GrainGenes

ITEMS FROM MEXICO

 

INIFAP, CAMPO EXPERIMENTAL VALLE DEL MÉXICO

km 38.5 Carr. MéxicoT_excoco, Chapingo, Edo. de México CP 56230.

INIFAP, CAMPO EXPERIMENTAL VALLE DEL YAQUI

Apdo. Postal 515, km 12 Norman E. Borlaug, entre 800 y 900, Valle del Yaqui, Cd. Obregón, Sonora, México CP 85000,

INSTITUTO TECNOLÓGICO DE SONORA

Dirección Académica de la División de Recursos Naturales, Depto. de Biotecnología y Ciencias Alimentarias, 5 de Febrero 818 Sur, Cd. Obregón, Sonora, México CP 85000.

Evaluation of head selections from 14 elite bread wheat lines for resistance to Karnal bunt. [p. 44-48]

Guillermo Fuentes-Dávila and Miguel Alfonso Camacho-Casas.

Introduction. Karnal bunt of wheat affects bread wheat (Mitra 1931), durum wheat, and triticale (Agarwal et al. 1977). Generally, kernels are partially bunted (Mitra 1935; Bedi et al. 1949; Chona et al. 1961). Control of this pathogen is difficult because teliospores are resistant to physical and chemical factors (Krishna and Singh 1982; Zhang et al. 1984; Smilanick et al. 1988). Chemical control can be accomplished by applying fungicides during flowering (Fuentes-Dávila et al. 2005), however, this measure is not feasible when quarantines do not allow tolerance levels for seed production. Resistant wheat cultivars are the best mean to control this disease. The susceptibility of bread wheat has been documented (Fuentes-Dávila et al. 1992, 1993) reaching infection levels above 50% under artificial inoculations; however, there also are reports of bread wheats which consistently have shown low infection levels (Fuentes-Dávila and Rajaram 1994). Maintaing an evaluation program of new lines that have reached an advanced stage of homocygosis and which are suitable for commercial release as a measure to avoid economic problems for farmers due to Karnal bunt is important. Our objective was to evaluate individual head selections of elite bread wheat lines for resistance to Karnal bunt.

Materials and methods. Head selections from 14 elite bread wheat lines (Table 1) were evaluated for resistance to Karnal bunt during the crop cycle autumn-winter 2003-04 in the Yaqui valley, Sonora, Mexico. Planting date was 10 December, 2003, using a 1-m bed with two rows. Inoculations were by injecting 1 mL of an allantoid sporidial suspension (10,000/mL) during the boot stage on two spikes from each of 106 head selections per line, with the exception of lines 6, 8, and 12, where 95, 94, and 103 spikes were inoculated, respectively. Harvest was manual, and the counting of healthy and infected grains was done visually to determine the percentage of infection. Evaluated lines originated from the collaborative project between CIMMYT and INIFAP.

Results and discussion. The range of mean infection of head selections was 1.02 to 14.7% (Figure 1). Sixty-five percent of the head selections had infection levels between 0 and 2.5%, 5.2% between 2.6-5%, 12.2% between 5.1-10%, 16.2% between 10.1-30%, and 1.3% had infection levels greater than 30% (Figure 2). Lines with less than 5% infection are considered resistant (Fuentes-Dávila and Rajaram 1994). Although the range of mean infection was rather low, there was quite of variation among head selections within single lines. The range of infection of head selections derived from lines 1 to 14 were 0-13.7, 0-15.2, 0-17.9, 0-13.9, 0-20.5, 0-18, 0-27.1, 0-38.1, 0-14.3, 0-38.9, 0-38.5, 0-22.8, 0-30.1, and 0-41, respectively (Figure 3, Figure 4, Figure 5, and Figure 6). Head selections with the highest levels of infection derived from the following lines: IRENA/BABAX//PASTOR, CNDO/R143//ENTE/MEXI_2/3/AEGILOPSTAUSCHII (TAUS)/4/WEAVER/5/ 2*PASTOR, FISCAL (27Y), FISCAL (11Y), and CNO79// PF70354/MUS/3/PASTOR/4/BABAX, with 30.1, 38.1, 38.5, 38.9, and 41%, respectively. These results indicate that a) escapes are more possible when inoculating a low number of spikes, and in only one planting date, as this was the case and b) segregation might be responsible in part for the variation in levels of infection within lines, because elite lines are bulked in F₆ and F₇. However, additional testing of greater number of spikes in more planting dates would be necessary to confirm the resistance shown by the group of head selections with infection levels between 0 and 5%.

