Introduction

Sweet green corn (Zea mays L.) is a very important cash crop in many parts of the word due to its great demand for freezing, canning, and table purposes (^{Mallikarjunaswamy et al., 1999}). Thus, sweet corn ear has typically ranked as one of the five most valuable vegetable crops in Florida, USA, where is fertilized up to 500 kg N^-1ha (^{He et al., 2009}). When green corn ear is harvested it is also possible to get high quality green fodder from the standing plants (^{Mallikarjunaswamy et al., 1999}) as a valuable byproduct. In addition, if green corn ear is unharvested, because low prices or for being outer of quality parameters, it still might produce a grain crop, thus giving an additional stability to this production systems.

Although sweet corn was fairly common among several North American Indian tribes as well as in South American ancient cultures, its consumption was rarely as boiled or roasted green corn ear (^{Mangelsdorf et al., 1939}; ^{Fussel, 1994}). In México there is a maize race named "maíz dulce" (sweet corn), which typically accumulates sugars in its endosperm so that the mature kernels appear wrinkled and translucent when dry.

This sweet corn is considered as an "Exotic pre Columbian" race because its affinities with South American corns, but this Mexican sweet corn is not consumed as a staple food but only for special uses: candies or morsels, soups and fermented drinks made of toasted (ground or whole) mature grains (^{Wellhausen et al., 1952}). In México and Central America the partially immature corn ear is called "elote", from the Náhuatl word elotl (^{Siméon, 1977}; ^{Santamaría, 1992}). This very old Mexican tradition of elote consumption, either boiled or roasted, has been mainly based on several early to late variants of different corn races with floury endosperm (Wellhausen et al., 1952; ^{Fussell, 1994}), but nowdays commercial grain crop variants are also used to produce green ears for urban customers.

Mexican government polices established during the eighties influenced negatively the production and profitability of corn grain. As a re sult of the growing demand of vegetables in the expanding cities, Mexican farmers of Rioverde region, in

San Luis Potosí State, changed from corn grain production to green corn ear production. This change allowed them to obtain a better price, lower production cost and profitability to an acceptable level. From that point onwards, the green corn ear production has acquired importance, and currently is the predominant crop in the 5000 hectares cultivated at Rioverde region where it is planted twice yearly (^{Charcas et al., 2000}; ^{Charcas et al., 2008}).

The potential of improved corn cultivars is manifested when they are sown in high densities, and are fertilized in adequate amount and time. On the other hand, traditional cultivars respond better to low plant density, especially in soils with low fertility levels (^{Evans, 1993}). At the same time, corn fertilization is determined by the crop purpose (grain, green corn ear or forage), the capacity of the cultivar, and weather and soil conditions. Thus, the most important agronomic factor for green corn ear yield and quality are nutrients, water, cultivar, planting date, location, and tillage practice, rather than years (^{Bekker et al., 1993}; ^{Mallikarjunaswamy et al., 1999}; ^{Makus, 2002}; ^{Kuabiah, 2004}; ^{Williams II et al., 2008}; ^{Balkom et al., 2010}).

Maize has reached high yields in developed countries of the temperate region where it is planted in optimal soil and weather conditions; furthermore, the technology used is highly developed. On the other hand, in developing countries of the inter-tropical zone even under irrigation and high technological management the average yields are low. This situation is due to varied factors; the tropical cultivars are not very efficient to produce grain, they show a low relation grain/ straw, and have tall and lodging susceptible plants; therefore they do not respond well to intensive management (^{Norman et al., 1995}; ^{Paliwal and Sprage, 1981}; ^{Dayanand, 2000}).

In México, until 1980, most of the corn improved cultivars recommended for warm and semi-warm zones were hybrid cultivars which presented the inconvenience of being too tall and susceptible to lodging (^{Reyes et al., 1961}; ^{Neve et al., 1962}). High costs of seed and the inefficiency of the government to produce and distribute them, resulted in their poor acceptance by the producers (^{Paliwal and Sprague, 1981}), in addition to their narrower adaptability. Thus, the official and private efforts to produce seed of improved variants have been now focused on producing cultivars with a wider genetic base that are adaptable to diverse production conditions (^{Sprague, 1981}; ^{Gerón et al.,1981};^{Aguilar et al., 1990}; ^{Sierra et al., 1990}; ^{Coutiño, 1994}). The objective of this work was to evaluate the yield and profitability of green corn ear of local and improved cultivars sown under traditional and intensive systems.

