**3. Selenium fertilization for tropical grazing**

The geochemistry of livestock-producing areas should be well understood to mitigate selenium-related disorders in animals [22]. In China, there are areas with selenosis occurrences; in contrast about 70% of China shows Selenium deficiency, as well as there are about 76% countries located in Se-deficient regions where the Se daily intake level is less than recommended [23].

Brazil is a country who owns the largest livestock in pastures; however, the majority of pastures are in marginal areas, under soils of low fertility. Brazil is one of the largest meat producers. In Brazilian food, the highest Se concentrations have been found to animal origin products, while vegetable food showed lower values [12]; however, some studies showed selenium deficiencies also in cattle, except in north areas [24, 25].

Cattle deficiencies have a likely relation between low Se in forage grass or supplements [26]. Thus, Se deficiency keeps as a concern, since this nutrient has been found in low levels in Brazilian diet.

Selenium fertilization could be a solution, but it is not realized in Brazil. Fertilizers containing selenium could support diet supplementation of grazing animals or animals feed with conserved forages (hay and silage), mainly considering Brazilian pasture areas with more than 160 million of hectares [27].

Selenium fertilization in pastures could increase its presence in animal and human diet besides its benefits to animal productivity. Feeding weaned beef calves for 7 weeks with alfalfa hay containing up to 3.26 mg kg<sup>−</sup><sup>1</sup> Se in dry matter harvested in fertilized fields with Se resulted to increasing whole-blood Se concentrations and body weights depending upon the Se application rate [28].

Grazing Se fertilizer has been shown to be more effective and safe treatment than animal dosing [13]. Selenium intake by ruminants in organic form, especially as selenoamino acids, needs to be release from proteins until through inorganic forms to be metabolized while Se in milk is from organic Se supply or converted by ruminal microorganisms [29].

Experiment with tropical grass has been shown large differences between Se contents in leaves and stem + sheath. *Urochloa brizantha* had desirable increased Se in leaves (0.4 mg kg<sup>−</sup><sup>1</sup> Se in DM) even in smaller evaluated dose of 10 g ha<sup>−</sup><sup>1</sup> Se up to 30 days, while the proportion of stem + sheath (0.1 mg kg<sup>−</sup><sup>1</sup> Se in DM) however for safe and required concentration in dry matter for cattle along with prolonged effect of two cuts, the authors recommended Se fertilization of 34.5 g ha<sup>−</sup><sup>1</sup> [16].

The fast answer of Se fertilization in tropical pastures can be positive to allow the low and safe doses of Se application along with nitrogen rates during rainy season, including for intercropped pastures with legumes. The concerning of intercropped pastures is explained by legume higher ability to produce protein, which can be apparently favorable to Se absorption.

Thus, this fact probably will require lower Se doses for fertilization in intercropped pastures, including caution with palatability and proportion of legumes in pastures. For example, in three different weathered soils carried out in pots, fertilization of 20 g ha<sup>−</sup><sup>1</sup> Se showed desirable concentrations to legume *Stylosanthes capitata* (136 μg kg<sup>−</sup><sup>1</sup> Se in DM), while it was insufficient to grass *Urochloa brizantha* (49.1 μg kg<sup>−</sup><sup>1</sup> Se in DM); however, it showed negative influence in content of crude protein from legume, probably by influence in nitrogen biological fixation (unpublished data).

High Se contents in leaves from tropical grass and in legumes comprise another fact to be analyzed for Se fertilization rate establishment although with few data. Usually, grass leaves and legumes are preference fractions for cattle according to its intake selective behavior, mainly in tropical pastures due to high accumulation of stem portions and its low digestibility. Selenium is one of few elements absorbed by plants in enough quantities which enable to intoxicate domestic animals [30].

Evaluating in vitro degradability of two cuts of *Urochloa brizantha* produced using fertilization rates of 0, 10, 20, 40, and 80, and 160 g ha<sup>−</sup><sup>1</sup> Se verified effects on gas production by truly degraded organic matter, amounts of acetic by propionic acids, some short-chain fatty acids and ammonia amounts [31]. The authors suggested that high levels of Se in forage can affect negatively ruminal microorganism activities but indicates positive effects of Se fertilization in 20 and 80 g ha<sup>−</sup><sup>1</sup> on chemical composition, in vitro degradability, short-chain fatty acids, and gas production.

In superior plants dual effects can be exerted by Se; at low concentrations it acted as an antioxidant, inhibiting lipid peroxidation, whereas at higher concentrations, it was a prooxidant [6]. Applied doses of selenate above 71 g ha<sup>−</sup><sup>1</sup> Se exceed maximum recommendation of 5 mg kg<sup>−</sup><sup>1</sup> Se in dry matter of the leaves of *Urochloa brizantha* to avoid toxicity problems in cattle [32] however with no damages in dry matter production [16]. Thus, even the plant is aparently normal, is necessary attempt that the recommendation for nutritional requirements of animals are values between 0.1 to 0.3 mg kg<sup>−</sup><sup>1</sup> in the dry matter required by cattle [20].

The effect of high Se diet concentration (60–70 μg kg BW<sup>−</sup><sup>1</sup> d<sup>−</sup><sup>1</sup> ) provided from wheat to steers indicated the negative effects of Se level used in this study on productive performance of feedlot which were not expected [25]. Low forage digestibility can contribute to low Se effects in animals, mainly in tropical forages, while high Se content in forage can reduce its digestibility. Undegraded residues from in vitro incubation contained 25–66% of Se from *Urochloa brizantha* Se enriched [31].

#### **4. Final considerations**

Selenium fertilization in tropical low Se soils, as Brazil agricultural areas, is an emergent necessity for animal and human health, also could be beneficial for plants. Thus, Se fertilization in pastures is an alternative to collaborate for animal supplementation and human nutritional demands.

Generally, Se quantities required as fertilizer are low, and there are already available technologies to application but is an extremely necessary soil monitoring.

Nevertheless, more researches in tropical environments is required to establishment of Se rates, plants, and animal answers and reduces or even neutralizes toxicity risks, even though the benefits already are known.

*Selenium Fertilization in Tropical Pastures DOI: http://dx.doi.org/10.5772/intechopen.89423*
