Rev. prod. anim., 26 (2): 2014 ISSN 2224-7920 Vertisol Soil fertility Indicators in Rotational grazing on the Cauto Valley Jorge Ray*, Diocles Benítez*, Rigoberto García**, M. Nieto* and D. García* * Institue for Agricultural Research Jorge Dimitrov, Bayamo, Granma, Cuba ** Institue for Animal Science, San José de Las Lajas, Mayabeque, Cuba jvray@dimitrov.cu ABSTRACT The behavior of some Vertisol soil fertility indicators was randomly studied for four years, at a milk production unit on the Cauto River Valley, Cuba. Brachiaria humidicola cv CIAT 679 pasture was used without irrigation and fertilization. Grazing intensity had mean variation values, of 243 UGM/ha in the rainy season and 190 UGM/ha in the dry season. The daily grazing stripe depended on pasture availability. The occupation time was 2 days. No signif- icant variations were observed in time, or associated to grazing intensity in terms of physical properties of the soil. The apparent density decreased according to the season (P < 0.05) and the highest value was observed in the dry sea- son (1.42 and 1.18 g/cm 3 ). The grazing stripe variable brought benefits to the balance of exchangeable ions, with a higher ratio of Ca 2+ /Mg 2+ and Mg 2+ /K + . Interaction between season and grazing was produced in mobile forms of soil nitrogen and NO 3 - , NH 4 + and NFH, higher during the rainy season in the grazing areas. Stripe grazing does not dimi- nish the main soil fertility indicators; however, it does favor nitrogen mobility and mineralization, depending on the most suitable grazing stripe variable for this kind of soil. Key Words: soil fertility, rotational grazing, season INTRODUCTION Several authors have reported that many of the factors that limit tropical pasture production are related to soil limitations (Pezo, 1997; Dias-Filho, 2001). When the pasture lands are subjected to inten- sive management systems of rotational grazing, the number of organisms living in the dead leaves is sharply increased; the same occurs to soil biota, which benefits soil fertility due to the contribution such organisms make by decomposing and incor- porating organic matter into the soil (Sánchez et al., 1996). Nevertheless, the literature reviewed on rotational grazing, does not offer complete in- formation on the variable of defoliation intensity. It has been acknowledged that increases in de- foliation intensity of tropical pastures can im- prove their structure, quality and purity (Baldo et al., 1998), but it is important to know how much soil, pasture and animal stability are affected by the sharp increases in defoliation intensity. Even by increasing defoliation intensity, whether by re- ducing the lot size or by increasing the number of animals, may cause several various effects on the behavior of these elements of the production sys- tem, which in the case of soil, lack of knowledge prevails. The purpose was to analyze the behavior of some fertility indicators of a Verstisol soil in rota- tional grazing conditions using the stripe grazing land variable, in order to adapt the procedures for system management. MATERIALS AND METHODS The study was conducted on a cattle farm at the Experimental Station for Pastures and Forages, of the Institute for Agricultural Research Jorge Di- mitrov, in Granma, 10 ½ km from the city of Bayamo (20º 18' 13'' north latitude and 76° 39' 48'' east longitude. It consisted in evaluating the influence of rotational grazing on the variation of some Vertisol soil fertility indicators, for four years that applied stripe grazing as a strategy, and where daily stripes of pasture land of variable siz- es are assigned to herds, depending on pasture availability (Ray et al., 1998). In the study area, the mean air temperature was 24.2 °C in the dry season (November-April) and 27.7 °C in the rainy season (May-October), with maximum values of 28.6 °C and 32.8 °C, respec- tively. Annual rainfall ranged between 815 and 1 052 mm. The base pasture was Brachiaria hu- midicola cv. CIAT 679, on dry land and without fertilizing. Vertisol soil, according to soil genetic classification (Hernández et al., 1999) is mainly characterized by the accumulation or large quanti-