Relationship between Maize Seed Productivity in Mexico between 1983 and 2018 with the Adoption of Genetically Modiﬁed Maize and the Resilience of Local Races

: Mexico depends on maize imports to satisfy its national demand. The use of native maize varieties among subsistence farmers can help to reduce the cereal’s imports. However, the agricultural policy in Mexico to improve the productivity per hectare has centered on the use of improved varieties; among them, the transgenic variety. In this study, the maize productivity in Mexico from 1983 to 2018 was analyzed to determine the inﬂuence of agricultural policies in the sector, and the factors that condition the adoption of transgenic maize. It was found that the agricultural policy improved the productivity of those regions with irrigation; however, for rainfed regions, the expected technological changes were not achieved because the ancestral tradition in cultivation, associated with the greater variety of native maize and to a larger indigenous population, was stronger. The adoption of transgenic maize also had low signiﬁcance in the rainfed regions, since the increase in ﬁeld yields is not economically proﬁtable with regards to the increase in production costs. Therefore, the agricultural policy to increase productivity ought to be directed at the protection of subsistence farmers, revaluing the use of native varieties that have shown higher resilience to technological and environmental changes.


Introduction
Maize (Zea mays L.) is the cereal of highest production worldwide, above wheat (Triticum aestivum L.) and rice (Oryza sativa L.); it constitutes the basis of the human diet for countries of Latin America [1], where Mexico is considered its center of origin [2]. However, Mexico is not the main producer of maize in the world; 57.35% of the world production is concentrated in two countries, the United States of America (USA, 35.42%) and China (21.93%) [1].
Unlike Mexico, USA bases its productivity on the use of transgenic maize varieties, which provide higher yields in the field than conventional varieties, which allows the USA to have yields higher than 10 t ha −1 while Mexico obtains on average 3.33 t ha −1 [3]. In Mexico, maize production is carried out using improved varieties and native varieties, since 64 native maize races are cultivated derived from the millenary domestication process
The following variables were obtained from the Agricultural and Fishing Information System [19]: production (t), surface planted (ha), field yield (t ha −1 ), and consumption (t) of grain maize at the state level from 1983 to 2019. The first three variables were obtained for the two production cycles that are found in Mexico: Spring-Summer (SS) and Fall-Winter (FW) in the two modalities, irrigation and rainfed. For the case of consumption (t), there was only information available starting from the period 2013-2018. The classification of Mexico's economic zones proposed by [20] was taken as a reference to obtain the average balances of 2013-2018 on production minus consumption per region.

Grain Maize Productivity in Mexico
To analyze the grain maize productivity at the national and state level, the decomposition of production growth factors model proposed by [14] was used. The model determines in percentage the influence that field yields (FY), surface planted (SP), and the FY×SP interaction have on the production growth in a final period compared to an initial period.
The periods analyzed were 1983-1988 versus 1989-1994, 1989-1994 versus 1995-2000, 1995-2000 versus 2001-2006, 2001-2006 versus 2007-2012, and 2007-2012 versus 2013-2018. In addition, the methodology proposed by [13] was used to classify the states where production increased and those where it decreased in 2013-2018 compared to 2007-2012 (Table 1). Table 1. Typology used to determine the production growth of an agricultural sector in function of the percentage variations of the surface planted and field yields.

Surface Planted (%) Field Yield (%)
A Increases  Increases  Increases  B  Increases  Increases  Decreases  C  Increases  Decreases  Increases  D  Decreases  Increases  Decreases  E  Decreases  Decreases  Increases  F Decreases Decreases Decreases * Production growth (%) = Surface planted (%) ± Field yield (%). The growth increases or decreases as a function of the factor that conditions it most; for example, for categories C and E, the surface planted decreases and the field yield increases. However, the production for category C grows since the field yields compensated the reduction in surface planted, which does not happen in category E, where the reduction of surface planted was higher than the efficiency in increase of field yields.

