Author: Albert Gallagher

Publisher:

Published: 1972

Total Pages: 0

ISBN-13:

Field experiments throughout Northern Alberta established picloram (4-amino-3,5,6-trichloropicolinic acid) as an excellent herbicide for the control of Canada thistle ( Cirsium arvense (L.) Scop.). Rates of 8 to 12 oz/A were sufficient to consistently reduce thistle stands under all conditions. Preplant herbicide treatments were tolerated by creeping red fescue ( Festuca rubra L.) and timothy ( Phleum pratense L.) if one growing season elapsed between treatments and planting of the grasses. Seeding of the grasses three weeks following herbicide application resulted in reduced grass stands and dry matter production in several instances. Seedling stands of creeping red fescue treated at the 1- to 5-tiller stage grew and developed better where thistle control was provided by single applications of picloram at 4,8, and 12 oz/A or dicamba at 24 oz/A. Fertile tiller production was increased substantially in plants selected from plots treated in the field with 4 and 12 oz/A of picloram at the 3- to 5-tiller stage, and transferred to a growth chamber. Seed production of timothy was not affected by 2 to 12 oz/A of picloram when applied at several growth stages. Dicamba and 2,4-D treatments were tolerated best at the post-flowering stage of growth. Creeping red fescue seed production was reduced by most herbicide treatments applied at 10 per cent anthesis in 1969. Plants receiving 8 and 12 oz/A of picloram in 1969 were prostrate and seed head numbers were reduced in 1970. In no instance did herbicide treatment result in more than a 9.4 per cent reduction in percentage germination of the seed from either grass species when compared to the untreated check. Dicamba applied on timothy two weeks after the peak flowering period did not significantly reduce the seed germination but seedlings produced were prostrate and bent. Picloram at 2 oz/A had no effect on seed germination irrespective of the growth stage of the plants at treatment time. In greenhouse experiments with picloram and dicamba-treated soil, creeping red fescue germinated and emerged more readily than did timothy. Surviving timothy plants grew and developed more readily, however, than did creeping red fescue plants. Dissipation of dicamba under field conditions was complete by the end of one growing season. Picloram soil residues which were phytotoxic to cucumbers were detected at all locations and at most rates of application after more than one growing season had elapsed. Dissipation of picloram was greatest at the St. Albert plot and this was attributed to the high organic matter content of the soil. The use of picloram or dicamba for thistle control in grass seed fields is feasible. Picloram applied to actively growing thistles in the bud stage on a spot application basis and prior to seeding at rates of 4 to 8 oz/A followed three to four weeks later by intensive cultivation is recommended. Thistle regrowth can be treated when seedling grasses are past the three to four-leaf stage with a mixture of 4 oz/A of dicamba and 8 oz/A of MCPA amine or 2,4-D amine.