Weed control in alfalfa can be complicated, particularly in the low desert areas where the crop never really goes dormant. Fortunately, growers have more options with the recent registrations of new herbicides and the easing of restrictions on others.
“Probably the closest we come to a dormant period is a ‘summer slump’ when the alfalfa has been cut eight to 10 times and just hasn't had time to recover, says Barry Tickes, University of Arizona Cooperative agricultural agent in Yuma, Ariz. “Another characteristic of the low desert is that weeds germinate almost every time you put water on the field, so you get multiple emergence.”
Additionally, the ground often cracks, and weeds emerge from below the surface. That makes weed control even more challenging, because growers need a herbicide that will maintain a barrier to prevent weed emergence.
“A lot of summer annual weeds will grow right through our mild winters,” Tickes says. “Sometimes they will look like they're dead, but then they will reemerge in the spring. A lot of times growers will use pre-emerge herbicides on these weeds and they won't get control and they don't understand why. The reason is the growth is coming from established weeds.”
Another factor could be poor cuticle absorption in the desert.
“I know that sometimes herbicides work in other areas that don't work in the desert,” Tickes says. “Often we can overcome this with surfactants, but then we lose selectivity of the herbicide.”
Rotational intervals are also a very important consideration when choosing a herbicide. During the winter, about 30 vegetable crops are grown in the desert. Alfalfa can be taken out at any time to make way for more profitable vegetable crops if the market looks promising.
“We want the herbicide to work to protect the alfalfa in the ground, but we want it gone completely if the grower rotates to some of these high value, specialty crops,” Tickes says. “That's a tough order.”
That challenge just recently got a lot easier with the registration of Chateau from Valent. “It's got a totally new mode of action in alfalfa,” Tickes says. “It's a PPO inhibitor. That's not a new mode of action in other crops, but it is in alfalfa. It controls both grasses and broadleaves. It has good crop safety and short crop rotational intervals. I'm very excited where this product will fit in the low desert.”
There are 20 herbicides registered on alfalfa in the low desert today, according to Tickes. “That more than we've ever had at any time in the past.”
“There's a perception that the heyday of ag chemicals was in the 1980s and 1990s, and then it was slowly being replaced by biotechnology,” Tickes says. “However, that's simply not true. There have been more products registered in the last eight years than there were in any of the previous decades. Some are different formulations of older products, but they're still new registrations.”
Raptor from BASF is an example. “It's very similar to Pursuit, but has a shorter soil residual and it's much better on grasses and some broadleaf weeds like lambsquarters.”
Roundup Ready alfalfa — if and when it's registered — will have a fit, according to Tickes. However, he cautions growers against thinking it's a cure-all for weeds in alfalfa.
“Roundup doesn't control all weeds,” he says. “Malva is one weed it will not control, and there are others. It has a fit, but it's like any other herbicide. It will select for those weeds that it doesn't control.”
Another consideration for using a Roundup Ready system on low desert alfalfa goes back to the fact that there are multiple germinations of weeds. A system solely on Roundup would require multiple applications at multiple times over multiple years.
“That's a prescription for resistance,” Tickes says. “I don't think we should look at Roundup as a replacement for a diversified weed management program.”
Prowl H2o is a herbicide that had been registered for seed production for several years. In 2006, it was registered for all alfalfa.
“This is a different formulation, Tickes says. “It's also registered to drip in the water. Prowl works remarkably well like that. Where Treflan didn't stay in suspension and didn't move down to the soil very well, Prowl stays in suspension and moves down to the soil very well.”
Another new registration in the low desert is Velpar from DuPont. It has been registered in other areas, but it wasn't considered safe in the desert. However, subsequent work was done and the desert restriction was lifted. Even so, it has to be timed correctly to prevent injury, Tickes says. “The safety with this product is probably not as much as we would like. It has a fit, but has to be used carefully.”
Another new registration is Sandea from Gowan. “It can be used in alfalfa in that summer slump period,” Tickes says. “It's going to be hard to use once we get past mid-September, but it controls a number of broadleaves as well as nutsedge. I think if we use it in the slow growing period during the late summer, it will have a fit.”
Zorial (Solicam) from Syngenta is another product that recently received a label amendment so that it can applied through drip. “It never really worked too well when we sprayed it over established alfalfa because we couldn't get good soil deposition, but through the drip it works real well,” Tickes says.
“The problem with this product is that you can get injury. It can turn the plants white. People are more sensitive to it because the injury is so much more apparent.”