle, dusts were considerably less efficient. To summarise:— 1. Nearly always the shoot of Californian thistle when treated with hormone weed killers is severely affected or killed. 2. Sinox and other DNOC compounds will check your thistles severely but mature plants usually recover fairly rapidly'. 3. Treatments have been unsuccessful when applied during showery weather, when applied to the mature thistle when lying down, and when used on cultivated land before the emergence of the thistle shoots in the spring. 4. The effect of hormone weed killers the year following treatment has nearly* always been a marked thinning of the weed but complete eradication is unusual. 5. There is no definite superiority of one form of hormone weed killer over another although per unit of active principle, sprays are considerably more efficient than dusts. 6. Rather heavy rates of hormone weed killers are required for satisfactory results: usually about 21b. per acre acid equivalent as sprays (2 gallons 10 per cent spray) or at least twice that rate as dusts. 7. Most of the applications of hormone weed killers were made when the thistle was flowering or approaching that stage. It seems, therefore, that the re J commendations to apply hormone weed killer at or near the flowering stage of Californian thistle is sound. Variegated Thistle (Silybum marianum): Reports are complete in from 8 out of 9 trials with this weed and in 6 trials out of 8, 100 per cent kill of thistle was obtained from hormone weed killers applied at the rates of from lib. per acre acid equivalent as sprays to 101 b. per acre as dusts. A 50-80 per cent kill was obtained in one of the remaining 2 trials from dusts whereas sodium chlorate lime dust (5 per cent at 3 and scwt. per acre gave 100 per cent kills). Variegated thistle thus appears quite susceptible to hormone weed killers and to sodium chlorate. In 2 trials application was made at 3 stages of growth with little ultimate difference. Nevertheless in 3 trials examined a year after application a thick seedling regrowth has appeared on all plots. This will necessitate re-treatment or, preferably some means of thickening-up the sward so that the bare patches left after treatment are covered by pasture grasses and thistle seedlings are prevented from coming away. Early treatment before the plants attain considerable size is also worth while. A noticeable feature with this plant as with many other weeds, was the fact that, if stock were allowed access to plants whose leaves had commenced to wilt as a result of, hormone treatment, they rapidly grazed these plants and the effect of this was sometimes to hasten the kill of the thistles. Scotch Thistle (Cirsium lanceolatum): Practically 100 per cent

kill of Scotch thistles has been obtained in those trials which have been reported on for long enough to be sure and the only trial in which a proportion of this' thistle was not killed was where heavy rain fell three days after dust treatment. Scotch thistle is very susceptible to hormone weed killers at all stages of growth. As the plant is a biennial and naturally dies down after seeding it would seem that application to the rosettes of the first year is the only worth while time. Nodding Thistle (Carduus nutans): In all trials 100 per cent kill of nodding thistle has been obtained except for a 70 per cent kill in a trial where showers • fell shortly after treatment. As it is a biennial thistle similar remarks apply as to Scotch thistle. Winged Thistle (Carduus pycnocephalus): Only two out of three trials on winged thistle were satisfactory. In both of these trials treatment with hormone weed killers were made at flowering and in one trial 100 per cent kill was obtained but in the other where the thistle was well advanced, wilting only occurred. Anyway, winged thistle is an annual and early treatment of newly established plants is the only sensible time to apply the weed killers.