Fodder yield

In order to evaluate the seasonal and annual forage yield, two fixed quadrants of 50*100 cm were randomly placed on each day before starting the study, which were harvested one day before each grazing at corresponding intensity and frequency. Subsequently, the forage was weighed fresh, placed in paper bags and dried in a forced air oven at 55 °C for 72 h. The dry weight of the forage was recorded and the yield was determined by surface unit (kg MS ha^-1).

Morphological components

From fresh forage, a sub-sample of approximately 100 g was taken, which was separated into components: leaf, stem and senescent material which were deposited in labeled paper bags and dried in a forced air oven at 55 °C, for 48 h and weighed on a Scout^® Pro digital scale.

Leaf: stem and leaf: not leaf ratio

The leaf: stem and leaf:non leaf ratios were estimated by dividing the yield of the leaf component between stem yield and the sum of stem and senescent material, respectively.

Nutritional value

In order to determine it, at the middle of each season, a sample of approximately 5 kg of fresh fodder was taken, which was washed and dried in a forced air oven at 55 ^oC for 48 h, and grounded (1 mm diameter mesh). Subsequently, the following determinations were made: protein by the Microkjendhal method (^{AOAC, 1990}). The digestible dry matter content was determined using the nylon bag technique described by ^{Ørskov et al. (1980)}. Which consisted of taking a sub-sample of 5 g of forage which was deposited in a polysed bag (5*10 cm), bags were grouped and secured with silk thread and then incubated in the rumen in duplicate for 48 h, two Bos taurus*Bos indicus fistulated cattle with a live weight of 300 kg were used. The incubated bags were then removed, washed and fed into a forced air oven at 65 °C for drying.

Statistic analysis

The data were analyzed by a completely randomized block design with factorial arrangement of 2*2 treatments, using the PROC MIXED procedure (^{SAS, 2009}), where the interval effects between cuts, time of year and their interactions were considered as fixed and block effect as random. The multiple comparison of means of treatments was performed using the adjusted Tukey’s test (α= 0.05).

Fodder yield

The seasonal and annual accumulation of Brachiaria humidicola cv. Chetumal forage subjected to different frequencies and intensities of grazing is shown in Table 1. The frequency of grazing affected forage yield throughout the experimental period (p< 0.01), regardless of the time of year and grazing intensity, as the interval between grazing increased forage yield. The annual yield increased 42% by increasing the interval between grazing from 21 to 28 d (p< 0.05). The rainy season accounted for 66% of the annual forage yield, with 4 298, 5 849 kg DM ha^-1 for the frequencies of 21, 28 d respectively and followed by the North and Dry seasons with 26 and 7% annual yield. It was observed that when harvesting with less intensity the annual yield of the forage increased by 16% (p< 0.01) compared to the severe cut.

Table 1 Accumulation of forage by season and annual (kg DM ha^-1) of Brachiaria humidicola cv Chetumal pasture, subjected to different frequencies and intensities of grazing. 

Nortes= noviembre-febrero; seca= marzo-mayo; lluvias= junio-octubre; severo= 9-11 cm; ligero= 13-15 cm; ns= no significativo; ^**= p≤ 0.01; ^*= p≤ 0.05, abcd= diferente literal minúscula en cada columna, indican diferencia (p< 0.05); EEM= error estándar de la media.

The light harvest exceeded forage yield in 10 and 56% (p< 0.05) to the severe in north and dry, respectively (Table 1). Rainfall and temperature in the rainy season stimulated forage growth and yield. In the northern season, the low temperatures were those that affected the growth of Chetumal pasture, resulting in a lower accumulation as stated by ^{Rojas et al. (2016)}, obtaining the lowest forage mass in the seasons with lower temperature, independently of the year of study. The response of forage yield to seasonal changes in precipitation and temperature was expected for the hot humid climate conditions of the state of Tabasco. ^{Candide et al. (2006)} mention that the greatest growth of forage species occurs between 25 and 35 ^oC. Other authors mention that good photosynthetic efficiency in tropical pastures occurs when the temperature is within the range of 15 to 45 ^oC (Sague and Kubein, 2007). The temperature and precipitation data recorded during the study period (Figure 1) show that the limiting factor in forage yield during the dry season was precipitation. In this paper it was observed that increasing the grazing interval increased the accumulation of forage (Table 1).

