The results shown in Table 1 correspond to the determination of the percentage of weed incidence. It is observed that the lowest incidence rates are exhibited by treatments T1 (cabbage), T2 (cabbage + lettuce), and T3 (cabbage + chard).

Table 1

It can be observed that at 25 days, the lowest incidence of weeds was obtained with treatments T1 (cabbage) and T2 (cabbage + lettuce). In contrast, at 40 and 55 days, the lowest results of weed incidence were observed in treatments T1 (cabbage), T2 (cabbage + lettuce) and T3 (cabbage + chard). It should be noted that all the identified weed species, observed both pre- and post-experimentation, are reported to be commonly occurring within the experimental area. The species include Commelina diffusa Burm., Cyperus rotundus L., Digitaria ciliaris (Retz.), Cenchrus equinatus L., and Amaranthus viridis L., along with the grass Echinochloa crus-galli (L.) O. Schulz.

In light of the findings of this determination, it can be posited that the coexistence of two species in a given area gives rise to competitive interactions with weeds, which ultimately prove deleterious to the weeds. Romero et al. (2024) define this phenomenon as phenotypic plasticity, defined as the ability of plants to respond to environmental stimuli with adjustments that generate different phenotypes over time or in different locations, modifying their functional traits. Consequently, plant growth is regulated by external factors, primarily through alterations in morphology, physiology, or biochemistry. In situations of stress, light and water are significant factors that exert a considerable influence on plant growth (Rahn et al. 2018).

As previously stated by Neupane et al. (2021) the practice of intercropping has been demonstrated to reduce the prevalence of weeds in agricultural systems. This is achieved by facilitating the capture of essential nutrients and other accessible resources. The presence of glucosinolate compounds in cruciferous plants has been identified as a contributing factor to weed suppression, due to their allelopathic effects. Furthermore, intercropping allows for the utilization of space and nutrients that would otherwise be unutilized by a single crop. This limits the space available for weed growth and proliferation.

However, to obtain efficient intercropping systems with bio-economic advantages, it is essential to manage the species to be cultivated effectively, as well as to consider production factors such as green manure, population densities of the component crops, and so forth (Lino et al. 2021).

As indicated in Table 2, T2 exhibited superior outcomes in terms of weight, although its results are statistically equivalent to those observed in T1. A comparison of cabbage yield in monoculture and polyculture demonstrated that the crop in the monoculture system produced 36.975 (ton ha^-1) and 50,008 (ton ha^-1) in association with lettuce. However, statistically significant differences were observed when comparing the cabbage yield in association with lettuce and cabbage with chard and parsley.

Table 2

According to Hernández (2019), determining the influence of polyculture on pests and productive indicators in Brassica oleracea L., where estimated cabbage yields were above 60 (ton ha^-1) in both treatments (monoculture and association). These data were higher than those obtained in this experiment.

It can be stated that the productivity and quality of cabbage heads are influenced by several factors, including plant population, genotype, climatic conditions, water regime and soil nutritional status (Tegen and Jembere 2022). It is thus important to consider that the intercropping species should not impede the growth of the main crop, should not compete with it, and should be a species that is advantageous to the main crop. This was demonstrated in the present investigation, where the combination of cabbage and lettuce resulted in a positive yield outcome. Therefore, when considering methods of increasing yield, cultivation practices such as intercropping are the optimal choice (Turan et al. 2022).

The analysis of variance (ANOVA), as presented in Table 3, revealed no statistically significant differences between the treatments in the determination of the equatorial diameter of the head of cabbage using two production systems (monoculture and association). The resulting data yielded the following measurements: 18.84 cm (T1), 21.18 cm (T2), 18.15 cm (T3), and 17.50 cm (T4).

Table 3

The fact that there were no statistical differences in cabbage head diameter based on the cropping system in this trial may be because cabbage and other leafy green vegetables have different requirements and do not compete for production factors. Head diameter is also considered to be an important component of cabbage yield.

El-Remaly et al. (2022) refer that cabbage is a relatively mid-season crop that grows slowly in the first growth phase, which provides the opportunity to intercrop a short-season crop such as lettuce with the main crop while maintaining yield and quality. Furthermore, lettuce is a short-season crop and its water and mineral requirements do not conflict with those of the main crop, cabbage. This reinforces the fact that there were no differences between the treatments applied in the present investigation.

Llomitoa et al. (2023) observed that the use of dolomite and earthworm humus in the cultivation of cabbage (Brassica oleracea L. var. capitata) resulted in head diameter measurements ranging from 19.70 to 22.67 cm, with an average of 20.92 cm. They found no statistical difference between these measurements, indicating that the replications within the experimental unit were properly distributed and homogeneous. These findings align with the results of the present experiment.

The ratio of the length of the polar and equatorial diameters defines the characteristic shape of the head of a given cabbage cultivar, and both dimensions also determine the weight of the cabbage head. Plant spacing is influential in any cultivation system and cultivar, mainly in cabbage cultivation (Girard and Osorio 1975). No significant differences were found regarding the effect of different planting densities on the variable polar diameter of the cabbage head (Table 4).

Table 4

In a study of intercropping lettuce and broccoli, Martinez et al. (2024) observed a significant improvement in broccoli yield and suggested that to mitigate nutrient competition in intercropping systems, it is recommended to select plant species with different rooting patterns, nutrient requirements, and periods of peak nutrient demand.

In contrast to the findings of the present investigation, Mladenova and Yordanova (2023) who evaluated the effect of intercropping on cabbage growth and yield, concluded that, regardless of small differences, the intercropping option influenced the height and diameter of cabbage heads, indicating that intercropping had the most complex effect on the analyzed indicators, evidencing that when two species are intercropped, the dominant species can increase its yield and nutrient uptake, while the yield of the other crop is reduced due to interspecific competition for nutrients (Shanmugam et al. 2022).