Parcel management

The grassland were established in July 2016, the sowing of cv. Cobra, Mulato II and Cayman, (Urochloa) were carried out in furrows at a separation of 50 cm and between plants at 5 cm, at a planting density of 8 kg ha^-1, while stargrass (Cynodon plectostachyus) was with plant material (stolons) to winnow. 48 experimental units of 10 x 10 m distributed in a randomized block design with three replications were used.

The plots were not fertilized, nor was irrigation applied throughout the experimental phase. At the beginning of the experiment, a uniform grazing was carried out in all the experimental units. The treatments were the grass genotypes: Cobra, Mulato II, Cayman and star grass and two grazing frequencies (FP: 28 and 35 d), with two intensities: severe (10 cm) and light (15 cm). Calves of approximately 180 to 200 kg SS, approximately F1 Brown Swiss x Brahman were used in each plot, only as defoliators, until grazing intensity was reached. The animals remained from 4 to 8 h depending on the treatment.

Forage yield

To evaluate the seasonal and annual forage yield, one day before each grazing, two fixed quadrants of 50 x 50 cm were randomly placed in each repetition, which were harvested at the corresponding intensity and frequency. Subsequently, the weight of the fresh forage was recorded, placed in paper bags and dried in a forced air stove (Memmert model UF 260) at a temperature of 55 °C, for 72 h. The dry weight of the forage was recorded and the yield per unit area (kg DM ha^-1) was determined.

Morphological components

From the green forage harvested to estimate forage yield, a subsample of approximately 20% was taken, separated into the components: leaf and stem and placed in labeled paper bags and dried in a forced air stove at 55 °C , for 48 hours or until constant weight was reached and weighed on a Scout^® Pro brand digital scale.

Leaf:stem ratio

The leaf: stem ratio was determined from the sample used for the morphological components and was obtained by dividing the yield of the leaf component by the yield of the stem.

Crude protein

To determine the crude protein, the leaf and stem morphological composition sample was ground (1 mm diameter mesh). Subsequently, subsamples were taken for protein determination using the Microkjendhal method (^{AOAC, 1990}).

Statistical analysis

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

Forage yield

Table 2 shows a higher yield of the Urochloa hybrids compared to star grass, regardless of the frequency and intensity of grazing. Grazing frequency affected forage performance throughout the experimental period (p< 0.05), regardless of genotype and grazing intensity. Increasing the interval between grazing increased forage yield.

Yield increased on average from 4 978 to 6 016 kg DM ha^-1 with increasing grazing interval from 28 to 35 d (p< 0.05). The grass Cobra, Cayman and Mulato II obtained the highest yield in the frequency at 35 light intensity with an average of 6 962 kg DM ha^-1, while, in this management, the stargrass obtained the lowest yield with 4 028 kg DM ha^-1.

The opposite case occurred in the frequency at 28 of severe intensity obtaining the lowest performance in all genotypes with the following descending order: Cobra ˃ Cayman ˃ Mulato II ˃ Star with 5 203˃ 5 045˃ 4 498˃ 3 405 (p< 0.05). Results similar to those of this investigation, presented by ^{Rojas et al. (2018)}, when obtaining a greater accumulation as the cutting frequency and intensity increased, this when evaluating growth curves in Cobra grass obtaining the highest dry matter yield at 56 days at an intensity of 15 cm with 2 550 kg DM ha^-1 and the lowest with intensity at 10 cm reaching a yield of 2 250 kg DM ha^-1 (p= 0.05).

On the other hand, ^{Garay-Martínez et al. (2018)} obtained in different Urochloa cultivars a variable DM accumulation between cultivars and regrowth age (p≤ 0.05). During the season of greatest precipitation, the Mulato II cultivar presented the highest leaf accumulation with an average of 8 400 kg DM ha^-1, followed by the Cayman and Insurgente cultivars with 7 740 and 7 250 kg DM ha^-1, respectively; while, the cultivar H-17 registered the least accumulation with a value of 6 210 kg DM ha^-1.

These results are similar to those obtained by different authors (^{Martínez et al., 2008}; ^{Cruz et al., 2017a)}, in the Brachiaria humidicola cv. Chetumal, obtained a greater accumulation of dry matter by increasing the frequency and intensity of grazing, this indicates that, with frequent defoliation, the density of stems in the grassland is increased, which do not manage to intercept 95% of sunlight (^{Rojas et al., 2017}).

On the contrary, with longer intervals the competition between plants for sunlight increases continuously, so that the grassland develops a low density of stems, with higher height and greater leaf area (^{Ramírez et al., 2010}; ^{Cruz et al., 2017b}). In accordance with the above, ^{Hirata and Pakiding (2004)} state that grasses that undergo frequent grazing and severe intensities decrease forage yield up to 50%.

Morphological components

The frequency and intensity of grazing of a grassland influences the growth rate, production, botanical composition and quality. This indicates that it is important to consider not only the forage yield, but also the proportion of leaves in relation to the stems (^{Joaquín-Cancino et al., 2019}). In the present investigation, there was an effect of grazing frequency and intensity (p< 0.05) on the component in the Urochloa and stargrass species (Table 3).

The greatest accumulation of leaves occurred in the grazing interval at 35 light intensity in the Cayman species with 6 342 kg DM ha^-1, while, with this same management, star grass obtained the lowest yield with 1 543 kg DM ha^-1 (p< 0.05). Inverse case with regard to the yield of stems, Stargrass and Cayman grass obtained the highest and lowest yield in frequency at 35 d and light intensity with 2 733 and 861 kg DM ha^-1, respectively (p< 0.05).

