Survey for weeds in December through January on a field-by-field basis. Be sure to check adjacent fields as they may have very different weed populations due to cropping history or soil type. In the Central Valley most winter annuals start germinating in late September or October and continue to germinate until late January whenever soil moisture and temperature conditions permit.
Weed surveys help in making decisions about weed management activities, including herbicide choice and cultivation practices. Information collected over a period of years tells you how weed populations are changing and how effective your management operations have been.
Weed identification
Identify common winter annual weeds and perennial weeds. The most dangerous winter annuals to livestock are fiddlenecks and common groundsel.
How to survey
Walk through each field in a random pattern, rating the degree of infestation for each weed species on your weed survey form. Use either a numeric scale of 1 to 5 (1 being the lightest, 5 being the heaviest), or rate weed species as "light," "medium," or "heavy."
- Check fencerows, ditch banks, field edges, and wet spots as these may be problem areas for weed growth.
- Check areas around field edges as these areas could be a potential source for wind-disseminated seeds.
- Pay particular attention to perennial weeds.
- Sketch a map of the field and mark areas with major weed infestations.
- Indicate the growth stage of the weed (seedling or mature).
- Record results on an established stands weed survey form .
The need for treatment depends on weed species, their competitiveness, the potential market for the alfalfa, and toxicity to livestock. In addition, vigor of the alfalfa stand is a complicating factor; weakened stands will require treatment when denser ones won't.