References.

• Agarwal VK, Verma HS, and Khetarpal RK. 1977. Occurrence of partial bunt on triticale. Plant Protect Bull 25:210-211.
  
• Bedi SKS, Sikka MR, and Mundkur BB. 1949. Transmission of wheat bunt due to Neovossia indica (Mitra) Mundkur. Ind Phytopath 2:20-26.
  
• Chona BL, Munjal RL, and Adlakha KL. 1961. A method for screening wheat plants for resistance to Neovossia indica. Ind Phytopath 14:99-101.
  
• Fuentes-Dávila G, Tapia-Ramos E, Toledo-Martínez JA, and Figueroa-López P. 2005. Evaluación de efectividad biológica de folicur 250 EW (Tebuconazol) para el control del carbón parcial (Tilletia indica) del trigo (Triticum aestivum), en el valle del Yaqui, Sonora, México, durante el ciclo de cultivo 2003-2004. Resœmenes, XIII Congreso Latinoamericano de Fitopatolog’a, III Taller de la Asociación Argentina de Fitopatólogos. 19-22 de Abril, 2005. Villa Carlos Paz, Córdoba, Argentina. Resumen HC-29, página 271. 640 pp. (In Spanish).
  
• Fuentes-Dávila G and Rajaram S. 1994. Sources of resistance to Tilletia indica in wheat. Crop Protect 13(1):20-24.
  
• Fuentes-Dávila G, Rajaram S, Pfeiffer WH, and Abdalla O. 1992. Results of artificial inoculation of the 4th Karnal Bunt Screening Nursery (KBSN). Ann Wheat Newslett 38:157-162.
  
• Fuentes-Dávila G, Rajaram S, Pfeiffer WH, Abdalla O, Van-Ginkel M, Mujeeb-Kazi A, and Rodríguez-Ramos R. 1993. Resultados de inoculaciones artificiales del 5o. vivero de selección para resistencia a Tilletia indica Mitra. Rev Mex Micro 9:57-65.
  
• Krishna A and Singh RA. 1982. Effect of physical factors and chemicals on the teliospore germination of Neovossia indica. Ind Phytopath 35:448-455.
  
• Mitra M. 1931. A new bunt of wheat in India. Ann Appl Biol 18:178-179.
  
• Mitra M. 1935. Stinking smut (bunt) of wheat with a special reference to Tilletia indica Mitra. Ind J Agric Sci 5:1-24.
  
• Smilanick JL, Hoffmann JA, Secrest LR, and Wiese K. 1988. Evaluation of chemical and physical treatments to prevent germination of Tilletia indica teliospores. Plant Dis 72:46-51.
  
• Zhang Z, Lange L, and Mathur SB. 1984. Teliospore survival and plant quarantine significance of Tilletia indica (causal agent of Karnal bunt) particularly in relation to China. Eur Plant Protect Bull 14:119-128.

Reaction of wheat cultivars WL-711 (Triticum aestivum) and Altar C84 (T. turgidum subsp. turgidum) to inoculation with Tilletia indica cultures obtained from infected wheat cultivars Baviacora M92 (T. aestivum) and Altar C84 under natural conditions in the Yaqui valley, Sonora, Mexico. [p.48-52]

Irazema Fuentes-Bueno (Instituto Technológico de Sonora) and Guillermo Fuentes-Dávila.

Introduction. Since the early 1980s, a project on breeding for resistance to Karnal bunt in the Yaqui Valley was initiated by the wheat program of CIMMYT (Metzger 1986). The project contemplated three main objectives: a) identification of sources of resistance, b) hybridization to incorporate resistance genes into suitable genotypes, and c) evaluation of advanced lines (Fuentes-Dávila 1997). During the course of the project, artificial inoculations in the field have been an essential component (Fuentes-Davila et al. 2001), because disease incidence is quite erratic in the Yaqui Valley (Lira-Ibarra 1992). The inoculum used has been a mixture of fungal cultures obtained from teliospores produced in wheat commercially grown and naturally infected in the Yaqui Valley (Fuentes-Dávila and Rajaram 1994). Our objective was perform preliminary testing of fungal cultures of T. indica for physiologic specialization.

