Weed Seed Decline in Irrigated Soil after Six Years of Continuous Corn(Zea mays)and Herbicides

The impact of two weed management systems on the weed seed reserves of the soil, on the yearly weed problem, and on corn (Zea maysL.) production was assessed where corn was grown under furrow irrigation for 6 consecutive years. In one system, 2.2 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was applied annually to the same plots as a preemergence treatment. In the other system, a mixture of 1.7 kg/ha of atrazine plus 2.2 kg/ha of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was applied preemergence, followed by a postemergence application of 0.6 kg/ha of the alkanolamine salts of 2,4-D [(2,4-dichlorophenoxy)acetic acid]. The response of weeds and corn is presented only where atrazine was applied annually because the results were similar between both weed management systems. Weed seeds from eight annual species were identified, with redroot pigweed (Amaranthus retroflexusL. ♯ AMARE) and common lambsquarters (Chenopodium album♯ CHEAL) comprising 82 and 12%, respectively, of the initial 1.3 billion weed seeds/ha that were present in the upper 25 cm of the soil profile. After the sixth cropping year, the overall decline in the total number of redroot pigweed and common lambsquarters seeds was 99 and 94%, respectively. Very few weeds produced seeds during the first 5 yr, and no weed seeds were produced during the sixth year where atrazine was applied annually. When the use of atrazine was discontinued on one-half of each plot at the beginning of the fourth year, the weed seed reserve in soil began to increase due to an increase in the weed population. After 3 yr of not using atrazine, the weed seed reserve in soil had built up to over 648 million seeds/ha, and was then within 50% of the initial weed seed population. In the fifth and sixth years, grain yields were reduced 39 and 14%, respectively, where atrazine had been discontinued after 3 yr.