Reflections of crop-livestock integration on nematodes

Crop-livestock integration (ILP), if well managed, makes it possible to diversify and increase food production per area, improving the income of rural producers and the quality of the environment. In the southern region of Brazil, a widely used system is that which alternates crops for grain production in the summer, especially soybeans, with annual winter pastures for cattle grazing, mainly black oats mixed or not with annual ryegrass. However, two aspects can affect the sustainability of ILP in the subtropical region of Brazil: the continued cultivation of the soy/oat succession and the excess animal load on pasture.

Direct planting is used by almost all rural producers, however a fundamental principle of this system, crop rotation, has been neglected for economic reasons. Approximately 15 million hectares have been cultivated in a succession of soybeans/black oats in the southern region of Brazil for 20 years. The lack of crop rotation has resulted in several problems, among which the increased occurrence or emergence of new pests, diseases and weeds stands out, both in soybeans and oats. One of the most serious problems is the increase in areas infested by phytonematodes, especially due to the use of susceptible soybean cultivars.

Another problem frequently observed in properties that adopt ILP is the excessive animal load in the pasture, which reduces the biomass of forage plants, increases soil compaction, reduces water infiltration and aeration, increases erosion, reduces the content of organic matter and nutrient cycling, etc. All of this results in high stress on forage plants and grain-producing plants grown in succession. It has already been proven that plants grown under environmental stress conditions are less tolerant to the incidence of phytonematodes.

To date, no study on the occurrence of phytonematodes in ILP systems in the subtropical region of Brazil had been carried out, which is why research was carried out to evaluate the infestation of nematodes in a crop-livestock integration, which for 15 years has been cultivated with the soy/oat + ryegrass succession is managed with four grazing intensities.

The study was carried out at Fazenda do Espinilho, which belongs to Agropecuária Cerro Coroado, located in São Miguel das Missões, Rio Grande do Sul, in an experimental area of ​​22ha (Figure 1). The soil is classified as a Red Oxisol, deep, well-drained and with a clayey texture (540, 270 and 190 g/kg of clay, silt and sand, respectively). In 2001, the ILP system was installed and since then the area has been continuously cultivated under the no-till system in a succession of soybeans in the summer (November to April) and black oats + ryegrass in the winter (May to October) for grazing cattle. The area was divided into 12 plots of 0,90ha to 3,53ha, where pasture is maintained at heights of 10cm, 20cm, 30cm or 40cm in winter by grazing three steers in each plot (average initial live weight of 200kg) ( Figure 1). There are also two plots without winter grazing, where oats + ryegrass were used only to produce straw for summer direct planting.

Soil collections for nematode analysis were carried out in summer and winter, when soybeans and black oats were flowering. In each plot, five composite samples were collected (the result of mixing nine subsamples each) with the aid of a drill bit, up to 20cm deep. Nematodes were extracted from 300cm3 of soil and identified at the Soil Biology Laboratory at the Federal University of Santa Maria.

Eight genera of phytonematodes were identified in the ILP soil, all below the level of economic damage: Helicotylenchus, Xiphinema, Tylenchus, Hoplolaimus, Hemicycliophora, Meloidogyne, Pratylenchus e Trichodorus. The genus Helicotylenchus was the most abundant, representing 77% of the phytonematodes collected from the pasture and 82% from soybeans (Figure 3). Despite widespread dominance, only the species Helicotylenchus dihystera was detected. The gender Xipinema it represented 17% of the phytonematodes collected in pasture, but only 5% in soybeans. Gender-contrary behavior Tylenchus, which harbored 4% of the phytonematodes collected in pasture, but 12% in soybeans. The genres Meloidogyne e Pratylenchus represented only 1% of soybean samples.

The level of economic damage from Helicotylenchus to soybean crops has not yet been established. But research indicates that population densities of around five thousand specimens per 100cm³ of soil did not cause economic damage. Even if Helicotylenchus is considered a secondary parasite for this crop, it has been reported with great frequency in several production systems. The secondary damage caused comes from its association with other phytonematodes and by facilitating the entry of pathogenic fungi and bacteria into open wounds in the root system.

The intensity of winter grazing altered the abundance of pasture and soybean phytonematodes (Figure 4). The pasture height of 40cm and the ungrazed area showed high densities in winter. On the other hand, treatments with grazing at 10cm and 20cm showed the lowest densities of phytonematodes.

In the analyzes carried out in the summer, it was observed that the grazed area with the lowest pasture height in winter (10cm) resulted in a higher density of phytonematodes in soybean (Figure 4), the vast majority of these being of the genus Helicotylenchus. Although soybeans tolerate high densities of this phytonematode, data indicate that intense grazing in winter increased the density of soybeans by 374%. Helicotylenchus in soybeans. If this management continues to be adopted, in the coming years these nematodes could become a phytosanitary problem for soybean crops. Ax et al. (2019) consider H. dihystera as a potential pathogen of soybeans in Brazil, where its dispersion and density have increased in crops.

This ILP was cultivated for 15 years with the same crop succession (soybean/black oats + ryegrass) and with soybean cultivars susceptible to phytonematodes. Therefore, it was expected that the phytonematode infestation would be greater. However, the characteristics of this ILP contributed to the reduction in the abundance of nematodes. Research indicates that black oats and ryegrass are not good hosts for nematodes, so growing these plants would help reduce nematode reproduction during the winter. The clayey texture (540kg) of the soil can also limit the development of phytonematode populations. Several authors report that nematodes of the genera Meloidogyne e Pratylenchus They prefer soils with a sandy to clayey texture.

The deposition of cattle manure on the soil during winter grazing can also help control phytonematodes. Ammonia is released during the microbial decomposition of manure and has a nematicidal effect. Furthermore, manure deposition increases the biological activity of the soil, increasing the number of predators and parasites of phytonematodes (natural enemies). In this same experimental area, Da Silva et al. (2014) determined during a pasture cycle the manure production of 669kg/ha, 478kg/ha, 366kg/ha and 213kg/ha (in dry mass) for pasture heights of 10cm, 20cm, 30cm and 40cm, respectively. Thus, it is possible that the lower abundances of phytonematodes in areas measuring 10cm, 20cm and 30cm are related to the greater deposition of manure in winter.

The conditions present in the ILP contribute to the fact that the incidence of phytonematodes in the soil is not high, however, adjusting the animal load is essential for the sustainability of this system. Several studies carried out in this area indicate that when grazing is carried out at 20cm and 30cm, it is possible to combine greater meat production, improved soil quality and reduced environmental impacts, without damaging the productivity of soybeans cultivated in succession. The data from this study confirm this, as a greater pasture height (40cm) increases the density of phytonematodes in the pasture, while a lower pasture height contributes to the growth of phytonematode density in soybeans.

 

Juliane Schmitt and Rodrigo Josemar Seminoti Jacques, Federal University of Santa Maria; Paulo César de Faccio Carvalho, Federal University of Rio Grande do Sul