These results are similar to those obtained by ^{Martínez et al. (2008)}, who obtained an annual accumulation of 11 583 kg DM ha^-1 for this same species when harvested every five and seven weeks. This indicates that, with frequent defoliation, the density of stems in the prairie, which can not intercept 95% of sunlight, is increased because of its higher leaf area index. In contrast, at longer intervals the competition between plants by sunlight increases continuously, so that the grasslands develop a low density of stems but larger and longer leaves (^{Ramírez et al., 2010}). In this regard, ^{Hirata and Pakiding (2004)} point out that grasses that undergo frequent grazing and severe intensities decreased forage yield.

Morphological components

The defoliation management of a prairie influences the growth speed, production, botanical composition, quality and its persistence (^{Nantes et al., 2013}). This indicates that it is important to consider not only the forage yield, but also the proportion of leaves in relation to the stems and the senesent material. There was an effect of frequency and intensity of grazing (p< 0.01) in this research, in the leaf component during all seasons of the year. There was no effect of interaction frequency and grazing intensity. The frequency of grazing of 28 d showed 35% more annual leaf yield compared to 21 d (Table 2). The highest accumulation of leaves occurred in the rainy season with 24% in the grazing interval at 28 d compared to 21 d. There were no differences between intensities in the frequency of grazing every 28 d in the leaf component in rainy and dry seasons (p> 0.05). Light grazing presented 20.5, 30.5 and 8.6% higher accumulation of leaves in the North, dry and rainy seasons, respectively, compared to severe grazing (p< 0.05).

Table 2 Accumulation of morphological components by season and annually (kg DM ha^-1) of Brachiaria humidicola cv Chetumal, subjected to different frequencies and intensities of grazing. 

Nortes= noviembre-febrero; seca= marzo-mayo; lluvias= junio-octubre; ne = no se encontró; severo= 9-11 cm; ligero= 13-15 cm; ns= no significativo; ^**= p≤ 0.01; ^*= p≤ 0.05, abcd= diferente literal minúscula en cada columna, indican diferencia (p< 0.05); EEM= error estándar de la media.

The changes in the morphological composition on the rainy season were due to the edaphic and climatic conditions favoring a greater leaves growth, which is similar with that reported by ^{Festo et al. (2003)}, who found that leaves increase their appearance when there are temperatures between 20 to 32.5 ^oC, but decrease if the temperature exceeds 35 ^oC. Regarding to the northern period, the changes were due to the fact that the growth of the plants was inhibited by the low temperatures, whereas in the dry season the water stress allowed little growth consisting mainly of leaves. The frequency of grazing had an effect on the accumulation of stems (p< 0.01) in the north and rainy season; while the intensity and interaction frequency by grazing intentions only had an effect in the rainy season (Table 2).

The highest accumulation of stems was obtained in the rainy season 76% more in the 28 d interval compared to 21 d; followed by the northern period. While with light grazing the accumulation of stems was 13.5 and 25% higher compared to severe grazing for periods of north and rainy, respectively. These results are similar to those reported by ^{Ramírez et al. (2009)}, who mention that the proportion of leaves in the harvested forage decreases as the interval between harvests increases, due to a higher growth of the stem, when the environmental conditions are favorable for plants growth as it happens in the time of rainfall; on the contrary, during the norths season the stem elongation is inhibited by the low temperatures, and in the dry season the water deficit (^{Sage and Kubein, 2007}) limits the growth of the plants and, consequently, the contribution of the stem to forage yield is zero.

The higher accumulation of forage in the grazing interval of 28 day compared to that of 21 day coincided with the greater number of leaves and stems. This behavior was observed by ^{Ramírez et al. (2009)}; ^{Difante et al. (2011)}; ^{Hernández et al. (2002)}; ^{Calzada et al. (2014)}; ^{Rueda et al. (2016)} who affirm that the plant’s age determines the distribution of dry matter in its different morphological components.

