This look at the yearly farrowing patterns among areas of Iowa and on 105 eastern Iowa farms may help in dealing with hog cycles. It also suggests some strategy you may want to consider in planning your yearly farrowings.
Beneke, Raymond R.; Kaldor, Donald R.; and Herendeen, James
"How Iowa Farmers Respond to Hog Cycles,"
Iowa Farm Science: Vol. 15
, Article 5.
Available at: http://lib.dr.iastate.edu/farmscience/vol15/iss11/5