Inoculum preparation. Infected grains from cultivars Baviacora M92 and Altar C84 were obtained from commercial fields in the Yaqui Valley, Sonora, Mexico, during the crop cycle autumn-winter 2002-03. Teliospores were scraped off infected grains with a dissecting needle and kept in a water-Tween 20 solution for 24 h, then the suspension was filtered through a 60 µm nylon sieve and centrifuged at 3,000 rpm. After discarding the supernatant, sodium hypochlorite (0.5% a.i.) was used to disinfect teliospores for 2 min while centrifuging again. Teliospores were then rinsed twice with sterile distilled water while centrifuging. Teliospores were resuspended in sterile distilled water in the centrifuge tube, and one mL of the teliospore suspension was spread on Petri plates with 2% water-agar (AA), which were incubated at 20°C in the dark. After 6 to 9 days, teliospore germination was evaluated using a compound microscope at 10X. Pieces of AA with germinated teliospores were removed and placed upside down on the lid of Petri plates containing potato-dextrose-agar (PDA). After 10 to 14 days, 2 to 3 mL of sterile distilled water were added to the plates and the colonies were scraped gently using a sterile spatula. Hyphae and sporidia were inoculated onto other plates with PDA using a sterile syringe, and the plates were incubated at 20°C in the dark for about nine days. After incubation, pieces of PDA with the different fungal propagules were transferred and placed upside down on the lids of sterile glass Petri plates, in order to induce production of allantoid secondary sporidia (Dhaliwal and Singh 1989; Fuentes-Dávila et al. 1993). Three mL of sterile distilled water were added to the bottom of the plates. Plates with fungal propagules derived from teliospores produced on Altar C84 and Baviacora M92 were kept separately. Water from the plates was collected every 24 h, secondary allantoid sporidia were collected and counted using a hemocytometer; then, the concentration was adjusted to 10,000/mL.

Artificial inoculation in the field. Twenty heads of the cultivars WL-711 (susceptible) and Altar C84 (resistant) were inoculated by injecting 1 mL of the allantoid sporidial suspension during the boot stage (stage 49, Zadoks et al. 1974), in five planting dates (10, 21, 22, 23, and 24 February). Harvest was done manually, and the counting of healthy and infected grains was done by visual inspection to calculate the percentage of infection (infected grains).

Results. The reaction of WL-711 and Altar C-84 to fungal cultures used in this study showed some consistency. In general, the percentage of infection was low in WL-711, perhaps due to high temperatures that predominated at the end of February (average 26.6ûC during the last four planting dates), and the low relative humidity. The culture obtained from T. turgidum subsp. turgidum caused the greatest infection level on WL-711 with 27.5% in the first planting date; however, the range of infection in the following dates was 0.14 to 3.31% (Figure 7A). The number of infected heads varied considerably among dates; in 10 February there were 16 infected heads, while in 21, 22, 23, and 24 February there were 1, 1, 8, and 3, respectively (Figure 8). The highest percent infection in individual heads was 90% for 10 February, and 1.89, 6.8, 22, and 10.3% for the rest of the dates, respectively. This culture also caused infection in Altar C84, but the levels of infection were low, which might be expected, because this cultivar is resistant to Karnal bunt (range of infection 0 to 1.47%) (Figure 7B). The number of infected heads was lower than in WL-711, with 0, 7, 0, 1, and 8 for the different dates. The highest infection levels in individual heads were 2.8, 5.5, and 13.3 for 21, 23, and 24 February, respectively (Figure 9).

The fungal culture obtained from T. aestivum caused the greatest level of infection on WL-711 on the third planting date with 14.12% (Figure 10A). This culture showed more consistency in the infection level of WL-711 than the previous culture, with a range of 2.82 to 14.12%. Also, this culture showed more consistency in relation to a greater number of infected heads, with 15, 10, 17, 6, and 13 for 10, 21, 22, 23, and 24 February, respectively (Figure 11). The highest levels of infection in individual heads were 37, 32, 45, 24, and 10.3 for the different dates. Altar C84 showed a resistant reaction to this fungal culture with a range of infection of 0 to 0.95% (Figure 10B). The number of infected heads was 2, 0, 3, 3, and 2 for 10, 21, 24, 23, and 24 February, respectively (Figure 12). The highest levels of infection in individual heads were 7.9, 14.3, 15.6, and 4.3 for 10, 22, 23, and 24 February, respectively.

