It may not have been a surprise 48th birthday party for Agriculture Secretary John Block at the Madison Hotel last night -- but there were two unexpected developments.

First, the entertainment.

After a subdued dinner of veal piccata, Jack "Kenny Rogers" Block astounded most of the 200 guests with his very own version of "Lucille" and "The Green, Green Grass of Home," and a half-hour medley of other country favorites. The birthday crowd, hosted by the Cooperative League of the USA, loved it, even at the late hour of 11 p.m. They whistled and hollered as though they were at a regular hoedown.

"The more you give him to drink, the better he gets," joked a White House aide, who thought better of allowing his name to be used.

Block, who has been singing for his family for the past 20 years, rarely takes his show public.

"Until I came to Washington I even used to be afraid to speak publicly, never mind sing country," said Block, who is an Illinois hog and grain farmer. "But after two years as secretary of Agriculture, I am not afraid of anything now."

Second, the phone call.

Around 8:30, the little white Princess phone on the dais started tinkling. Everyone knew who that was. But they feigned surprise.

"Now Jack, I know I caught you out socializing tonight," President Reagan's voiced boomed over the speakers. Everyone laughed. "I called to wish you a happy birthday. Most people I know, I recommend they start having 39th birthday anniversary parties, but you can go right on having birthdays for a while...

"I want you to know," the president continued, "that this is a very special year for you. In China, this year is the year of the hog..."

More laughter.

"That's tremendous that you're aware of that, Mr. President," Block said teasingly.

The Cooperative League, established in 1916, is a powerful umbrella group mostly for farm co-ops, but also for rural electric and telephone co-ops throughout the country. It has 350 member groups that reach about 40 million people.

Last night's group seemed to be in particularly high spirits, considering the state of the farm industry. The administration and Congress are frantically seeking ways to rescue agriculture from its near-depression.

"The farmers are suffering from over-production, low prices and worldwide competition," said League president Morgan Williams. "We would like the government to support more export-promotion programs..."

"It's a very complicated mess," declared House Agricultural Committee chairman Rep. Kika de la Garza (D-Tex.). "I don't think the administration is doing enough possible in the area of government support... the new budget is reducing funds for research, rural electric and telephone, and a lot of other things the farmers need to produce by themselves... But this isn't the place to talk about it. It's a birthday party."

He was right. Everyone ate chocolate birthday cake and cherries jubilee, and some told jokes about Block. The secretary even got a number of gifts: A pickax for his farm and a large stuffed hog to add to the 7,000 live ones in his Illinois herd.