The value of U.S. agricultural exports is expected to jump 22 percent to a four-year high of $34 billion this fiscal year, up from $27.9 billion in 1986-87, the Agriculture Department said Thursday. In terms of actual quantity, shipments were forecast at 146 million metric tons in the year that will end Sept. 30, up 13 percent from last year. Both the value and quantity estimates were up from the previous USDA forecasts on May 26 of $33.5 billion and 145.5 million tons. Although the report cited no figures for subsequent years, it said export values may increase in 1988-89 due to higher prices but the volume of shipments may decline. The report said the rise in estimates since May stemmed from larger-than-expected exports of animal products, slightly larger subsidized wheat exports, and higher corn prices due to the drought. Noting that 1987-88 exports are now expected to be up $6.1 billion and 16.8 million tons, the report said, ``This largely reflects reduced competitor supplies; higher prices for grains, oilseeds and cotton; and increased U.S. export competitiveness'' helped by a lower-valued dollar and the Export Enhancement Program, or EEP. Under EEP, selected countries _ including the Soviet Union and China _ can get wheat and other designated commodities subsidized by the U.S. government. The new forecast showed the value of grain exports at $12.2 billion, up from $11.8 billion expected in May and 1986-87 shipments worth $9.33 billion. Oilseeds, mainly soybeans, were forecast at $7.6 billion, down slightly from $7.7 billion indicated three months ago. But that was up from $6.47 billion last year. Exports of livestock products this year were raised to $4.7 billion from the May forecast of $4.5 billion. In 1986-87, those were valued at $3.96 billion. Other major categories included: horticultural products, $3.7 billion this year, the same as indicated in May but up from $3.15 billion in 1986-87. Cotton export values were shown at $2.2 billion, the same as the earlier forecast. Last year's cotton shipments were worth $1.43 billion.