Traditional milpa system (SMT)

The SMT is a polyculture that has different variants according to biophysical and cultural conditions. In the case of Tlaxcala, various types of associations were found in all of them, maize (Zea mays L.) with one or more of the following species and varieties: beans (Phaseolus vulgaris L.); ayocote (Phaseolus coccineus L.); broad bean (Vicia faba); and pumpkin (Cucurbita sp). The SMT is a result of the preservation of traditional Mesoamerican practices that, in this zone, receives different names like “milpa” or “Traditional Milpa System” (^{Sánchez and Castro, 2011}; ^{Sánchez and Hernández, 2014}). The SMT ranked 20% of the total producers surveyed (59 people); however, only 5% are peasants who were found at random, the rest were placed in a targeted manner, due to the low proportion of peasants still planting milpa.

The age of the producers of this group varies from 35 to 86 years (with average and median of 58 years), they possess on average 3.7 ha, although the range fluctuates between 0.5 and 9 ha. The group of respondents is made up of 14% women and the rest are men. In general, average schooling is almost five years, and 6.8% of the total do not know how to read or write. In addition, 28% of producers have another job for the generation of complementary income, and the average number of members per family is five, although 57% of families are made up of two to four people and the rest are made up of five to 11.

Soil preparation

According to the collected information, in order to prepare the soil before sowing, fallow and dredge are basically carried out, depending on the economic conditions of the producers. Occasionally dredge is not carried out, due to its high costs and soil is only traced; in other cases, up to two dredges and three or four pre-sowing trawls are carried out when the producer has his own tractor. For dredging the rental cost of tractor varies between $600.00 and $1 200.00 ha^-1, and for harrow $400.00 to $1 200.00 ha^-1 varying on every State region. The proportion of producers that dredge with tractor is 57.6%, while 76.4% harrows once and 8.3% twice. To calculate the amount of fuel used in the soil preparation, a factor of 36.7 was used to convert the liters of diesel (Ld) to MJ.

Sowing

Here two possibilities were placed in the use of tractor: through the furrow (to later plant using shovel, a tapapié or a yunta planter) and planting with tractor equipment. In the SMT 40.7% of producers use tractor for furrowing; 45.7% planted with yunta seeder and 13.6% used tractor seeder. By replacing the data in the formula the following results were obtained.

Tillage work

In this area the fertilization, weeding, tillage, asegunda and fumigation were considered. In the SMT only four producers fertilize with tractor at the time of sowing. In addition, 11.9% of peasants weed, 8.5% worked the soil and 15.3% asegundan with tractor; the rest use yunta and some do not perform these tasks. Finally, 11.9% apply some insecticide; however, only 3.4% use gasoline engine sprinkler and the rest, manual backpack. To convert the liters of gasoline (Lg) to MJ, it was calculated with a factor of 32.

Harvest

In the SMT, the proportion of producers that use combined is 6.8% and 94.9% grind or threshes their grass, for the purpose of selling or saving it for their livestock. In these activities the fuel used was diesel. The proportion of peasants that threshed with gasoline machines was 28.8% and 83% used freight. Finally, producers using electric motor thresher represent 8.5%. The calculated energy is shown below.

With these results the total energy used per hectare in the STM was calculated, being as follows.

The results show that 2 114.42 MJ ha^-1 are used, corresponding to 57.62 liters of diesel equivalent (Lde) per hectare among members of the SMT. Harvested dry biomass (BSC) was calculated by adding the yield per hectare of maize, bean and pumpkin seed, dry maize grass, bean and pumpkin thatch. Maize yield was 978 kg ha^-1 and 140 pieces of bales per hectare (21 354 kg each). The average BSC was 5 638 kg ha^-1, and 29.13 Lde are used to produce a tm of maize. With these data direct emissions were calculated, and the result for the SMT was 163.3 kg of CO₂e ha^-1.

Maize monoculture system (SMo)

In the SMo only maize is sown. In this way, the crop conditions for the mechanization in terms of planting, tillage tasks, irrigation, herbicides application and harvest are adapted.This type of cultivation is a result of the RV technological package. 80% of the sampled producers belong to this group in the state of Tlaxcala (320 people), ranging in age from 23 to 93 years and an average of 62 years. The amount of land they cultivate is 5.5 ha on average, however, the range fluctuates between 0.5 and 60 ha. Of the people surveyed, 15% are women; the illiteracy level is 6.2% and each family has five members on average. The practices carried out are mentioned below.

Preparation of soil

The proportion of peasants who plough using a tractor is 85.3%, and 6.9% do not plough prior to planting due to the high costs involved. In the case of the dredge, when these activities are carried out using animals, the peasants call it “contlapanear”; occasionally it is carried out at the time of harvest and then before planting, even, a biga is passed over to undo the lumps and seal the soil so that it does not lose much moisture. In this group 8.8% uses a yunta and 90.9% use tractor for this purpose. Below is the calculation of fuel and energy used.

Sowing

In the SMo, 29.7% furrows with tractor and 12.8% with yunta; the rest (57.5%) does not furrow previously, precisely because they sow with tractor equipment. In addition, 18.4% do so with sowing machine pulled by yunta; 23.1% sows using shovel and 1% sows to tapapié. The fuels use is as follows.

