Wheeling-Pittsburgh Steel Corp. yesterday reported a net second-quarter loss of $60.3 million, compared with a loss of $50.1 million in the period last year.

At the same time, Weyerhaeuser Co. of Tacoma, Wash., and Kellogg Corp. of Battle Creek, Mich., reported higher earnings.

For the first half, Wheeling-Pittsburgh said it lost $56.3 million on net sales of $465 million, compared with a 1985 loss of $78.9 million on $486.6 million in sales.

Wheeling-Pittsburgh attributed the improvement in the 1986 first half to greater productivity reflecting its cooperative partnership agreement with labor, increased production and shipping levels, reduced interest expense and significant reductions in steel-making costs due in part to 1985 plant and mine closings.

* Weyerhaeuser, despite a strike by 7,500 workers late in the quarter, reported an earnings increase of 15 percent for the second quarter and 21 percent for the first six months of the year.

Weyerhaeuser posted net earnings of $68.4 million (46 cents a share) on sales of $1.5 billion for the 13 weeks ended June 29. For the second quarter of 1985, the company had net earnings of $59.7 million (41 cents) on sales of $1.4 billion.

For the first six months, Weyerhaeuser reported net earnings of $118.4 million (77 cents) on sales of $2.8 billion. A year ago, the company posted net earnings of $97.9 million (65 cents) on sales of $2.7 billion.

The company said the results reflect continuing improvement in pulp and container board markets.

* Kellogg profit rose in the second quarter 11 percent above the corresponding period last year on soaring sales of the company's breakfast cereals, the firm said yesterday.

Kellogg reported net earnings of $69.2 million (56 cents a share) in the quarter, compared with $62.2 million (50 cents) in the year-ago period. Second-quarter sales totaled $867.2 million, up 22 percent from $713.7 million a year earlier, Kellogg said.

For the first half of this year, Kellogg reported net earnings of $145.7 million ($1.18), up 9 percent from $133.3 million ($1.08) in the corresponding period last year.