The effects of composted swine manure (0, 8, 16 or 24 mg C/ha) on maize (cv. Pioneer 3563), soyabean (cv. Pioneer 92B84), winter wheat (cv. Arapahoe), velvetleaf (Abutilon theophrasti), giant foxtail (Setaria faberi) and common waterhemp (Amaranthus rudis [Amaranthus tuberculatus]) emergence and early growth were studied. Compost amendment had no effect on crop emergence but reduced weed emergence. The inhibition of seedling emergence was between 15 and 57% for giant foxtail, 0 and 23% for velvetleaf, and 16 and 76% for common waterhemp. Soil amendment enhanced weed growth but not crop growth. A response surface regression analysis indicated that while large-seeded crops have constant relative growth rates, small-seeded weed species increase their relative growth rates with compost amendment (P<0.0001, r2=0.5252).