Use of recombinant bovine somatotropin in Wisconsin more than doubled between 1995 and 1999. About 15 percent of the dairy cows in Wisconsin are now treated with rBST, and about 15 percent of the state”s dairy farms have adopted the technology, according to researchers at the UW-Madison”s College of Agricultural and Life Sciences.

Farms with larger herds have been much more likely to use rBST than smaller farms. More than 70 percent of herds with more than 200 cows now use rBST. Larger farms have consistently been more likely to adopt rBST since its introduction in 1994, according to Marcy Ostrom, an outreach specialist at the college”s Program on Agricultural Technology Studies.

On farms with fewer than 50 cows, which still comprise more than 40 percent of the dairy farms in Wisconsin, just 4 percent of the operators are using rBST. About 10 percent of farms with 50 to 99 cows used rBST in 1995, rising to 16 percent in 1999. On farms of 100 to 199 cows, use went from 21 percent to 31 percent, according to Ostrom.

Adoption rates may be highest among large farms because their management practices maximize the production benefits of rBST, Ostrom says. These practices include confinement feeding, computerized production records, total mixed rations, and ration balancing.

In 1997, farmers who used rBST reported using it on 48 percent of their cows. Combining the larger average herd size of adopters with the 48-percent use rate indicates that about 15 percent of the cows in Wisconsin are now being treated with rBST. Presuming a 10 percent increase in milk supply from use of rBST, the hormone may be accounting for less than a 2 percent overall increase in Wisconsin milk supplies, according to Ostrom.

Nationally, because of higher adoption rates elsewhere (especially among large Western herds), rBST has probably caused a 4-percent to 5-percent increase in the milk supply. However, improved management and breeding have also increased dairy herd productivity. The additional milk from rBST use is, by historical standards, relatively minor, Ostrom says.

Lower than expected adoption rates have eased concerns that rBST will cause major price declines. However, given recent increases in price volatility in dairy markets, the supply-augmenting effect of rBST use may be contributing to greater downward price swings, according to Ostrom.

This analysis is based on data gathered by PATS in regular surveys of Wisconsin farmers from 1994 to 1999. For a copy of the report, The use of rBST in America”s Dairyland: an update, contact Nancy Carlisle at (608) 265-2908, carlisle@ssc.wisc.edu.