A modest increase in livestock output is under way, and Agriculture Department economists say it could take some of the upward pressure from retail meat prices in 1991.

Early projections by the department's Economic Research Service point to 1991 gains in beef production, up 1 percent from this year; pork, 3 percent; and broilers and turkeys, about 5 percent.A major reason for the stepped-up livestock production is the bumper harvest of corn and other crops this fall, which has helped reduce feed costs. Improved grazing has helped, too.

For example, range and wheat pasture conditions a year ago deteriorated in many areas, pushing larger number of lighter-weight cattle into feedlots.

Prices of stocker and feeder cattle are expected to "remain well above a year ago" this fall, and with strong demand for lighter-weight animals to put on pastures, fewer will be moving into feedlots.

Market prices of Choice-grade steers averaged about $77 per 100 pounds in the first two weeks of September but have been declining seasonally, the report said. One factor has been the record weights of cattle going to slaughter.

Some of that extra weight has been due to producers holding back cattle because of the uncertainty over the federal budget and the possibility of a furlough of meat inspectors.