Farming methods are improved, the yield of wheat per acre is being increased. Some of the main factors causing the increase are: the use of drills in seeding results in greater immunity against drought and winter-killing, especially if press drills are used; crop rotation; improved methods in plowing and cultivation; improvement of seed by natural and artificial selection, and by hybridization; fertilizing; irrigation; and tile drainage. The yield is always low under extensive methods of farming. The cost of production varies at different times and on different farms. The rapid improvement in farm machinery has favored extensive cultivation. It has also cheapened the cost of production, so that comparatively poor grades of land which it was unprofitable to work can now be farmed at a fair rate of profit.