Indicators of GMO Maize Adoption
Data were obtained from the Agricultural and Fishing Information System [19] for grain maize production (t) of the municipalities that showed continuous operations from 1983 to 2018 in SS and FW. To determine the areas of highest productivity, the annual production per municipality for the period of analysis was added, and through tools of Geographic Information Systems, it was represented spatially. The results were grouped into five class intervals: very low, low, medium, high and very high production, both for SS and FW. Finally, to determine whether producers from the main grain maize-producing regions in Mexico are willing to adopt GMO maize, a survey was applied between August and November, 2017. The criteria used to select producers were their availability and references in the region, and they were called on by state agencies from the Ministry of Agriculture, Livestock Production, Rural Development, Fishing and Food. The survey included questions about age (years), schooling (years of study), experience in the activity (years), surface planted (ha), field yield (t ha −1 ) in SS and FW, and family income represented by the activity (%), as well as the question: Would you adopt GMO maize?

Grain Maize Production and Consumption at the Global Level
Out of the global production of grain maize from 2007-2018, 73.76% was concentrated in 5 countries, with USA (35.42%) and China (21.93%) as the main producers, followed by Brazil (9.34%), Argentina (4.59%) and Mexico (2.48%) ( Table 2). The production from USA and China in the analysis period was based on the broad surface planted that was destined to the maize crop, and to the good field yields, which for the case of China (5.71 t ha −1 ) were only exceeded by Argentina (6.85 t ha −1 ).
The intensive model of maize production that has been developed in USA is the result of the interaction of mechanized systems with the adoption of transgenic varieties, which allows for obtaining the best field yields of the world (9.93 t ha −1 ) [7]. In Latin America, Brazil (third world producer) and Argentina (fourth) also base their production in the use of transgenic maize varieties [21]. For the case of Mexico, the low field yields that it presented (3.33 t ha −1 ) are the result of the combination of native varieties with traditional rainfed production systems, which according to [22,23] are explained by the ethnography and culture of the country, since it is the center of origin of maize.
The maize that is imported in Mexico comes from the USA [7], so authors such as [24,25] maintain that the aversion of the Mexican population toward transgenic maize is unjustified, since the deficit between production and consumption is actually covered with transgenic maize. In this regard, [4] found that the cultivation of native races in Mexico contributes to imports of the grain being lower and ensures food security of the regions with greatest social backwardness, where, in addition, the native races show higher resilience to the rainfed environmental conditions, which has allowed them to remain in the preference of subsistence producers [26,27].

