Redroot Pigweed (Amaranthus retroflexus) Seedling Emergence and Growth in Soils Amended with Composted Dairy Cattle Manure and Fresh Dairy Cattle Manure under Greenhouse Conditions

Abstract

Organic soil amendments are known to affect the composition and density of annual weed communities. The objective of this research was to measure the effect on emergence and growth of redroot pigweed seedlings when soil was amended with composted dairy manure at 18, 36, and 54 T/ha, or with raw dairy manure at 41, 82, and 123 T/ha. Data recorded (1) seedling emergence over 12 days, (2) number of leaves and total leaf area, (3) shoot and root dry weight, and (4) seed number. Maximum seedling emergence (32%) occurred in nonamended soil (the control). Emergence declined in a linear fashion when soil was amended with manure or with compost. Compost additions affected seedling emergence more severely than did manure additions. For every measure of redroot pigweed growth except seed production, amendment with manure at 123 T/ha retarded growth compared to soil alone or compost-amended mixes. Manure applied at 82 T/ha reduced leaf area and plant height relative to other treatments. Growth of redroot pigweed in soil amended with compost at 36 and 54 T/ha was always equal to or greater than growth in soil that was not amended. Seed production in one of two runs of the experiment was more than double in soils amended with compost at 36 and 54 T/ha compared to the nonamended soil. These results suggest that amending soils with raw dairy manure may decrease the competitiveness of redroot pigweed, whereas amending with composted manure is likely to increase competitiveness.