Answer: Controlling bahiagrass in any scenario is extremely difficult. Bahiagrass reproduces by rhizomes (primarily) and seeds, is very tolerant of grazing, and tolerates droughty conditions. It isn't very cold tolerant, which makes it an ideal pasture species for the Gulf Coast region. Some grasses you can "mismanage" in order to reduce their incidence in the field, but bahiagrass usually thrives on rough treatment. Several chemical means are available that purport to control bahiagrass, but this treatment is extreme, questionable in its efficacy, and results in upsetting the ecology of the field by reducing biodiversity and subsequent long term viability.
Bahiagrass can certainly play a part in a diversified pasture or hayfield system. Being different from bermudagrass, it can express itself at times when the conditions for bermudagrass growth are compromised. This principle of agricultural biodiversity is extremely important for low input systems that rely on nutrient cycling, natural pest immunity, and plant species diversity to maintain ecological balance and productivity in perpetuity.
Monocultures of any crop are problematic from an ecological and often economic standpoint. That being said, market constraints or yield differences often dictate the species of grass grown in an area. Hybrid bermudagrass can yield greater than 10,000 pounds per acre per year, and bahiagrass encroachment can reduce yields and affect per-acre profitability. If a monoculture bermudagrass hayfield is your goal, then setting up a management regime that favors bermudagrass proliferation at the expense of other species is the best bet. Hybrid bermudagrass has been genetically selected for nitrogen use efficiency and rapid, voluminous growth. Careful attention to soil fertility and timely watering would be the first line of defense in selecting against bahiagrass proliferation in bermudagrass. Granted, bahiagrass uses nitrogen and water much the same as bermudagrass does, but hybrid bermudagrass can out-compete bahiagrass given high levels of fertility.
Another method of bahiagrass control would be limited tillage in spots where bahiagrass is dominant. Rhizomes can be tilled, raked up, and removed from the field, and if the spot isn't too large, the bermudagrass should be able to spread into the bare spots. Careful attention should be paid to other invasive species like sand bur, ragweed, carpetgrass, crabgrass, and nightshade. Spot sprigging with bermudagrass sprigs or tops might be appropriate in some areas.
Again, controlling a persistent perennial grass in a warm-season hayfield is very challenging. Short of wiping the field out with glyphosate and starting over, which I do not recommend, you'll need to resign yourself to the slow process of changing the environment so the bermudagrass proliferates at the expense of the bahiagrass.
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