The change in the morphological composition was due to the handling frequency and grazing intensity that favored a greater growth of the leaves, which agrees with what was reported by ^{Cruz et al. (2017a)}, found that the leaf increases its appearance in Brachiaria humidicola cv Chetumal when grazing with cattle at a frequency of 28 intensity of 13 to 15 cm in the rainy season with 7 271 kg DM ha^-1 and less when it decreases the frequency and intensity with 4 734 kg DM ha^-1.

Those who mention that the proportion of the morphological components in the harvested forage decreases the leaf with increasing the interval between harvests, due to a greater growth of the stem, when the environmental conditions are favorable for the growth of the plants as it happens at the time of rains due to favorable climatic conditions for growth (^{Sage and Kubein, 2007}).

The higher forage accumulation in the 35 d grazing interval compared to the 28 d grazing interval coincided with the greater number of leaves and stems. This behavior was observed by different researchers (^{Difante et al., 2011}; ^{Calzada et al., 2014}; ^{Rueda et al., 2016}) who affirm that the age of the plant determines the distribution of dry matter in its different morphological components.

Leaf:stem ratio

In general, it is observed that the pastures of the genus Urochloa have a better leaf: stem relationship, compared to star grass (Table 4). Grazing frequency and intensity effect was presented, as well as its interaction, in all species (p< 0.05). The leaf: stem relationship was variable depending on the genotype and management given in the grassland, in frequency and grazing intensity.

The leaf: stem ratio in Mulato II grass was the highest (p< 0.05) when grazing every 28 of severe intensity with a leaf: stem ratio of 10.1 and less in star grass, regardless of the frequency and intensity of grazing with 0.6 (p< 0.05). This same behavior is reported by ^{Cruz et al. (2017a)} in the grass Brachiaria humidicola cv. Chetumal as leaf: stem ratio increases as grazing frequency and intensity increases.

The Cobra and Cayman grass increased in the leaf: stem ratio at a higher frequency and light intensity from 1.7 and 4.6 to 2.2 and 7.3, respectively. The higher leaf: stem ratio was due mainly to the management given in the grassland, as demonstrated by ^{Ramírez et al. (2009)}, when increasing the cutting age from 3 to 7 weeks, they found a lower leaf: stem relationship in Mombaza grass (Panicum maximum Jacq.).

The highest values in the leaf: stem ratio were associated with the sampling technique, since forage harvesting was carried out at predetermined grazing heights, which avoided harvesting a greater number of stems, which are located near the surface of the soil (^{Cruz et al., 2017b}). In addition, when considering the stoloniferous growth habit of star grass compared to the tufts of the Urochloa, the lower leaf: stem ratio and the absence of senescent material are explained, since it was concentrated in the lower strata of the grassland.

Crude protein

The crude protein of leaf and stem of hybrid Urochloa and star grass grasses, when varying the frequency and intensity of grazing is observed in Table 5. In general, the highest crude protein in leaf, in all grasses, was found in the severe grazing and intensity at 28 d with 20, 20, 18 and 10% of crude protein in the Cobra, Mulato II, Cayman and star grass pastures, respectively (p< 0.05).

While, at higher frequency and light intensity, there was a decrease in the percentage of crude protein, independently of the pastures evaluated with the following descending order: Mulato II, Cobra, Cayman and star grass with 16, 14, 14, 6, respectively (p< 0.05). On the other hand, the amount of protein in stems was highly variable depending on the type of grass and management given in the grassland. The highest percentage is reported in Cobra grass, with a frequency of 28 of severe intensity with 13% of crude protein, while the lowest was in star grass in the frequency at 35 d with 4% of crude protein (p< 0.05).

^{Hernández et al. (2002)} have indicated that the environment and management in terms of grazing frequency and intensity, are the main factors that affect the yield, quality and persistence of a grassland. Therefore, when a grassland is grazing frequently and at a severe intensity, forage production is lower, but with higher digestibility and crude protein (^{Ramírez et al., 2009}), since the largest amount of forage harvested is leaf.

Whole plant in grass Estrella-^{Molina et al. (2015)}, reported 11.8% protein very similar in protein content in this research. On the other hand, ^{Villalobos and Arce (2014)} report the average crude protein content in star grass with 20.27%. This result is greater than that reported in this investigation, however, the crude protein varies depending on the climatic conditions and management given in the grassland (^{Rojas-García et al., 2018})

^{Cano et al. (2004)} reported a lower crude protein concentration, with increasing cutoff frequency. On the other hand, ^{Vergara and Araujo (2006)} state that there is an increase in crude protein in Brachiara humidicola grass by decreasing the cutting interval. ^{Cruz et al. (2017a)} mention in Chetumal grass, a decrease in the interval between grazing in the crude protein content, such behavior is due to the fact that, in the plant, the older the cell wall accumulates, which is known as fiber.

They observed that the amount of crude protein was higher at the younger sprout ages resulting in more frequent cuts as in this research. This behavior has been observed by other researchers (^{Lara and Pedreira, 2011}; ^{Rojas-García et al., 2018}) who mention that the nutritional quality of forages is closely linked to their maturity at harvest and the environmental conditions where the crop is developed.