Although differences were found in levels of infection and in number of infected heads of WL-711 and Altar C84, after artificial inoculation with individual fungal cultures obtained from naturally infected durum and bread wheat, the results do not indicate physiologic specialization. Further studies should be conducted, including molecular characterization of fungal cultures.

References.

• Dhaliwal HS, Randhawa AS, Chand K, and Singh D. 1983. Primary infection and fruther development of Karnal bunt of wheat. Ind J Agric Sci 53:239-244.
  
• Fuentes-Dávila G. 1997. Carbón parcial del trigo: situación actual y perspectivas. Memorias del primer simposio internacional de trigo, 7 al 9 de Abril de 1997, Cd. Obregón, Sonora, México. pp. 105-118 (In Spanish).
  
• Fuentes-Dávila G, and Rajaram S. 1994. Sources of resistance to Tilletia indica in wheat (Triticum aestivum). Crop Protect 13(1):20-24.
  
• Fuentes-Dávila G, Rajaram S, Pfeiffer WH, Abdalla O, Van-Ginkel M, Mujeeb-Kazi A, and Rodríguez-Ramos R. 1993. Resultados de inoculaciones artificiales del 5o. vivero de selección para resistencia a Tilletia indica Mitra. Rev Mex Micro 9:57-65 (In Spanish).
  
• Fuentes-Dávila G, Rajaram S, and van-Ginkel M. 2001. Artificial inoculation: An efficient tool in breeding for resistance. In: Proc IVth International Scientific Seminar of Plant Health-41st Annual Meeting of the American Phytopathological Society-Caribbean Division. Varadero, Cuba, 11-15 June, 2001.
  
• Lira-Ibarra M. 1992. Karnal bunt (Tilletia indica) of wheat in the state of Sonora, Mexico. pp. 6-9. In: Update on Karnal Bunt Research in Mexico (Fuentes-Dávila G and Hettel GP, Eds). Wheat Special Report No. 7. CIMMYT. Cd. Obregon, Sonora, Mexico. 38 p.
• Metzger RJ. 1986. Screening for resistance to Karnal bunt. In: Proc 5th Biennial Smut Workers' Workshop, 28-30 April, 1986. Cd. Obregón, Sonora, México. p. 38
  
• Zadoks JC, Cheng TT, and Konzak CF. 1974. A decimal code for the growth stages of cereals. Weed Res 14:415-421.

Reaction of advanced lines and cultivars of wheat to Karnal bunt artificial inoculation. [p. 52-54]

Guillermo Fuentes-Dávila, Héctor Eduardo Villaseñor-Mir (Campo Experimental Valle de México), and Pedro Figueroa-López.

Introduction. Karnal bunt is caused by the fungus Tilletia indica. This disease occurs in bread wheat (Mitra 1931), durum wheat, and triticale (Agarwal et al. 1977). In general, infected kernels are partially affected (Mitra 1935, Bedi et al. 1949, Chona et al. 1961). The susceptibility of bread wheat has been documented (Fuentes-Dávila et al. 1992, 1993) reaching infection levels above 50% under artificial inoculations, therefore, it is important to have in place a program of evaluation of wheat germplasm, in order to provide to farmers wheat cultivars which are tolerant to this disease. The objective of this work was to evaluate the reaction of new wheat advanced lines and cultivars to artificial inoculation with Karnal bunt.

Materials and methods. Thirty-nine advanced bread wheat lines and eleven cultivars produced in the collaborative project CIMMYTÐINIFAP, were evaluated for resistance to Karnal bunt during the crop cycle autumn-winter 2005-06 in the Yaqui Valley, Sonora, Mexico. Planting dates were 28 November and 19 December, 2005, using approximately 10 g of seed in 1-m beds with two rows. A mist irrigation system was used 3 to 5 times each day for 15 min each time, to provide high relative humidity in the experimental area. Inoculations were by injecting 1 ml of an allantoid sporidial suspension (10,000/mL) during the boot stage on 10 heads/line. Harvest was done manually, and the counting of healthy and infected grains was done by visual inspection to calculate the percentage of infection (infected grains).

