Reducing herbicide rates can save Wisconsin corn and soybean producers $10 per acre, say researchers at the University of Wisconsin-Madison. Reduced rates of post-emergent herbicides can control giant foxtail, pigweed, and lambsquarters without affecting yields, when combined with rotary hoeing or cultivation.

“The key is putting a management package together,” said Chris Boerboom, extension weed specialist at the College of Agricultural and Life Sciences. Generally, herbicide rates can be cut in half when weeds are half the maximum size listed on the herbicide label. Then one cultivation will often control weed escapes until the canopy closes. However, if weeds can”t be sprayed early, farmers should use the full rate, Boerboom said.

For example, in corn, giant foxtail and lambsquarters could be treated with a half-rate herbicide program before they are three inches tall. Herbicide costs for this program will be about $15 per acre, plus application costs. Cultivation would add another $4.50 per acre. A herbicide program that follows the labeled rate will cost $30 per acre plus application costs, and mechanical control may still be needed.

In fact, according Boerboom, integrating mechanical control with full rates is just as important as with reduced rates to delay the development of herbicide resistant weeds.

“If we are relying on full rates without herbicide rotation or without any cultivation we are more likely to see the development of herbicide resistant weeds,” said Boerboom. “No herbicide resistant weeds are resistant to the steel of a cultivator.”

Reduced rates work because herbicides rates are selected to perform over a variety of climatic conditions, several weed species, and heavy weed pressures.

“Compared to other climates, Wisconsin”s climate provides adequate rainfall and decent temperatures for actively growing weeds at the time of application, increasing the efficacy of herbicides,” said Alvin Bussan, weed science graduate student.

However, the researchers recommend using reduced rates only on more easily controlled species, not on perennials or tough annuals such as wild proso millet and woolly cupgrass. Fields chosen for the program should also have good crop stands and a history of low weed pressure. Drilled beans will do better under this program than row beans, because the canopy closes faster, said Bussan.

One risk when using reduced rates is that herbicide companies will not guarantee the performance of their products. However, farmers skilled in integrated crop management can adjust a reduced-rate program to lessen potential risk on their farms.

“Growers can adapt the program to their farm conditions, and vary rates for just broadleaf or just grass herbicides or for certain fields.” said Boerboom. “There is an endless combination of options depending on labor, time and risk management.”

Boerboom and Bussan base their recommendations on previous CALS research, which showed that corn and soybean yields were the same with reduced rates as with labeled rates. They have also completed research on pigweed, giant foxtail, and velvetleaf seed production after reduced pre- and post-emergence herbicide rates at the Arlington Agricultural Research Station and the Rock County Farm in Janesville.

The researchers found that half rates reduced weed seed production similar to full rates on pigweed and giant foxtail in corn and soybeans. Velvetleaf was slightly harder to control with reduced rates.

Even without the recommended cultivation, weed seed production with half rates was reduced the same as with full rates in pigweed (90 percent) and in giant foxtail (98 percent). Velvetleaf seed production was reduced 88 percent in corn and 72 percent in soybeans with half rates, while full rates averaged a 98 percent reduction in corn and soybeans.

“Increased weed pressure after using reduced rates doesn”t appear to be a problem based on our weed seed data,” said Bussan. “Wisconsin farmers should test reduced rates to see how well they fit into their operations.”