Materials and methods

The experiments were conducted during the winter-spring growing season (1998), at San José del Tapanco (well-water) and El Refugio (spring-water) locations of Rioverde, San Luis Potosí (altitude, 900 masl; mean annual temperature, 21⁰C; mean annual rainfall, 500 mm), or irrigated plots of volunteer peasants. In each location, an experiment was established to evaluate the yield of green corn ear of nine cultivars (five land races or traditional cultivars and four commercial improved cultivars: Luis Hipólito, Chón, Prisciliano and Gabino, and Asgrow 7573, Aspros 910, Huracán and Dekalb 880, respectively) by using a traditional and an intensive sowing systems. In San José del Tapanco yield of green corn ear, grain, and green silage forage after green corn ear harvest; plant and ear height; and weight, length and diameter of the ear were measured. In El Refugio only data on yield of green corn ear and grain were possible obtained.

The seed of traditional cultivars was acquired from local outstanding peasants known as good green corn ear producers; these cultivars are quite typical of Celaya race described by ^{Wellhausen et al. (1952)}. Seed of improved cultivars were obtained from local dealers. The traditional corn production system consisted of 34 000 plants ha^-1, planting two seeds per hole every 0.7 m, in furrows 0.84 m apart; a single dose of fertilizer (100-60-00) was applied at the first weeding; this system is used by the most advanced green corn ear farmers from the region. For the intensive crop system, a density of 59500 plants ha^-1 was used by planting two seeds per hole every 0.4 m, in furrows 0.84 m apart; the total fertilization was 150-60-00 applied in two parts, 50-60-00 at the sowing time and 100-00-00 at the first weeding. This last approach of cropping was based on ^{Hernández et al. (1988)}. For green corn ear four waterings were given and five waterings for corn grain. Herbicide was not used because it is considered to be unnecessary during winter growing season.

The treatments resulted from the combination of nine cultivars and two production systems were randomly distributed in complete block experimental design with five replicates. The 18 treatments were arranged in a split-plot way where plots were the production systems and sub plots the cultivars. The experimental unit comprised four rows of 10 m length to 0.84 m spacing. Only the fresh yield of the green corn ear and remaining green forage was simultaneously estimated, 90d after sowing date, in the two central rows, excluding a meter of each extreme. The grain yield (air dried) was evaluated on the lateral rows and also excluding a meter from each extreme, 120d after sowing time. The total plant and ear height were measured in ten plants. The average weight, length and middle diameter of green corn ear were estimated from three attributes were measured in part of the experimental unit material used to estimate the yield of green corn ear. The statistic treatment of the data consisted in a variance analysis and multiple comparisons of means (Tukey 0.5) using the SAS software.

Results and discussion

(a) San José del Tapanco. According to results obtained from the variance analysis (Table 1) the yield of green corn was not statistically different between production systems, but cultivars and the interaction production system x cultivars were statistically significant (p≥ 0.05). However, the yield of green forage presented high significant differences (p≤ 0.01) between production systems and between cultivars, but the interaction between these factors was not significant. The fact that yield in both production systems was statistically similar was probably due to the good level of soil fertility of the field where this experiment was established. The soils of Rioverde region, which are rotated with vegetable commercial crops, are naturally rich in phosphorus and potassium (^{Charcas et al., 2012}), and present some residual effects of nitrogen from vegetable crops fertilization, which ranges from 90 to 160 kg of nitrogen (^{Martin et al., 1976}; ^{Charcas et al., 2008}). Also, since green corn ear crop is harvested earlier, it consumes less nutrients than forage and grain crops, so the yields obtained seem to be reasonable.

Table 1 Variance analysis of yield, plant height, and characteristics of green corn ear cropped in San José del Tapanco, Rioverde, San Luis Potosí. 

*, **p≤ 0.05, and 0.01, respectively. NS= no significant difference.

The production of green corn ear under the intensive system renders in a loss of US $40 ha^-1, amount that corresponds to the additional cost of 90 kg of N (NO₃) and its application. Thus, this system is less profitable than the traditional one. As for the green forage and grain, the average yield under intensive production was 37.4 and 5 t ha^-1 and under the traditional 29.9 and 4.3 t ha^-1, this constitutes a difference of 7.5 and 0.7 t ha^-1 respectively. So, it seems possible to increase the yield of grain through intensive production; however, the profit obtained by selling 0.7 t of grain hardly covers the expenses of additional fertilization and harvest.

Also the yield of green forage in the intensive production system renders gross profits similar to those obtained by the traditional one but turns out to be more expensive.