Results. The range of infection for the first planting date was 0 to 13.9%, with a mean of 4.31. In this date, 34 genotypes had infection levels below 5% (Figure 13). The range of infection for the second planting date was 0 to 23.9%, with a mean of 5.6; 29 genotypes had infection levels below 5% (Figure 14). Considering the highest levels of infection obtained in each genotype, the distribution was the following: 9 genotypes were in the 0.1-2.5 infection category, 17 in 2.6-5.0, 11 in 5.1-10.0, and 13 in 10.1-30 (Figure 15). The mean of the three highest infection scores

References.

• Agarwal VK, Verma HS, and Khetarpal RK. 1977. Occurrence of partial bunt on triticale. Plant Protection Bull 25:210-211.
• Bedi SKS, Sikka MR, and Mundkur BB. 1949. Transmission of wheat bunt due to Neovossia indica (Mitra) Mundkur. Ind Phytopath 2:20-26.
• Chona BL, Munjal RL, and Adlakha KL. 1961. A method for screening wheat plants for resistance to Neovossia indica. Ind Phytopath 14:99-101.
• Fuentes-Davila G and Rajaram S. 1994. Sources of resistance to Tilletia indica in wheat. Crop Protect 13(1):20-24.
• Fuentes-Davila G, Rajaram S, Pfeiffer WH, and Abdalla O. 1992. Results of artificial inoculation of the 4th Karnal Bunt Screening Nursery (KBSN). Ann Wheat Newslet 38:157-162.
• Fuentes-Dávila G, Rajaram S, Pfeiffer WH, Abdalla O, Van-Ginkel M, Mujeeb-Kazi A, and Rodríguez R. 1993. Resultados de inoculaciones artificiales del 5 vivero de selección para resistencia a Tilletia indica Mitra. Revista Mexicana de Micología 9:57-65 (In Spanish).
• Mitra M. 1931. A new bunt of wheat in India. Ann Appl Biol 18:178-179.
• Mitra M. 1935. Stinking smut (bunt) of wheat with a special reference to Tilletia indica Mitra. Ind J Agric Sci 5:1-24.

Tillage methods for wheat cultivation in Southern Sonora, Mexico. [p. 55-57]

Juan Manuel Cortés-Jiménez and Guillermo Fuentes-Dávila.

Introduction. Farmers in the Yaqui Valley, Sonora, Mexico, practice at least four tillage methods for wheat cultivation: conventional tillage, reduced tillage, minimum tillage, and conservation tillage (Cortés 1997). In the first method, subsoiling and plowing are considered the primary tillage operations. In the second method, both operations are eliminated, and the soil is prepared only with disking. In the third method, beds are used for several seasons; after each harvest beds are reformed and residues generally are burned. For conservation tillage, crops are established on a variable amount of harvest residues, which generally cover about 30% or more of the soil surface.

As a definition, Jasa et al. (2000) indicate that conventional tillage is the sequence of operations commonly used in a given geographic area in order to prepare the planting bed to be able to produce a specific crop. Because such activities vary under different conditions, the definition of conventional tillage varies from region to region and even within a region. Conventional tillage operations leave much less than 30% of residues after planting.

In the case of reduced tillage, the same authors indicate that this term refers to any system which is less intensive than conventional tillage. The number of operations is lower or the tillage implements require less energy by unit area than those commonly used in the conventional system. In the case of minimum tillage, they indicate that it is not a useful term, because in most cases it refers to reduced tillage (Jasa et al. 2000). In conservation tillage, most researchers coincide in pointing out that the objective is to provide a proper environment for crop development, while minimizing soil erosion caused by wind and water. Basically, this can be achieved leaving a residue cover on the soil or establishing a cover crop (Jasa et al. 2000).

Although emphasis has been given to soil conservation, water and energy conservation, as well as less depreciation of equipment and agricultural machinery, represent some additional benefits of using this technology.

An economic analysis about the most used tillage systems is in Table 2 and describes the cost of each operation involved in soil preparation. The sequence of operations for each one of the tillage methods and the total cost of soil preparation varies from one farmer to the next.

As general criteria, in the case of conventional tillage, after harvest a disking operation followed by plowing or subsoiling, then one or two diskings, planking, and bed formation is common. The cost of the most used tillage methods is described in Table 3.

In reduced tillage, preparing the soil with one or two disking operations is possible if harvest residues are burned; otherwise, 2-3 diskings are necessary to properly incorporate residues. The cost of this method with and without residue burning is $81.87 and $106.71/ha, respectively.