Maize Production and Consumption in Mexico
The deficit between production and consumption, which Mexico has historically showed, had its origin in the 1970s when the agrifood policy of the country encouraged the import of the grain in face of high production costs [28]. Since 1994, the difference between production and consumption shows a growing trend (Figure 1), and this fact coincides with the signature of the North American Free Trade Agreement (NAFTA) that Mexico established with USA and Canada.
According to [29], the signature of NAFTA caused Mexico to increase the imports of grain maize, by reducing the import customs fees. However, NAFTA only evidenced the productivity problems of the Mexican farmland with negative MAGR, which have been worsened by the sustained increase of the population that demands more grain maize with a per capita mean of 196 kg (Figure 1) [5].  * Millions of tons, ** Millions of hectares. The quantity of consumption for the period analyzed was higher than that of production, which was due to the initial stock in each of the years, considering the amount of grain in storage.
Agriculture 2021, 11, 737 6 of 15 six-year period of 2001-2006 (MAGR, 1.41%). However, in the period of 2007-2012 with the national policy of allowing GMO maize cultivation, the productivity decreased (MAGR, −1.05%) due to the uncertainty generated among small-scale producers [24]. In the last six-year period (2013-2018) with the MasAgro program, national production increased through the use of native maize varieties (MAGR, 3.07%) with small-scale producers [32]. Despite all these efforts, grain maize production in Mexico is insufficient to guarantee food security of a population in continuous growth [33].  The agrifood policy in Mexico in the six-year period of 1983-1988 caused the MAGR of this period to be negative (−3.59%), since imports were prioritized over the programs to strengthen the farmland [30]. The largest increase in production happened in the six-year period of 1989-1994 (MAGR, 8.87%) with the Program for Direct Support to the Farmland (Programa de Apoyos Directos al Campo, PROCAMPO) [9], which not only entailed strategies to improve competitiveness but also regularized the property rights over land. The social and economic crisis that Mexico experienced in the period of 1995-2000 had an impact on the national agricultural productivity, and maize (MAGR, −0.74%) was not the exception [30].
The with the national policy of allowing GMO maize cultivation, the productivity decreased (MAGR, −1.05%) due to the uncertainty generated among small-scale producers [24]. In the last six-year period (2013-2018) with the MasAgro program, national production increased through the use of native maize varieties (MAGR, 3.07%) with small-scale producers [32]. Despite all these efforts, grain maize production in Mexico is insufficient to guarantee food security of a population in continuous growth [33].
Grain maize production in Mexico is performed in two periods: Spring-Summer (planted in May and harvested in October) and Fall-Winter (planted in December and harvested in May), both in two modalities, irrigation and rainfed. The technological gap between the regions of irrigation and rainfed production is evident in the field yields (Table 3); while for irrigation they are higher than 7 t ha −1 , for rainfed they are lower than Out of the surface planted with grain maize in Mexico, 73.98% depends on summer rains and contributes 49.83% of the total production, and it develops in a context of subsistence, with low technological level and inadequate agronomic management [32] resulting in field yields of 2.42 t ha −1 (Table 3). This is different from irrigation zones where 46.75% of national production was obtained in 19.91% of the surface planted, with average yields higher than 7 t ha −1 . In Mexico, for every 1 ha of irrigation cultivation, 4 ha are planted in rainfed conditions, and for every ton of grain maize in rainfed conditions, 3.76 t were obtained with irrigation.
The problem of low rainfed grain maize productivity has been documented by [7,28,32]. These authors agree that the problem is spatial, seasonal and cultural, since more than 80% of the producers, particularly in the southeast of the country, cultivate grain maize of native varieties in plots of less than 2 ha, in the ancestral milpa system (association of native maize with bean -Phaseolus vulgarisand squash -Cucurbita spp.-), with rudimentary low-cost farming tasks (cultivation management).
The growing trend in the production of grain maize for the SS-Rainfed cycle can be reverted in the short term, as consequence of the reduction of the surface planted since the year 2000 and a null growth of field yields in the same cycle ( Figure 2). In this regard, [33] considers that the constant production of grain maize that Mexico has maintained in the last decade is due to the increase in productive efficiency, particularly in the irrigation areas, which compensates the low productivity per surface planted in the rainfed areas.
To determine the regions in Mexico with the highest deficits between production and consumption, the classification proposed by [20] was used, which divides the country into eight economic zones based on the existence of natural resources that allow certain productive activities and on the degree of development of the production forces. The Center East (6.81 million tons, which represents 51.65% of the national deficit), Center West (2.99 million tons, 22.68%) and Southeast (1.41 million tons, 10.72%) were the zones with the highest deficit of grain maize; in total, the three regions added up to 85.05% of national deficit (Figure 3).
According to [6], the highest grain maize deficit shown by the Center East region (51.65% of the national deficit) is due to two factors: it is the region with the highest number of inhabitants in urban zones (33.81% of the national total), and mostly represents industrial, non-agricultural activity. In contrast, the South region was the only zone in the country without deficit; this zone was characterized by containing the highest diversity of native maize races, with the highest levels of indigenous (rural) population, in addition to being a region with subsistence agriculture and rudimentary production systems that depend on summer rains [22,23]. The demographic and economic differences between both regions expose the capacity of rural zones to produce their own food, so the challenge is: How to feed the cities? [34,35]. To determine the regions in Mexico with the highest deficits between production and consumption, the classification proposed by [20] was used, which divides the country into eight economic zones based on the existence of natural resources that allow certain productive activities and on the degree of development of the production forces. The Center East (6.81 million tons, which represents 51.65% of the national deficit), Center West (2.99 million tons, 22.68%) and Southeast (1.41 million tons, 10.72%) were the zones with the highest deficit of grain maize; in total, the three regions added up to 85.05% of national deficit ( Figure 3).  To determine the regions in Mexico with the highest deficits between production and consumption, the classification proposed by [20] was used, which divides the country into eight economic zones based on the existence of natural resources that allow certain productive activities and on the degree of development of the production forces. The Center East (6.81 million tons, which represents 51.65% of the national deficit), Center West (2.99 million tons, 22.68%) and Southeast (1.41 million tons, 10.72%) were the zones with the highest deficit of grain maize; in total, the three regions added up to 85.05% of national deficit (Figure 3).

