Role of Seed Reproduction in the Population Ecology of Sorghum halepense in Maize Crops

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Abstract
(1) Sorghum halepense (L.) Pers., a perennial weed, reproduces by seed and from rhizomes. This study examines the contribution of seed reproduction to population growth in maize fields. (2) Seed production in dense patches was very high (8080 .+-. 4300 full seeds m-2). Ninety per cent of the total seed yield was produced during December and early January. (3) Shedding of seed began 30 days after panicle emergence. Daily seed shedding in dense patches ranged from 75 to 100 m-2 during most of the summer. (4) After an initial rise in January, seed density on the soil surface decreased about five-fold during the summer months. Using seed sources of known size, seed losses from the soil surface were found to vary between 3 and 15% per day during the crop cycle, and between 20 and 50% per day during the fallow period. Loss of seed was substantially reduced by seed burial and completely prevented if seeds were covered with 5 mm plastic mesh. Results indicated that predators (most likely rodents) were the agents of seed loss. (5) Seedlings emerged during the month after sowing maize. Average seedling survival was 25%, between-row cultivation being the main cause of mortality. Forty per cent of the established plants produced seeds. The average fecundity of seed plants was forty-one seeds per plant. (6) A simple model indicated that the potential for multiplication during the ''plant phase'' is about four seeds per seedling per year. However, when losses during the ''seed phase'' are incorporated in the model, the multiplication rate of a population reproducing solely by seeds is substantially lower than one. It is inferred that reproduction by seeds would not maintain the population under continuous maize.