In minimum tillage, the bed from the previous crop is used, and the cost of preparation is equivalent to ridge-till, that is $18.39/ha. This operation should be performed once or twice to properly make the bed for the following crop. However, with the machinery available in the valley, this can only be possible after burning residues, because the amount left after the wheat harvest is proportional to the yield obtained, which fluctuates from 1.1 to 1.4 ton of residue per ton of grain produced. Nevertheless, results of research and validation of technology indicate that with minor adaptations it is possible to reuse the bed with all the residue present.

Progress. From 1996 to the present, several evaluations were made on tillage methods. In the first evaluation, plowing did not affect wheat production since yield in the check (without plowing) yielded only 35 kg less than the treatment with plowing (Table 4).

Subsoiling is considered a component of the conventional tillage method. The evaluation indicated that this operation did not have any effect on yield, which makes the relationship between benefit/cost more favorable for the treatment without the use of this implement (Table 4).

After analyzing the effects of primary tillage, the following evaluations consisted in testing secondary or reduced tillage schemes: disking, planking, and bed formation. We observed during the evaluation, a greater economic benefit upon using the minimum tillage scheme which consisted in burning residues of previous crop and reforming the bed (Table 4).

However, burning residues is considered an unacceptable practice because of its effects on the environment. Therefore, methods for reforming the bed with residues were evaluated, and it was observed in the short term that burning residues decreased wheat yield (Table 4).

This intermediate technology consists in bed tillage, which implies using the same bed for several years, and it is only reformed after each harvest, or after the rains when there is weed infestation. With this system, it is possible to reach the planting time with a residue cover of about 30%, which does not limit the use of conventional planters.

Results of using conventional tillage in wheat, indicated that disking operations did not have any effect on yield and it only contributed to increase production costs (Table 4). Results also indicated that the cost of disking was $115.45, whereas conservation tillage was $43.23. We observed that conservation tillage increased in $60.25 the economic return per ha, in relation to the disking treatment. According to the results, the use of tillage implements did not have any impact of economic importance on wheat production. Therefore, we concluded that the minimum number of operations should only be made in order to establish the crop. These results agree with reports by other researchers on conservation tillage under drip irrigation (Félix et al. 2003).

Even when efforts by national or international institutions to promote the use ot conservation technologies have been important, this has not been reflected in a significant increase of area planted under this system.

To reform the bed with all residues, it is necessary to adapt to the traditional implements a corrugated disk which cuts the straw before the pass of a moldboard which reforms the bed; this is done with the objective that the straw does not agglutinate or makes balls. The straw must be dry for a more efficient work. This system, similarly to conservation tillage, should be visualized as a system for which one should be prepared from the previous crop, leveling the land, controlling perennial weeds, using adequate seeding methods, and harvesting with the necessary adjustments in the combine.

Conclusions. The use of tillage implements did not have any impact of economic importance on wheat yield; therefore we can conclude that a minimum number of operations should be carried out in order to establish the crop; it is possible to implement tillage methods that are not so aggressive to the environment that allow to reduce production costs and increase the profitability of wheat in the Yaqui valley; and aside from the technical and economical aspects, information should be generated in order to determine the environmental impact that burning crop residues has on soil, air, and human health.

References.

• Cortés JJM. 1997. Efecto del manejo de la paja de trigo sobre el rendimiento de maíz de verano. Reporte técnico, CEVY-INIFAP, Cd. Obregón, Sonora, México (In Spanish).
  
• Cortés JJM. 2003 . Validación de métodos de labranza en la parte central del Valle del Yaqui. Reporte técnico, CEVY-INIFAP, Cd. Obregón, Sonora, México (In Spanish).
  
• Cortés JJM. 2006. Evaluación de métodos de labranza en el cultivo de trigo. Reporte técnico, CEVY-INIFAP, Cd. Obregón, Sonora, México (In Spanish).
  
• Félix VP, Ortiz JE, and Cortés JM. 2003. Producción y análisis económico de cultivos en labranza de conservación con riego por goteo en el Valle del Yaqui, Sonora. VI. Congreso Internacional de Ciencias Agrícolas, Mexicali, Baja California, México. Pp. 325-330 (In Spanish).
  
• Jasa P, Siemens J, Hofman V, and Shelton D. 2000. Tillage systems definitions. In: Conservation Tillage Systems and Management, Second Ed (Reeder R, Ed). Iowa State University, Ames, Iowa, USA. Pp. 5-9.