Effects of an Agropastoral System on Soybean Production

Increase of the cereal production in the world is desired for increase of global food demand. The savanna agro-ecosystems of South America are one of the most important potential areas for expansion of agricultural production in the world. Therefore, the opening of some 12 million ha of this area has been conducted over the last 40 years (Kerridge, 2001). For example, in the Brazilian savanna, grain crops cover 12-14 million ha and introduced pasture area is over 50 million ha (Maceado, 2001). However, under continuous cropping, soil productivity declines due to soil loss, soil compaction, loss of organic matter and increase in pests, diseases and weeds. For example, the soybean yield in Yguazu, Alto Parana, Paraguay, increased to over 3 t/ha after the introduction of a no-tillage cultivation system in 1983. However, the yield increase stopped in the early 1990’s, and recently the yield seems to be decreasing (Seki, 1999; Shimoda et al., 2010). As a means to solve these problems, Kerridge (2001) suggested that integrated agropastoral systems with no-tillage appeared to be the key to sustainable development. But, since the period used as pasture and the pasture manegement methods have various combination in the system, few studies have been evaluated them synthetically. The Japan International Research Center for Agricultural Socience Japan (JIRCAS) was conducted the joint reserchs of agropastoral system with the National Beef Cattle Research Center of the Brazilian Agricultural research Corporation (EMBRAPA-CNPGC) from 1996 to 2006 and many results were obtained from several experiments (Macedo et al., 2004; Kanno et al., 2004; Miranda et al., 2004 etc.). In addition, JIRCAS was conducted with the Japan International Cooperation Agency’s Paraguay Agricultural Technology Center (CETAPARJICA) from 2003 to 2008 (Shimoda et al., 2010; 2011). In this report, I clarify the positive effects of an agropastoral system synthetically.


Experimental design
For Exp.-1, part of a field at CETAPAR-JICA, where soybean and wheat had been continuously cropped in a no-tillage system since 1993, was converted to Guinea grass (Panicum maximum cv. Tanzania) pasture in 1996. Established as a permanent pasture, it was maintained without fertilizer, cutting, or renovation for 7 years after establishment, and was used as a complementary pasture. In October 2003, the pasture was converted into an agropastoral plot where soybean and wheat were cultivated. the agropastoral plot was 2.97ha. In another part of the field adjacent to the agropastoral site, where soybean and wheat had been continuously cultivated in a no-tillage system since 1993, the non-converted treatment was replicated in three plots (control plots). Each plot was 0.68 ha. For Exp.-2, 15 plots were arranged at the study site, each plot was 0.68 ha (124 m × 55 m) where soybean and wheat had been continuously cultivated in a no-tillage system since 1993. Twelve plots were randomly converted to Guinea grass (Panicum maximum cv. Monbasa) pasture in November 2003. These pastures were managed as intensive grazing pastures under high grazing pressure. The strip grazing was conducted in the pasture year round, and cattle were fed supplement during four months in dry season. Fertilization was also conducted (ammonium sulfate). The stocking rate was from 4.5 to 6.0 UA/ha for 3 years. Three plots of these pastures were also reconverted to soybeanwheat fields in October 2007 as no-tillage system (agropastoral plots). The non-converted treatment was replicated in three plots (control plots). Control plots in Exp.-1 and Exp.-2 were same plots.

Chemical and physical properties of soil
To investigate the chemical properties of the soil, samples from depths of 0-10, 10-20, 20-40, and 40-60 cm were collected independently from each plot, and the concentrations of phosphate, the percentage organic matter and pH, were measured. The concentrations of phosphate were analyzed using the Mehlich-III method, and percentage organic matter was analyzed using the Walkley-Black method. The pH of soils was measured using a pH meter (Horiba Co. Ltd.). Moreover, soil was sampled from 0-5, 5-10, 10-20, 20-40, and 40-60 cm depths for measurement of physical properties, three phases of soil, bulk density, and soil aggregates (Only Exp.-1). Three phases of soils and soil aggregates were measured using a three-phase meter and an aggregate analyzer (Daiki Rika Kogyo Co., Ltd.), respectively. We analyzed soybean and wheat production and soil chemical and physical data between the agropastoral plots and control plots using t-test, and the annual variation of chemical data in both plots using the Tukey-Kramer method. Details of the study methods were shown in Shimoda et al (2010Shimoda et al ( , 2011.

Soybean production
Since the yield of a soybean had a large change every year, the effect of agropastoral system was evaluated by using ratio of the soybean yield in agropastoral plots to that in control plots ( Table 1). As a result, the ratios of the first year when reconverted into soybean field from the pasture were 2.35 in Exp-1 and 1.02 in Exp-2, respectively. In addition, the ratios of the second year were 1.86 in Exp-1 and 1.42 in Exp-2, respectively. The effect of Exp-1 was larger than that of Exp-2. www.intechopen.com The first, second and third year in Exp.-1 were drought years and the second year in Exp.-2 was drought year too. The ratios of drought years were larger than the ratio of normal year in both experiments. Moreover, during the experimental period in Exp.-1, the relative yield of soybean decreased year by year from 2.31 t/ha in the first year to 1.11 t/ha in the fourth year. Experiment 1 (Exp-1) 1 Experiment 2 (Exp-2) 2

Pasture condition
Period as a soybean field before rotation More than 3 years More than 10 years

Chemical properties
In Exp-1, the soil organic matter content in the agropastoral plots was 1.20 times higher than that in the control plots with significant difference at the reconversion from the pasture. However, in Exp-2, the soil organic matter content in the agropastoral plots was same (1.04 times) as that in the control plots. Under intensive grazing (Exp.2), even if soybean field was converted to the pasture, the accumulation of organic matter was not promoted (Fig. 1).
On the other hand, the phosphate concentration in agropastoral plots was 0.28 times than that in the control plots in Exp.-1 and 0.19 times than that in the control plots in Exp.-2 (Tble 2). Under extensive and intensive grazing, the accumulation of phosphate at soil surface was dissolved. After conversion to the pasture, phospate concentration was reduced by half in only two years (Fig. 2). But, phosphate accumulation was promoted in control plots. In addition, pH of soil surface was also increased with dissolution of phspate accumulation, the acidic soil was improved by nutral soil (Fig. 3).