Grain Maize Productivity in Mexico
When analyzing the production of grain maize in Mexico from 1983 to 2018, through Tukey's means tests, a continuous significant increase was found in each six-year period, as a result of an improvement in the field yields that compensated the reduction of the surface planted (Table 4). However, when using the decomposition of production growth factors model proposed by [14], the apparent efficiency in the production system did not consider that the growth of grain maize productivity between six-year periods presented a decreasing trend, since it went from 16.4% in 1995-2000 compared to 1989-1994, to 6.9% in 2013-2018 compared to 2007-2012 (Table 5).   According to [8,25] the growth from 1983 to 2000 is explained by the strengthening of the policies for centralized support in agricultural production, technological research, guarantee prices, use of improved varieties and fertilizers, supports for commercialization, credit, and agricultural insurance. The decrease in productivity since 2001 coincides with a stage of policies for decentralized support to the farmland, where the private sector participated in technological changes, research, dissemination, and promotion of input use; and where producers made their own decisions about the destination of the subsidies contributed by the government [25]. In this regard, [18] found that the agricultural competitiveness of the local and subsistence production systems is strengthened as the government establishes mechanisms to monitor and evaluate, since in most cases the producers channel part of the subsidies as a complement to the family income.
In 2013-2018 the productivity of grain maize at the national level had a growth of 6.9% compared to 2007-2012, as a response to an increase in the efficiency of field yields (10.5%) ( Table 5). When analyzing the productivity of grain maize by states, it was found that 20 out of the 32 states had growth, from which 11 were managed under a rainfed water regime and nine under irrigation (Table 6).
Villegas et al. [11] consider that the growth in grain maize productivity of rainfed areas is explained by the inclusion of federal programs such as MasAgro that develop regional strategies of sustainable intensification of maize production, through innovation networks composed by research platforms, demonstrative modules, and extension areas where sustainable agronomic technologies and practices are evaluated, developed and adapted, which promote the use of improved maize seeds [36]. Carpentieri-Pípolo et al. and Sales-Rocha et al. [37,38] found that native maize varieties cultivated in a traditional system tend to stand out in comparison to commercial varieties in the long-term, because the native varieties have genotypes capable of responding better to abiotic and biotic stress, providing a better stability of the yield.
However, [33] found that the rainfed areas with grain maize production in Mexico are the most vulnerable to the effects of the future climate. This scenario is especially critical because nine out of the 12 states that presented reductions in their productivity are managed under rainfed conditions (categories D, E and F). The states that reduced their productivity are located mostly in the center and south of the country (Table 6), in the South economic region [20], where there is currently no deficit between production-consumption and where the greatest variety of native maize and indigenous population are concentrated [22,23].
The reduction in grain maize productivity of 12 states (categories D, E and F) represented 45.10% of the surface planted for the period of 2013-2018, with a production of 26.90% of the national total, and with average field yields of 2.15 t ha −1 . On the contrary, eight states (Category C) produced 52.45% of the national total in 31.28% of the surface planted with field yields higher than 10 t ha −1 ( Table 6).
The increase in grain maize productivity of the states located in category C was because they managed to make their field yields more efficient, since their surfaces planted decreased. Two factors explained the increase in the field yields: use of improved maize seeds [11] and use of irrigation [39], whether in their totality as in the case of Sinaloa (main maize producer in Mexico) or with irrigation that aids the summer rains (rainfed) in Jalisco (second producer) and Estado de México (third producer).
However, two states that are a reference in grain maize production at the national level presented a reduction in their productivity: Guanajuato (fifth producer, category D) and Chiapas (7, F), which compromise 11.1% of the national production ( Table 6). The low productivity showed by Guanajuato was due to the reduction of its field yields because of the strong competition over the use of water with other cyclic crops such as berries and vegetables [40]. For the case of Chiapas, the reconversion of the maize production areas to more profitable crops explains why both the surface planted and the field yields have decreased [41].

