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Introduction
In tropical livestock, the low quality of grasses and forages, do not allow to express the ruminants production potential. Being a main objective of researchers to find a grass that apart from producing a sufficient biomass throughout the year, provide the amount of nutrients that can cover the animal requirements.
The star grass (Cynodon plectostachyus) fulfilled in its moment, with being good alternative, due mainly to its rusticity and persistence, even in spite of the bad management; however, grasses from Brachiaria genus opened new expectations for tropical livestock, due to its wide range of adaptation. This has allowed the farmer to choose the grass that best suits the conditions of his land and the type of exploitation he manages, giving a greater efficiency and profitability (Guiot 2017).
In production systems where the grass is the fundamental food, the laws of grazing must be clearly understood and a management that allows flexible resting times must be used to take advantage of the grass at the moment that guarantees adequate quality and at the same time, that do not compromise the grassland stability. In this study we will focus on the performance of Brachiaria decumbens cultivar Basilisk, under dry conditions and without fertilization, with two grazing intensities, during the rainy season.
Materials and Methods
The study was developed at the Instituto de Ciencia Animal (ICA), located at 22° 58 '00 "north latitude and 82° 09' 21" west longitude at 92 m o.s.l, in San José de Las Lajas municipality, Mayabeque province, Cuba. The average general weather conditions were: annual precipitation 1 475 mm and relative humidity 80.20 %.
A total of 2.66 hectares were used, divided into 20 paddocks of 0.133 hectares; the predominant soil is red ferralitic (Hernández et al.2015), which was sowing two years earlier with the variety Brachiaria decumbens vc. Basilisk. Twenty Siboney crossbred cows were used in milking with a body weight of 486 ± 8.25 kg, between the second and third lactation. The animals were divided into two groups (10 animals group-1); taking into account body weight, milk production and days of lactation.
The grassland was subjected to two grazing intensities (GI) GI1 = 75 and GI2 = 150 LAU ha-1 day-1. For the GI1 treatment, 13 fixed paddocks were used, an ocupation of one day paddock-1, while for GI 2, the remaining 7 paddocks were used with an ocupation of two days paddock-1. The animals grazed from 5:00 pm to 3:30 am and from 6:30 to 11:00 am, for an average of 15 hours a day. The rest of the time, they stayed in shade building, all together, where they had water and mineral salts at will.
The measurements made were:
Availability of the grass in each rotation, both at the entrance and exit of the animals, by the method described by Haydock and Shaw (1975). The cut height was 10 cm from the soil, taking between 80 to 100 observations hectarea-1.
Bromatological composition of the grass, for each rotation: 200 g of fresh matter were taken of the proportion of each of the five points. The bromatological analyzes of the grass, were made by sample in parallel. To estimate the dry matter percentage, a BINDER forced air oven brand model FP 720 was used at 60°C for 48 hours. The crude protein content and mineral elements were determined using the AOAC (2016), techniques while for the NDF and ADF the Van Soest (1994) technique was used. Likewise, OM digestibility was determined.
Phenological structure of the grass, by rotation, was determined with the same base of samples as for the bromatological composition and leaves, stems and senescent material of the reference samples were taken into account, at the entrance and exit of the animals from the paddocks.
Botanical composition of the grassland was carried out in the entire area at the beginning and end of the experiment by the "range in dry weight" method (Mannetje and Haydock 1963).
Resting times, records of the dates of entry and exit of animals in each paddock, by rotation were performed.
The information corresponding to the bromatological variables were jointly analyzed, for the different fractions of the plant (whole plant, leaf and stem), seeking to know which of them made a higher contribution to the total variability of the system, under the conditions in that the experiment was carried out. Subsequently, a completely randomized design was used independently for each component.
The statistical software InfoStat Version 12.0 (Di Rienzo et al. 2012) was used, using the Duncan (1955) test, to establish differences between means.