The cultivars mean yield varied from 6.0 to 16.1t ha^-1 for green corn ear; from 26 to 45.3 t ha^-1 for green forage and from 3 to 6.4 t ha^-1 for grain yield (Table 2). The yield ranges of green corn ear (7.5 - 15.3 t ha^-1), residual green forage (29.9 - 37.4 t ha^-1), and grain (3.9 - 6.3 t ha^-1) obtained for both locations seem to be reasonable considering the climate conditions (lower temperature season), contrasting cultivars and moderate fertilization used. Thus, our results are between the low and high average limits of green corn ear yield of six very different experimental conditions, 7.6 and 17.3 t ha^-1, respectively (^{Bekker et al., 1993}; ^{Abu-Awwad, 1994}; ^{Mallikarjunaswamy et al., 1999}; ^{Makus, 2002}; ^{Williams II et al., 2008}; ^{Balkom et al., 2010}). The low limit of the average range (16.3 - 32.2 t ha^-1) of green forage yield, after green corn ear harvested, from two reports (^{Abu-Awwad, 1994}; ^{Mallikarjunaswamy et al., 1999}), is smaller than our results, probably due to the great size of traditional cultivars. Regarding grain yield, the values obtained agreed closely with the range (3.7 - 6.5 t ha^-1) reported for a set of10 experiments made in Colombia and Costa Rica (^{Norman et al., 1995}). Based on the multiple comparisons of means, the cultivars can arbitrarily be grouped into: improved cultivars with yields of green corn ear and grain higher that 15 and 5.2 t ha^-1, respectively, and of green forage lower than 32.2 t ha^-1. Traditional cultivars with yields of green corn ear, grain, and green forage in the range of8.0 to 9.9; 3.5 to 4.5 and from 32 to 37.8 t ha^-1, respectively.

Table 2 Average yield (t ha^-1) of green corn ear, grain and green forage in corn traditional and improved cultivars in San José del Tapanco, Rioverde, San Luis Potosí. 

**Values with the same letter each column are statistically equals (Tukey, p≤ 0.05).

Traditional cultivar Gabino that rendered lower yields of green corn ear and grain (6 and 3 t ha^-1) but produced the highest in green forage (45.3 t ha^-1). It should be noticed that the improved cultivar have an advantage over traditional cultivars in grain production and green corn ear yield, but their production levels of forage were lower. This could be attributed to their lower efficiency of allowance of dry matter to the grain of traditional cultivars and a low grain harvest, as ^{Norman et al. (1995)} asserts. In effect, the best commercial cultivar (Asgrow 7573) is superior to the best traditional one (Luis) in 6.2 and 1.9 t ha^-1 of green corn ear and grain yield, respectively, but it is inferior in 10 t ha^-1 of green forage production.

Regarding the significant interaction between production systems and cultivars, while the improved cultivars increased the yield of green corn ear under the intensive form, the traditional ones, except Hipólito, decreased it. These results seem to agree with the information about the interaction between management and cultivars presented by ^{Loomis and Connor (1992)}, ^{Evans (1993)} and ^{Dayanand (2000)} and may be due to the contrasting environments of selection (production conditions) for both cultivar groups. Plant characteristics related to hand harvested easiness, problems of pollination and lodging. The analysis of variance for the means of overall plant and ear height showed highly significant differences between treatments (p≤ 0.01), due to the production systems and cultivars; however, the interaction lacked statistical meaning (Table 1). Under the intensive production systems, the overall plant height was 13.5 cm higher than the traditional systems; the same occurred with the ear height, which presented 40.0 cm more under the intensive production systems. Higher plant density and height rendered in higher yield of forage. In the traditional cultivars, it also meant an observed higher number of unproductive plants; this was owing to partial or total failures in pollination, which in turn was derived from a deficient development of stigmas (Loomis and Connors, 1992), and to a higher observed frequency of laid plants. Thus, plant density and fertilizations doses for intensive production can be adverse to produce green corn ear with some of these traditional cultivars.

Based on the average plant height of ear position, and their comparison performed by the Tukey test (p≤ 0.05), the cultivars were classified into four groups (Table 3). (i) Tall size; the traditional cultivar Gabino of this single formed group presents some serious problems to be harvested by hand, because the plant has to be bent to rip the ear, and this also complicates the harvest of forage; (ii) intermediate size; this group includes the traditional cultivars Luis, Chón, Prisciliano, and Hipólito; these also presented some problems to be harvested the green corn ear, in smaller scale; and (iii) short size; this group was formed by the improved cultivars in which the ears are harvested fast and easily. In this last group it is also possible to increase the plant density and fertilization without observing infertility and lodging problems.

