Plot management

Sowing was carried out on August 10, 2017, 8 kg ha^-1 of viable pure seed of three grasses of the genus Urochloa, Mulato II, Convert 330 and Convert 431, were used. The method of sowing was direct at a distance between furrows of 50 cm and continuous lines between plants. Four plots of 10 × 5 m were sown, these plots were divided into eight areas of 2.5 × 2.5 m, in order to evaluate a growth analysis with eight cut stages. The soil had a clay-sandy texture, pH of 4.8 to 5, deficient in organic matter with 1.5%.

Weeds were controlled manually with the help of a hoe, two nitrogen fertilizations were carried out, comprising 100 and 150 kg ha^-1 with urea (46-00-00) deposited in the soil in strips 3 cm deep and 10 cm away from the plant, being applied at 48 and 67 days after sowing, respectively. Eight days before starting the growth analyses, on October 12, 2017, a homogenization cut was made at a height of 10 cm in all the experimental plots manually with the help of pruning shears. Four irrigations were provided with the drip method in November and December every ten days up to field capacity with a net layer of 200 mm for the good regrowth of the grasses (Figure 1).

Variables evaluated

Stem population dynamics

The highest stem density was recorded from the cut age of day 49 to 56, regardless of the grass, with an average of 410 stems m^-2 (Figure 2). The Mulato II grass showed the highest stem density at the end of the curve with 540 stems m^-2, with continuous and accelerated growth, while the grassland with Convert 330 grass had the lowest stem density throughout the research, with an average of 220 stems m^-2 and the population trend was slower in time than the other treatments (p< 0.05). For its part, the Convert 431 grass was the intermediate with a stem population of 390 stems m^-2 and a drastic increase in stems in the stage of 21 days, and then it slowly increased as shown in Figure 2.

Figure 2 Changes in stem dynamics of three species of the genus Urochloa. a) Mulato II; b) Convert 330; and c) Convert 431. 

The three grasses had lower stem density at the start of the growth curve (day 7) and it can be attributed to the start of the growth curve since, at the beginning of regrowth, the tillers tend to have a smaller population of stems and increase with the time of establishment of the grassland, greater leaf area and therefore, greater weight as stated by several authors (^{Ramírez et al., 2011}; ^{Rueda et al., 2020}; ^{Maldonado et al., 2020}a). Another factor was competition since the stems regrowth in a grassland as a usually dense population, where the surrounding vegetation exerts a very strong influence on the inherent characteristics of each species (^{Matthew et al., 1996}; ^{Rueda et al., 2018}).

Studies such as those of ^{Maldonado et al. (2020}b), in the dry tropics in the population density of Urochloa stems, obtained results similar to this study, with an average of 450 stems m^-2. ^{Rodolfo et al. (2015}), when assessing elephant grass (Pennisetum purpureum), observed 293 stems m^-2 in the initial regrowth state, followed by a significant increase and the highest values in the intermediate state of 420 stems m^-2 and a decrease in the final state with 331 stems m^-2. In gamba grass (Andropogon gayanus Kunt), ^{Ramírez et al. (2020}) report an average of 350 stems m^-2 in a growth curve, results similar to those of this trial. For their part, ^{Maldonado et al. (2019)}, in Cuba 22 grass in the tropics of Mexico, obtained a population smaller than that of this study, with an average of 76 stems m^-2, being very variable over time.

Tissue replacement

Figure 3 shows leaf elongation, net growth and senescence by stem of three Urochloa grasses, Mulato II, Convert 330 and Convert 431, as the days of cut days vary. The grass that obtained the highest net growth was Convert 330, followed by Mulato II and finally Convert 431, with an average of 169, 133 and 104 cm stem^-1, respectively (p< 0.05). The Convert 330 grass showed rapid and higher leaf elongation on all days of cut. Mulato II began to decline from day 41, while Convert 330 and 431 did so from day 49 (p> 0.05). Regarding senescence, in the three grasses it started from the cut stage of 35 days; with Convert 330 showing the highest senescence with 9.2, followed by Mulato II and Convert 431 with 6.2 and 5.6 cm stem^-1, respectively (p< 0.05).

Figure 3 Tissue replacement, leaf:stem ratio and weight per stem (g) of Urochloa grasses, Mulato II. a) Convert 330; b) Convert 431; and c) in growth curves. 

The greater growth and trend of these Urochloa grasses is related to a promotion of cell division and elongation (^{Taiz and Zeiger, 2002}; ^{Wilson et al., 2008}). In Urochloa grasses, ^{Mandonado et al. (2020b)} mention that at the end of the forage accumulation curve, losses due to senescence increase and therefore, the net accumulation of forage decreases, as in this research. For their part, ^{Rojas et al. (2018}); ^{Maldonado et al. (2019}), in tropical grasses, mention that the maximum leaf growth occurs in the cut stage of 49 days, then the growth rates decrease due to a progressive increase of senescent stems and material. After a defoliation, the rate with which a forage recovers depends on the reserve carbohydrates in roots, remaining leaf area and the number of vegetative buds initiated, which increases the photosynthetic capacity of the leaves and greater elongation and net foliar growth (^{Li et al., 2016}; ^{Wang et al., 2017}).