Results and Discussion
The analysis of the evolution of the botanical composition (table 1), showed that important changes occurred in the composition of the grassland, in the area destined to the lower grazing intensity, the Brachiaria percentage was increased (P <0.05) 7.9 %; while it was possible to increase (P <0.001) 13.2 percentage units the areas subjected to the highest grazing intensity. These increases in the proportions of Brachiarias, in both intensities respond to the decrease (P <0.001) of other plant species that were identified in the paddocks.
Table 1 Changes in the grassland botanical composition with the application of two grazing intensities
* P<0.05, ** P<0.01, ** P<0.001
This can respond to the positive effect caused by intense defoliation, on the increase in the number of regrowth per plant (Vélez and Berger 2011), especially with the increase in the intake of plants with lower acceptance, due to the scarce or null selection of the animal, which favors the growth of the improved species that has better response to grazing, provided that the resting time is sufficient (Senra 2005). Previously, Cevallos et al. (2008) perceived the characteristics of this species, to tolerate intense grazing and manifest good persistence under intense or frequent defoliation.
The resting days for each grazing intensity (figure 1), showed low intervals (P <0.01), 23.7 % with the lowest grazing intensity (25.39 ± 0.28 and 33.28 ± 0.49 resting days, for 75 and 150 LAU has-1 day- 1, respectively). The higher number of resting days that the grass was needed when, it was submitted to the higher grazing intensity, is due to the higher use of it according to the increase of grazing intensity, which causes a lower amount of green residues of the grass and, therefore, of the photosynthetically active material. (Cevallos et al. 2008).
Figure 1 Resting days of the grass Brachiaria decumbens vc. Basilisk, according to grazing intensities.
Therefore, by increasing the use of grass, the use of reserves accumulated by the plant is increased in order to be able to carry out its new growth, which causes the grass to need a longer time to be grazed again, affirming what was stated by Vélez and Berger (2011), with respect to the response of the regrowth based on the energy reserves accumulated in the root and strains.
The areas subjected to the highest grazing intensity, showed higher availability of DM ha-1 (P <0.01) in 39.9 %, in relation to the areas of lower intensity (1 831.05 vs 2 562.11 kg DM ha-1 rotation-1, for 75 and 150 LAU ha-1 day-1, respectively); likewise, the grass yield (initial availability minus rejection of the previous rotation) was favored (P <0.05), 23.9 % when the area was submitted to 150 LAU ha-1 day-1; 870.45 and 1078.61 kg of DM ha-1 rotation-1, for 75 and 150 LAU ha-1 day-1, respectively (figure 2).
These yields expressed by the grass, are below to those reported by Pizarro, et al. (2013), who when harvesting this type of grass between 22 and 24 days of regrowth in the rainy season, obtained between 2700 and 3 200 kg of DM ha-1 cut-1; similarly Suchini (2015) and Guiot (2017), reported productions higher than 3 700 kg of DM ha-1cut-1, but with cuts after 60 days and using nitrogen fertilization in the order of 150 kg N ha-1; being this increase according to the age of the material.
Figure 2 Availability and yield of the grass Brachiaria decumbens vc. Basilisk, with two grazing intensities.
The percentages of the senescent material, leaves, stems and the index that relates these last two fractions, are also important when characterizing a grassland. There was no difference between the proportions of stems and senescent material between the two intensities neither in the supply nor rejection; however, the proportion of leaves and the leaf / stem ratio in the initial availability increased (P <0.05), in 5.2 and 20.93 %, under the effect of the highest intensity, while, in the rejection of grass, these same indicators were increased (P <0.05), 6.33 and 40 % but with the lowest intensity (table 2).
Table 2 Performance of the percentage of leaves, stems, senescent material and leaf/stem index, in the supply and rejection of Brachiaria decumbens vc. Basilisk, in two grazing intensities.
