University of Arizona
With the cooler temperatures of fall and optimum planting times for alfalfa approaching, winter weeds will soon be germinating in established alfalfa fields. It is human nature to think that winter weeds germinate later in the year because that is when they are large and visible along roads and in fields. In reality, they germinate earlier than we often realize. Now is a good time to be thinking about preemergence herbicide options for established alfalfa fields. Some winter weeds such as common groundsel (Senecio vulgaris) are toxic while others impart off-flavors to milk when fed to dairy cows.
Apply preemergence herbicides to alfalfa fields shortly after cutting and removal of the forage. This minimizes the amount of preemergence herbicide bound to leaf and stem surfaces and the loss of weed control this causes. Some preemergence herbicides such as Chateau, Velpar AlfaMax Gold, diuron, hexazinone and metribuzin have contact-burndown activity if mixed with an adjuvant such as a surfactant or a crop oil concentrate. To avoid alfalfa damage do not mix them with these types of adjuvants. Imazethapyr (Pursuit) has both preemergence and postemergence activity and is selective in alfalfa. Other preemergence herbicides such as norflurazon (Solicam), EPTC (Eptam), Prowl H2O, and trifluralin granules do not have significant foliar activity. Velpar AlfaMax Gold, diuron, hexazinone and metribuzin are mobile enough in soil that in warm conditions alfalfa can absorb enough herbicide into the roots with soil water to cause injury (see labels). Chateau, Prowl H2O and trifluralin granules have very limited mobility in soil, just enough to control weeds. Observe the pre-harvest intervals on herbicide labels to avoid illegal residues in the harvested forage. There will always be weeds that escape preemergence herbicide applications so you may need to spray a postemergence herbicide later in the fall. Be sure to consider potential crop rotations and plant-back intervals when using soil active herbicides.
Tickes, B. and M.J. Ottman. 2008. Alfalfa Weed Control in the Low Deserts of Arizona. https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1266.pdf. Accessed September 26, 2017.