Indicators of GMO Maize Adoption
When adding the annual production in t per municipality from 1983 to 2018 per production cycle SS and FW, and grouping this production in class intervals: very low, low, medium, high and very high; it was found that the main grain maize-producing regions in the SS cycle were located in the states of Jalisco and Chiapas, while for the FW cycle they were located in Sinaloa and Tamaulipas. In these regions, a total of 141 surveys were applied to key producers: Jalisco (45), Chiapas (33), Sinaloa (36) and Tamaulipas (27) to determine the factors that condition the adoption of GMO maize in Mexico (Figure 4).
Mwangi and Kariuki [42] found that the factors that condition the adoption of new technologies in the production systems of small-scale agricultural producers are associated with age, schooling, size of the plot, and importance that the activity represents in the family income. When characterizing the producers from the main grain maize-producing regions in Mexico, it was found that the size of the plots and the % that the activity represents for the family income (FI) seem to condition the adoption of GMO maize (Table 7).
From the producers, 56% answered that they are willing to adopt GMO maize particularly in the regions that are managed with irrigation (Sinaloa and Tamaulipas), the remaining 44% sustained their negative in the environmental conditions (rainfed) of their production areas (Jalisco and Chiapas). Of the total producers, 94% said they were aware that the use of transgenic varieties increases production costs. Noriero-Escalante and Massieu-Trigo [43] found that the regions with commercial maize production are more prone to the adoption of GMO maize because the increase in their production costs is less significant than in the areas with subsistence production.

Indicators of GMO Maize Adoption
When adding the annual production in t per municipality from 1983 to 2018 per production cycle SS and FW, and grouping this production in class intervals: very low, low, medium, high and very high; it was found that the main grain maize-producing regions in the SS cycle were located in the states of Jalisco and Chiapas, while for the FW cycle they were located in Sinaloa and Tamaulipas. In these regions, a total of 141 surveys were applied to key producers: Jalisco (45), Chiapas (33), Sinaloa (36) and Tamaulipas (27) to determine the factors that condition the adoption of GMO maize in Mexico (Figure 4).

Figure 4.
Main maize-producing zones in Mexico from 1983 to 2018 obtained from the sum of the production at the municipal level in the spring-summer and fall-winter cycles [19]. The circles indicate the regions where surveys were applied to key producers: Jalisco (45), Chiapas (33), Sinaloa (36) and Tamaulipas (27).
Mwangi and Kariuki [42] found that the factors that condition the adoption of new technologies in the production systems of small-scale agricultural producers are associated with age, schooling, size of the plot, and importance that the activity represents in the family income. When characterizing the producers from the main grain maize-producing regions in Mexico, it was found that the size of the plots and the % that the activity represents for the family income (FI) seem to condition the adoption of GMO maize (Table  7).
From the producers, 56% answered that they are willing to adopt GMO maize particularly in the regions that are managed with irrigation (Sinaloa and Tamaulipas), the remaining 44% sustained their negative in the environmental conditions (rainfed) of their production areas (Jalisco and Chiapas). Of the total producers, 94% said they were aware that the use of transgenic varieties increases production costs. Noriero-Escalante and Massieu-Trigo [43] found that the regions with commercial maize production are more prone to the adoption of GMO maize because the increase in their production costs is less significant than in the areas with subsistence production. Main maize-producing zones in Mexico from 1983 to 2018 obtained from the sum of the production at the municipal level in the spring-summer and fall-winter cycles [19]. The circles indicate the regions where surveys were applied to key producers: Jalisco (45), Chiapas (33), Sinaloa (36) and Tamaulipas (27). González-Estrada and Alferes-Varela [28] found that the main problems in maize cultivation in Mexico are associated with the low productivity per the surface planted, a problem that GMO maize does not resolve, since the increase in field yields is not economically profitable with regard to the increase in production costs [24]. Hernández-Hernández et al. [25] found that the economic and agronomic potential of the adoption of genetically modified maize seeds in Mexico has been unviable because the improved seeds proposed solve problems related to resistance to glyphosate and corn budworm, factors that historically have not been a problem in the production of grain maize in Mexico.

Conclusions
The agricultural policy in Mexico during the period of 2013-2018 fostered grain maize production through the Sustainable Modernization of Traditional Agriculture, compared