Physical properties
In soil samples from deeper than 10 cm, the gaseous phase percentage in the agropastoral plots was significantly higher than in control plots (Fig 4), but was lower at the soil surface (0-10 cm depth). The bulk density of samples from the soil surface of agropastoral plots was higher than that of control plots, and the bulk density of soil samples from deeper than 20 cm was lower than that of control plots (Fig. 4). But, they did not have significantly difference. In addition, the percentage of large aggregates in the soil of the agropastoral plots was 3 to 7% higher than that in control plots at each depth (Fig. 4). Especially, at the soil surface, it was significantly higher than that in control plots.

Discussion
In the first and second year after reconversion from the pastures, the ratio of the soybean yield in agropastoral plots to that in control plots in Exp.-1 was higher than that in Exp.-2. Therefore, it was thought that the positive effects on soybean productivity of the agropastoral system with extensive grazing and long term pasture was higher than that with intensive grazing and short term pasture. Macedo et al. (2004) reported that the mean ratio of the soybean yield in the first year between all 4 years agropastoral plots and control plots was 1.12 (calculated from their table) under a conventional grazing system which the weight gain of cattle per hectare was one-third that of our intensive grazing system. But, it had higher grazing pressure than that of our extensive grazing system. Therefore, it was considered that the positive effect on soybean productivity was large as the grazing pressure was low. However, it was thought that it would be lost in about four years even if agropastoral system with the extensive grazing and long term pasture (seven years) was conducted. So, it was important for this system to convert a field into a pasture continually.
In the drought year, the positive effect on soybean productivity was clear. It was considered as a reason that the phosphate accumulation at soil surface was dissolved. Many studies reported that the root of soybean was distributed within a shallow soil layer with no-tillage system (Iijima et al., 2007;Izumi et al., 2009). In addition, the phosphate accumulates near the soil surface, which restricts a crop root distribution within a shallow soil layer with a notillage system (Holanda et al., 1998;Seki et al., 2001). Plants with shallower root systems have a disadvantage for uptake and sensitive to drought (Schwinning ,1988). In general, soybean does not grow well in acidic soil, and a pH range of 6.0 to 6.5 is best for soybean cultivation (Kokubun, 2002). In our system, soil pH in top soil improved from 5.89 to 6.11 over 3 years in the pasture, and conversely, soil pH became lower and the soil acidified in the control plots. Therefore, possibly the improvement of soil pH had same effect on the increase of the soybean yield. Studies have reported that the accumulation of organic matter in soil is promoted by introducing agropastoral systems Salton & Lamas, 2007;Shimoda et al., 2010). In general, organic matter develops the soil aggregate structure and improves the water-holding capacity (Uwasawa, 2002). However, the accumulation of organic matter was promoted in Exp.-1 and not promoted in Exp.-2. Ogawa & Mitamura (1982) also reported that the accumulation of organic matter was not promoted by grazing. It was a reason that the root growth was inhibited by cutting the aboveground part of the grass (Davidson & Milthorpe, 1966a;1966b). In addition, a lot of grass was grazed and much cattle meat (1.54 ton/ha) was carried out from the pasture every year under our intensive grazing. Therefore, it was thought that the positive effects on soybean productivity in Exp.-1 was larger than that in Exp.-2 by the promotion of organic matter accumulation. In Exp.-1, at soil surface (0-10 cm at depth) in agro-pastoral plots, the percentage of gaseous was lower and bulk density was higher. Soil compaction of soil surface inhibits soybean production (Ae, 1997). However, since soil sampling was carried out immediately after killing off Guinea grass by herbicide, soil compaction at the surface would disappear rapidly by decomposition of the root of Guinea grass after that. And, since the percentage of gaseous phase of soil in agropastoral plots was higher in the soil layer from 10 cm to 50 cm at depth, it was thought that soil compaction occurred in no-tillage cultivation had improved. In addition, percentage of large aggregate of soil of agro-pastoral plots was www.intechopen.com higher than that of control plots in each depth. Higher percentage of large aggregate may promote inflow of air to underground. Inflow of air to underground promotes nitrogen fixation of soybean (Ae 1997). Therefore, it seemed that the improvement of physical properties of soil has contributed to recovery of soybean productivity.

Conclusion
The effects on soybean productivity of the agropastoral system with extensive grazing and long term pasture and with intensive grazing and short term pasture were positive. In addition, the positive effect on soybean productivity was clear in the drought year. It was thought the positive effects were promoted by the dissolvation of phosphate accumulates near the soil surface, accumulation of organic matter, improvement of soil compaction, and etc.. The investigation which took in intensive grazing was carried out for the income compensation of soybean farmers during the period used as a pasture. However, it was thought that productivity recovery of a soybean and the productivity of livestock had a relation of a trade-off. So, it waits for research to shorten the period used as a pasture.

Acknowledgment
All of the field tests described in this chapter were conducted in the joint research of JIRCAS and CETAPAR-JICA, Yguazu, Alto Parana, Paraguay. I deeply thank Mr. Toshiyuki Horita, Mr. Ken Hoshiba, and Mr. Jorge Bordon for their helpful advice and support. Special thanks are also due Dr. Kazunobu Toriyama and Dr. Ana Kojima for their helpful suggestion and encouragement.