Table 3 Average height (m) in traditional and improved corn cultivars in San José del Tapanco, Rioverde, San Luis Potosí. 

**Values with the same letter are statistically equal according to the Tukey test to p≤ 0.05.

Green corn ear characteristics in relation to commercial quality requirements. In México the standards for green corn ear remain to be established. In general, the green corn ear obtained (Table 4) oversize (in weight, length and middle diameter) the standards of marketable size for sweet green corn ear improved variants (^{Mallikarjunaswamy et al., 1999}; ^{Makus, 2002}; ^{Kuabiah, 2004}; ^{Williams II et al., 2008}), as well as those weight and length estimates obtained by ^{Ortiz-Torres et al. (2013)} for several traditional cultivars in the Tehuacán Valley. According to the variance analysis (Table 1) the average weight of ears with husks shows highly significant differences (p≤ 0.01) due to the production systems and cultivars, but the interactions between these factors was not significant.

Table 4 Average green corn ear weight (with husks) and size (husks removed) from traditional and improved cultivars in San José del Tapanco, Rioverde, San Luis Potosí. 

**Values with the same letter are statistically equal according to the Tukey test to p< 0.05.

The difference in length of ears without husks was highly significant (p< 0.01) due to cultivars, but not to the production system and their interaction was not significant either. The differences in diameter of green corn ear without husk were highly significant due to cultivars (p< 0.01) as well as to production systems (p≤ 0.05); the interaction between these factors was meaningless. In relation to the traditional production system, the intensive form generated an average decrement of 38 g in the weight of ears, which is related to a reduction in their diameter. This reaction is explained by changes in relation to sown density: as the number of plants by area increased, the number and weight of kernels by ear diminishes (Looms and Connor, 1992). Thus, the plant density recommended for grain production can be inadequate for the production of green corn ear.

Based on the weight and average length of ears and their comparison by the Tukey test (p≤ 0.05), two groups can be identified (Table 4): (i) Improved and traditional cultivars with an average weight that ranges between 450 and 517 g, and an average length between 20.4 and 21.6 cm; in this group Asgrow 7573 and Luis stood out. (ii) Improved cultivar Aspros 910, with the smaller ears had an average weight of 423 g and length of 19.2 cm.

(b) El Refugio. According to the analysis of variance, in this location the yield of green corn and grain showed no significant differences owing to the production forms, but highly significant owing to cultivars; the interaction between both factors was not significant (Table 5). In accordance with their mean yield and the multiple comparison (Table 6), the cultivars can be grouped in: (i) improved cultivar Aspros 910, with the highest mean yield of green corn ear (14.5 t ha^-1) although its grain yield was intermediate (5.8 t ha^-1); (ii) improved and traditional cultivars, which showed an intermediate yield of green corn ear (10.9 to 12.2 t ha^-1) and grain yield (5.4 to 6.2 t ha^-1); and (iii) improved cultivar Dekalb 880, with intermediate yield of green corn ear (11.6 t ha^-1) but a low yield of grain (4.9 t ha^-1).

Table 5 Analysis of variance of corn green ear and grain yield with corn traditional and improved cultivars in El Refugio, Rioverde, San Luis Potosí. 

**p≤ 0.01. NS= no significant difference.

Table 6 Mean yield (t ha^-1) of corn improved and traditional cultivars of green ear and grain in El Refugio, Rioverde, San Luis Potosí. 

**Values with the same letter are statistically equal according to the Tukey test to p≤ 0.05.

Finally, the average yield of green corn ear, in the experiment established in El Refugio, was of11.5 t ha^-1, and the average yield obtained in San José del Tapanco was of 11.2 t ha^-1. Therefore, in both locations the overall yield and profitability were similar, but greater than the national mean yield (9.8 t ha^-1) for 2010 (Anonimous, 2012), as well as than the average yield (9.9 t ha^-1) of the 16 best traditional cultivars evaluated in the Tehuacán Valley by ^{Ortiz-Torres et al. (2013}).

Conclusions

The traditional production system allows the farmers to obtain a similar and cheaper yield of green corn ear comparable to that obtained by the intensive one; however, with the traditional system the yield of grain and green forage is slightly lower.

In both production systems at least one improved cultivar significantly surpassed the traditional ones in the yield of green corn ear and grain.

It is possible to improve easily the yield of green corn ear in the region; Asgrow 7573 and Aspros 910 improved cultivars or traditional cultivars Hipólito and Luis can be used for this purpose.