* P<0.05
The values of proportion of leaves are inferior to those reported by Suchini (2015), in Brachiaria hybrid cv. CIAT BR02/1794, which exceeded 60 %. This suggests that more intensive grazing favored the grassland performance with an increase in leaf percentage, since, being grazed more thoroughly, it needs more nutrients and energy for its regrowth and these elements are produced in the leaves (Villalobos and Longhi 2015).
After grazing, the leaf/stem ratio was precipitated to values of 0.56 vs. 0.40 and the percentages of senescent materials and stems increased to 15.63 vs 16.88 and 54.24 vs 59.33 for paddocks with 75 and 150 LAU ha-1 day-1, respectively. Both in the samples of the supply and in the rejection, the variations in the leaf/stem ratio were linked to the changes in the leaf proportion, denoting a higher intake of them, for being the most nutritious parts of the grass; so Guiot (2017), when comparing Mulato II with the hybrid CIAT BR02/1752 with three grazing frequencies (2, 4 and 6 weeks) the values for the crude protein (CP) and in vitro digestibility of the dry matter (IVDDM), were higher in the leaves.
The chemical composition indicates the concentration of the main nutrients of a given food (FAO 2015). The effect of the applied grazing intensities on the bromatological variables of the grass is shown in table 3, where there is no variation between the intensities and the different fractions of the grass, only an increase (P <0.05) of the ash and a decrease (P <0.01) of the NDF in the stem of the grass subjected to 150 LAU ha-1day-1. Similarly, Ramírez et al. (2009), did not report differences for the grazing intensities evaluated by them.
The levels of ashes did not exceed ten percent, maintaining unchanged when studying leaf and whole plant, according to the different intensities. An aspect that can influence on the ash content is the ability of the plants to absorb the mineral elements of the soil, which would be determined, among other factors, by their content in the soil and by the development of their root system (Caballero-Gómez et al. 2016).
Table 3 Performance of some bromatological indicators of Brachiaria decumbens vc. Basilisk, in whole plant, leaf and stem, with two grazing intensities.
* P<0.05, ** P<0.01
The proportion of crude protein of the grass expressed values between 8.7 to 8.9 %, which are below the reports, in this plant species, performed by CIAT in Colombia that established the crude protein indicator between 12 and 15 %, which is similar to what was published by Ramírez et al. (2009) at 15 regrowth days in conditions similar to those of the study.
The premise that the protein content is higher in samples of leaves than in stems was fulfilled, regardless of the intensities used. This was also reported by Hare et al. (2013) and Guiot, (2017), and is due to the lower fibrous content of this fraction, which gives it a better quality. The inverse relation with the protein is the fiber and one of its constituents, the NDF, which its determination provides the estimate of the total concentration of the food fiber and is closely and inversely related to the intake capacity (Roca-Fernández et al. 2013).
Independently to the fact that fiber fractionation has not responded adequately to the nutritional characterization of food (Choct 2016), the NDF is the most used and recommended method for estimating the content of cell walls of animal foods (Choct 2015 and Molina et al. 2015). The results obtained from NDF were within the normal ranges (Hare et al. 2013), although, these values were closer to the upper limits, which seems to be the key to the affectation that showed the apparent digestibility of the grass. Dervin (2014) reported that high fiber content in grass and forage diets are responsible for the incomplete digestion of cellulose and hemicellulose, making carbohydrates less available to rumen microorganisms.
The obtained results show that the variety Brachiaria decumbens cv. Basilisk subjected to the grazing intensity of 150 LAU ha-1 day-1, showed higher stability, achieving an increase of 13.24 % in the covered area, as well as higher availability and yield of the grass, although they needed an average of 18.9 % more of resting time; showed a better leaves proportion, although it did not exceed 44.55 %. The grass quality did not show differences between the intensities used, showing values of a grass from medium to good quality, with crude protein values between 86.7-89.2 g kg-1DM-1 and digestibilities 51.6-51.7 %.
