A weed that is showing up more and is being blamed on direct seeding is dandelion. This weed is well adapted to field conditions when there is no tillage. In order to set a management practice to control dandelion in a direct seeding system we must first understand it's growing habits.
Dandelion is a perennial that grows from a taproot many feet long. It reproduces from seeds to produce new plants while the original plant sends up new shoots from the root crown. The seeds are produced throughout the growing season and germination of these seeds can occur anytime throughout the summer. At any particular time of the year you may have newly germinated dandelion plants less than 1 inch in diameter as well as large perennial plants greater than 1 foot in diameter. Once dandelions have developed into a large perennial plant, shallow tillage will not control them. All that happens is the top of the plant is cut off and it regrows from the crown.
In a direct seeding system your control options are: Preseeding, Incrop, Preharvest and Post harvest.
Preseeding: Rob Neyedley, Product Development Representative from Monsanto, has been looking at rates of Roundup required for dandelion control. He has found that 1 litre per acre of Roundup as a spring preseeding treatment gave good control of dandelions up to 6 inches in diameter. When he added 2,4-D amine at 6oz. active/acre he did not see any increase in control. Neyedley also used 1 litre of Rustler, this is the same as 0.4 litres Roundup + 2 oz active Banvel per acre. This treatment gave unsatisfactory control, less than 50%. Spring rates of 1.5 - 2.0 litres of Roundup per acre were required to get any control of the dandelions larger than 6 in. in diameter.
Incrop: Topgrowth control or suppression can be achieved in cereal crops with 2,4-D or MCPA. However there is no registered control for dandelions in pulse crops, oilseed crops or other specialty crops.
Preharvest: Roundup at 1 litre per acre provides excellent control of dandelions, 87%, 10 to 12 months after treatment, according to Monsanto's research. At the Indian Head Experimental farm Dr. Doug Derksen found similar results. Using preharvest Roundup at 1 litre/ac. on lentils and wheat in the 1992, 1993 and 1994, he achieved a 90% + control. When he added a postharvest 2,4-D application he showed an improvement in control of close to 100% the following spring as this would control late germinating seedlings. Both Derksen and Neyedley found reduced control in crops with a heavy canopy. Neyedley recommends the use of water volumes of 7 to 10 gal/acre when spraying cereals or canola with heavy canopy cover. This will improve coverage of the Roundup and improve control of the dandelions.
Post Harvest: Dandelions are a low growing plant and are usually not disturbed with swathing or combining. Most of the leaves are still intact after harvest and Neyedley has found excellent control with 1 to 1.5 litres per acre of Roundup in a postharvest application, providing the plants are not covered with dust or straw.
If dandelions were not controlled this fall the following steps should be taken next spring on your direct seeded fields.
This may be a bit costly in the first year, but once we get the large plants under control the smaller ones that are usually seedlings are not as tough to manage. Spring applications of 0.5 to 0.75 litre of Roundup per acre should control seedlings less than two inches in diameter.