Tillage work

In this group 10% of producers fertilize maize with a tractor; 83.4% do it manually and the rest does not fertilize. Regarding weeding, 38.1% performed it with tractor, 20.9% tillage and 33.4% asegunda with machinery; the rest does it with yunta or does not carry out these tillage works. In addition, 67.5% used herbicides and 12.8% apply some insecticide; however, most of them do it with a manual backpack, 8.4% apply it with a backpack and 4% do so with a tractor. The calculations are as follows.

Harvest

For the harvest, in this group the mechanization is more feasible than in the SMT. The proportion of producers using a combined machine to harvest their maize is 13.4% and the rest do it manually and the grass is packed by 89.1% of peasants. In addition, producers using gasoline engine grinders represent 62.8%, with a 12.5% electric motor and 77% used freight to move their harvest. The energy calculated for the harvest in this group is shown below.

The concentration of the above parameters allowed to calculate the total energy used per hectare in the system of maize production in monoculture, being as follows.

SM_O uses about 81.8 Lde ha^-1; for calculating the average BSC performance, the average maize yield 1 942.8 kg ha^-1 and 142 bales ha^-1 were contemplated. The average BSC was 4 975 kg ha^-1 and for this group 41.35 L of diesel are used per ton of maize grain. Regarding the calculation of direct emissions of CO₂e, the results were 231.9 kg of CO₂e ha^-1.

Fossil energy use and CO ₂ e production of compared cultivation systems

Student’s t-Test was applied to data related to the use of fossil fuels (and consequent generation of CO₂e) from agricultural activity, the results for soil preparation were F= 6.464, p= 0.116, this situation indicates that there is no significant difference between the groups (SMT and SMo). For sowing cases (F= 223, 615, p= 0.001) and tillage tasks (F= 34 928, p= 0.002) there is a highly significant difference which shows that SMo uses more fuel and generates more CO₂e than the SMT in these activities (^{Doménech et al., 2012}; ^{Guevara, 2013}). However, the task that used more fuel was harvesting, where F= 3.068, p= 0.255, no significant difference exists (Table 1).

Table 1 Comparison of energy use and emissions of CO₂e between production systems. 

Fuente: elaboración con datos de campo febrero-mayo 2015. nμSMT= 59; nμSMo= 320; ns= no significativo; **= altamente significativo.

Material and energy balances are important to know if there is a high or low efficiency in a system. According to ^{Odum (1983)}, agricultural systems are open and dissipative, a situation that involves energy use for growing, maintaining and establishing a flow of matter and energy in its environment. This paper shows the results of SMT that employs 2 114.4 MJ ha^-1 equivalent to 57.6 Lde ha^-1 of ef_Total and produces 82.6 kg of CO₂e tm^-1 of maize grain. The SMo used 3 002.6 MJ ha^-1 corresponding to 81.8 Lde ha^-1 that generates 119.4 kg of CO₂e tm^-1 of maize.

^{Guevara et al. (2013)}; Reyes-Muro et al.(2013) reported on a paper carried out in Frailesca Chiapas region, that for that maize area whose average yields were 3.47 tm ha^-1 producers used 1 788.2 MJ tm^-1 of maize approximately. However, the direct use of fuel (for tractor) represents 6% of the total, which involves the use of 108 MJ tm^-1.The situation is that in that region 85.7% of producers use “improved” seeds that generate relatively high yields and only 11% use soil preparation machinery; the rest do the work manually. In this case, most of energy used for production systems is biological (human and yoke), which is why they emit only 8.1 kg of CO₂e tm^-1.

While in a research with different crops in La Rioja, Spain, the most similar for the point of comparison with maize are wheat and barley; according to ^{Domenech et al. (2012)} agricultural activities generate 1.42 tm of CO₂e ha^-1 for the concept of direct emissions; the sum of direct and indirect emissions exceeds 2 tm CO₂e ha^-1 for those crops.

In both cases, the results of reported CO₂e emissions are extreme. While in ^{Guevara et al. (2013)} paper direct use emissions represent 6%, ^{Doménech et al. (2012)} report is 11 times higher (67%) in relation to the same concept. However, in the first one the crop is maize and in the second are barley and wheat. In addition, the context is very different between them, since the case of Chiapas is carried out in conditions of a high use of labor for maize cultivation, and in the case of La Rioja apparently it is of intensive use of machinery and equipment in barley and wheat crops.

Other references are Wayne (1990) and ^{Pimentel and Pimentel (2005)}, which assert that in the United States of America 142.5 L of diesel are used for each tm of maize grain produced.This amounts to 7 425.8 MJ tm^-1 maize, generating 437 kg of CO₂e tm^-1, which are higher values than those obtained in this paper. According to SIAP (2015), for 2014 in the state of Tlaxcala 115 603 ha of maize were sown and 114 453 ha were harvested with an average yield of 3.18 tm ha^-1. The total amount of grain produced at state level was 364 450 tm during that year, considering rain-fed and irrigation conditions.

If all Tlaxcala maize had been produced in conditions of fossil energy use similar to that used in the United States of America in 2014, it would have been 2 706 332 810 MJ or 2 706 333 TJ of energy and 151 631 tm of CO₂e would have been generated. But if all the tlaxcalteca maize production had been under similar conditions to SMT on the use of fossil fuels, the energy used must have been 389 582.5 TJ and 30 103.6 tm of CO₂e would have been emited. However, making a calculation close to reality of how it occurred, and considering that only 5% of producers planted in SMT and 95% in SMo, the energy used was approximately 554 577.2 TJ and generated around 42 844.7 tm of CO₂e.