The growth cycle is coming to an end and the leaves are in full flush. This and still active growth are important when using systemic herbicides.
Plants absorb this type of herbicide through their leaves and translocate it through their system to growing points.
When they have stopped growing due to colder temperatures in winter or are stressed by drought or water logging translocation happens at a very slow rate and spraying will most likely be ineffective.
Our two problem pasture weeds here are native tussock and bracken. A distant third is snake vine but it is nowhere near the problem of the other two.
For tussock we spot spray with glyphosate, more commonly known as Roundup. Since Monsanto's patent ran out on their product in 2000, there are a myriad of equivalents on the market, many from China. As a result the price has fallen through the floor.
It takes around a week for the evidence of 'a kill' to become obvious so it is easy enough to do a second sweep to take care of tussock missed during the first.
For bracken we use metsulfuron-methyl more commonly known as Brush-Off made by DuPont. Again their patent has run out and what used to be a hellishly expensive chemical is readily available from many other suppliers at a quarter of the price it used to be. I guess when I say expensive it's worth noting the application rate is only 10g/100L so a little bit goes a long way.
It is a systemic compound with foliar and soil activity, and it works rapidly after it is taken up by the plant. Its mode of action is by inhibiting cell division in the shoots and roots of the plant.
It is very effective on weeds that include bulbs or tubers. Bracken grows from rhizomes.
The major problem is it takes a long time for 'a kill' to become obvious. The first symptom is the upward curling of leaves about two weeks after application but many weeks before browning off occurs. As a result you have to concentrate on getting good overall coverage the first time around which is not always easy when in a big patch. I have tried dye markers in the spray to show me where I have been but always ended a lovely shade of pink on the face which takes a long time to wear off.