Leaf:stem ratio

The leaf:stem ratio is a measure that represents the quality of the grassland, if it is greater than 1, the leaf will be higher in yield compared to the stem component in the grassland. In the three grasses, the leaf:stem ratio decreased as the cut stage passed (p< 0.05). The highest leaf:stem ratio was reported in Mulato II grass in the cut stage of seven days with a value of 6, decreasing until day 56 with 2.3, while the lowest value occurred in Convert 330 grass, starting with a ratio of five in the cut stage of seven days and ending in the cut stage of 56 days with a leaf:stem ratio of 1.6 (p< 0.05).

The decrease in the leaf:stem ratio as the cut age increased is related to the decrease in net growth and increase in senescence, as shown in Figure 3. The high leaf-stem ratios and the minimal appearance of foliar senescence can be explained by the habit of tillering and semi-erect growth of Urochloa grasses, because it is concentrated in the lower strata of the grassland, in this case, 10 cm of leaf area was left remaining (^{Rojas et al., 2020}; ^{Maldonado et al., 2020}b).

In Mombaza grass (M. maximum), ^{Ramírez et al. (2009}), when evaluating different cut frequencies in the rainy and dry seasons, reported a decrease in the leaf:stem ratio in the dry season (80%) compared to the rainy season; in addition, when the age of regrowth increases, the leaf:stem ratio decreases, they obtained on average 8.4 on day 21, decreasing on day 49 with a leaf:stem ratio of 3.8, trend and results similar to those of this study. In Mulato I (hybrid U.), other researchers ^{Cruz et al. (2011}) report similar results, they mention that the leaf:stem ratio depends on the management given to the grassland, height and cut stages. ^{Maldonado et al. (2020}b) obtained similar results in grasses of the genus Urochloa, having a higher leaf:stem ratio in early phenologies and decreasing as the time and senescence of the plant increase, obtaining a leaf:stem ratio of 8 at the regrowth age of seven days in Piata grass.

Weight per stem

Regardless of the grasses, the weight per stem increased as the evaluation time passed (Figure 3). The highest weight per stem was obtained in the Convert 330 grass at 49 days, with 3 g, followed by Mulato II and Convert 431 with 2.1 and 1.6 g (p< 0.05) on that same day. The highest weight per stem was obtained by the Convert 330 grass with 0.5 g in the cut stage of seven days (p< 0.05). The weight per stem is closely related to forage yield and stem dynamics since as the weight per stem increases, the dry matter yield increases; however, the population of stems decreases, as is the case of the Convert 330 grass in this research. Several authors record this trend (^{Rueda et al., 2018}), due to inter-specific competition for resources such as water, light and nutrients.

In this regard, several researchers mention (^{Cruz et al., 2011}) the age of the plant as a factor that determines the increase in weight per stem and senescent material, decreasing the formation of leaves, appearing in the most advanced cut stages. However, it does not only occur due to the age of the plant, but also due to the management given to the grassland and selected species (^{Maldonado et al., 2020}b).

Dry matter yield

Table 1 shows the dry matter yield of the grasses Mulato II, Convert 330 and Convert 431 as the days of cut of the grassland vary. In the three grasses, the yield in the first days of growth was slow and then accelerated and then decreased. The highest yield of dry matter, on average, was obtained in the three grasses at 49 days of regrowth with 3 531 kg DM ha^-1 (p< 0.05). The genotypes obtained the following yield, from highest to lowest: Convert 330 ˃ Convert 431 ˃ Mulato II with 1 845, 1 689 and 1 502 kg DM ha^-1, respectively (p< 0.05). On day 49 after cutting, the Convert 330 grass obtained the highest yield with 4 091 kg DM ha^-1 (p< 0.05).

Several authors report the same behavior in dry matter yield in the analysis of growth in tropical grasses, an increase in the first days of regrowth to later to begin to decline as the leaves in the lower layers senesced (^{Maldonado et al., 2019}; ^{Rueda et al., 2020}). In grasses of the genus Urochloa, Maldonado et al. (2020a) report the highest dry matter yield in three grasses, at 49 days of regrowth with 6 732 kg DM ha^-1 in the Insurgente grass, followed by Piata and the lowest in Señal with 3 320 and 2 675 kg DM ha^-1, respectively (p< 0.05), results similar to those of this research. On the other hand, ^{Rojas et al. (2018}), in Cobra grass (hybrid Urochloa), obtained the highest dry matter yield at 56 days at a defoliation intensity of 15 cm with 2 550 kg DM ha^-1.