54 1:0, 1:1, 1:2, 1:3 and 1:4 of C. argentea to A. spinosus. A distance of 6 cm was maintained between and among stands of combining species. In the field trial, the same treatment combinations were established but in a randomized complete block design with three replicates. Each block was 2 x 1 m 2 . The plots were bed-raised with hoes. The plots were 0.5m apart. Seeds of C. argentea were drilled at 15cm spacing between rows and later thinned to 15cm within rows to have a plant density of 444,444 plants per hectare.. Seeds of A. spinosus were ring-broadcast at 6cm intervals and later thinned to the required density ratios at 2 WAP. The performance of both C. argentea and A. spinosus was monitored in terms of plant height, stem diameter, number of leaves/plant, leaf area and plant biomass at harvest. S. OGUNYEMI et al. Experiment II. Corchorus olitorius versus Amaranthus spinosus: The design of the experiment followed the two component model as described by Hall et al. (1992). The 1 st component is the maximum length of time weeds emerging with the crop can remain before they reduce yield (weedy and subsequently weedfree, wd-wf) and the 2 nd component is the length of time that a crop must be kept weed-free after planting so that weeds emerging later do not reduce yield (weedfree and subsequently weedy, wf-wd). A. spinosus was allowed to associate with C. olitorius for varying periods of time (as indicated in Table 1). The same treatments were established in pots and in the field. In the pots, two seeds of C. olitorius and 4 of A. spinosus were sown in each pot to give a ratio of 1:2 crop to weed. Weed-crop distance was 8cm in all pots. The pot trial was a completely randomized design while the field was a randomized complete block design with 3 replicates. Each block was (12 x 2) m 2 and each plot was 2 x 1 m 2 . C. olitorious was planted in rows with 30cm inter-row spacing and later thinned to the required population densities. Seeds of A. spinosus were planted in rows 8cm from C. olitorious and a crop to weed ratio of 1:2 was also maintained. Data recorded for C. olitorius at 12 weeks after planting (WAP) were plant height, stem diameter, number of leaves per plant and plant dry weight at harvest. All dry weights were obtained by oven drying harvested materials for 48 hours at 70 0 C before weighing. All data collected were subjected to ANOVA and mean differences were compared using DMRT at 5% level of probability (Gomez and Gomez, 1983). Results Density and Duration of Interaction between A. spinosus and C. argentea. In the presentation of results the four control treatments were pooled into one treatment (D0). Different densities of A. spinosus growing in association with Celosia argentea exerted significant pressure on the performance of the potherb. In the pot trial, regardless of duration of weed association, density presssure caused significant reduction in the stem height and the number of leaves per plant of Celosia argentea (Table 2). However, the treatments were not significantly different with regards to stem diameter. In the field trial, A. spinosus induced significant reduction in stem height, stem diameter and number of leaves per plant of Celosia argentea (Table 2). Dry matter production was also significantly reduced by increasing weed density regardless of duration of competition especially in the field trial, although fresh weight of Celosia a rgentea was not significantly affected by interference from A. spinosus (Table 3). It was informative that dry weight suffered as high as 50% reduction when four A. spinosus competed with one Celosia argentea for eight weeks. Critical period of interference of A. spinosus with Corchorus olitorius. In the two trials at 12 WAP the C. olitorius exposed to weedy treatment (Treatment 7) had significantly reduced height (Table 4). They were similar to plants that were weedy for at least four weeks (Treatments 3, 4, 5 & 6). The weedfree plants (Treatment 1) and those that were weedy in the first two weeks and subsequently weedfree (Treatment 2) had good height growth and were similar to those plants that were weedfree for at least two weeks (Treatments 8, 9, 10, 11 & 12). The treatments followed the same trend in terms of all growth parameters considered. With regards to dry matter accumulation in the field trial, plants that were weedy for at least the first 2 weeks (Treatments 1 & 2) and those that were weedfree for at least the first four weeks (Treatments 9, 10, 11 & 12) were similar and had significantly (P
Competitive impact of Amaranthus spinosus and Corchorus olitorius in Nigeria. 55