Leaf:stem and leaf:non leaf ratio

In general, Chetumal grass has an excellent leaf: stem ratio (Table 3). There was an effect of frequency and grazing intensity, as well as interaction, during the rainy season (p< 0.05). The leaf-stem ratio during the rainy season was 27% higher (p< 0.05) when grazing every 21 day, which decreased as the interval between grazing increased, although climatic conditions were favorable for the grass growth, during rainy season the leaf: stem ratio was due mainly to the management, as demonstrated by ^{Ramírez et al. (2009)}, when increasing the cutting age of 3 to 7 weeks there was found a lower leaf: stem ratio in Mombaza grass (Panicum maximum Jacq.).

Table 3 Seasonal changes in leaf: stem and leaf: non-leaf ratio of Brachiaria humidicola cv Chetumal, by time of year, subjected to different frequencies and intensities of grazing. 

Nortes= noviembre-febrero; seca= marzo-mayo; lluvias= junio-octubre; &= 100% hojas; severo= 9-11 cm; ligero= 13-15 cm; ns= no significativo; ^**= p≤ 0.01; ^*= p≤ 0.05; abc= diferente literal minúscula en cada columna, indican diferencia (p< 0.05); EEM= error estándar de la media.

The intensity of grazing had an effect (p< 0.01) on the leaf: stem ratio during the rainy season (Table 3), and it was observed that severe grazing was greater in 6% in rains and 1% in north compared to light grazing. Regarding the leaf: non-leaf ratio, frequency and intensity effects, as well as interaction frequency x intensity (p< 0.05) were shown in the rainy season. The higher values in the leaf: stem ratio were associated with the sampling technique, since the harvested forage was performed at predetermined grazing heights, which avoided harvesting a larger number of stems, which are located close to the surface ground. In addition, when considering the stoloniferous growth habit of the Chetumal grass, the high leaf-stem ratio and the absence of senescent material are explained since it was concentrated in the lower strata of the prairie.

Nutritional value

The frequency and intensity of grazing had a significant effect on crude protein content (p< 0.05) only in the rainy season where the content of the Brachiaria humidicola grass protein was observed to decrease by 20% with increasing grazing frequency 21 a 28 day. A similar behavior was observed during the period of norths and dry (Table 4).

Table 4 Dry digestible matter and crude protein content of Brachiaria humidicola cv Chetumal, by time of year, subjected to different frequencies and intensities of grazing. 

Nortes= noviembre-febrero; seca= marzo-mayo; lluvias= junio-octubre; DIGMS= materia seca digestible; severo= 9-11 cm; ligero= 13-15 cm; ns= no significativo; ^*= p≤ 0.05; ab= diferente literal minúscula en cada columna, indican diferencia (p< 0.05); EEM= error estándar de la media.

In general, crude protein with severe grazing was 10, 9, and 15% higher in seasons of norths, dry and rainfall, respectively, compared to light grazing. It has been pointed out that the environment and the management in terms of frequency and intensity of grazing are the main factors affecting the yield, quality and persistence of a prairie cultivated with grasses (^{Hernández et al., 2002}). Thus, when a pasture is frequently grazed and at a severe intensity the forage production is lower but with higher digestibility (^{Ramírez et al., 2009}), since the greatest amount of harvested forage are leaves.

In this research it was observed that increasing the interval between grazing decreased the crude protein content and digestibility of the dry matter of Chetumal pasture, this behavior is due to the fact that in a plant, the older the plant the thicker the cell wall gets, best known as fiber. The highest protein content during the dry season was attributed to the low leaf growth, where the highest amount of nutrients were found.

It was observed that the amount of crude protein was higher in the ages of smaller regrowths that result with more frequent cuts. This behavior has been observed by other researchers who mention that the nutritive quality of forages is closely linked to their maturity and environmental conditions at the time of harvesting (^{Lara and Pedreira, 2011}). In this regard, ^{Cano et al. (2004)}, found that the concentration of crude protein decreases with increasing age of cuts. In this regard, ^{Vergara and Araujo (2006)} report for Brachiara humidicola that increasing the cutting interval from 14 to 28 d during the rainy season decreased crude protein content from 8.5 to 6.7%, but increased NDF content of 72.54 to 77% and ADF of 43.2 to 41%.