Stem population density

The average stem population density was higher at the intensity of 15 cm by 11.4, 4.7 and 11.0% in the hybrids Mulato II, Cayman and Mavuno, respectively, compared to the intensity of 25 cm (Table 1). In addition, there was a decrease at the end of the evaluation (February-2021), compared to the initial population (March-2020), of 31 and 51% at the intensity of 15 and 25 cm, respectively, in Mulato II, 39 and 75% for Cayman and 23 and 32% for Mavuno at the same intensities (Table 1).

The evaluation over time with established cutting interval defined the behavior in the density of stems in the three hybrids, the hybrid Mulato II showed differences between the intensities from October, with the intensity of 15 cm remaining higher by 20% on average until January, Figure 2, therefore, it is probably a function of the environmental conditions present in January and February, Figure 1. In this regard, the behavior of the stem population density with prolonged cutting interval in the cultivar Mulato II in conjunction with four cultivars of Urochloa presented densities greater than 4 000 stems m²; however, no significant differences were found between cultivars (^{Lucio et al., 2023}), therefore, for this experiment the cutting intensity is crucial to determine the effect on the stem population density, since it has been established that, after mechanical or grazing defoliation, stem density growth is induced, which lasts until 21 and 28 days in Andropogon gayanus, for example (^{Ramírez et al., 2020}).

Figure 2 Effect of cutting intensity (15 and 25 cm) on stem population density (stems m²), with a fixed interval of four weeks. 

Regarding the hybrid Cayman, it did not show differences between intensities (p≤ 0.05) during the evaluation, except for February, in which the intensity of 15 cm was 53% higher compared to the intensity of 25 cm, the same behavior occurred in another study when evaluating the cultivar Cayman in the period from June to December with a fixed intensity of 15 cm and an interval of four weeks (^{Lucio-Ruiz et al., 2021}), as in this experiment there were no differences between these dates, so it is proposed that the cutting intensity for this cultivar is 15 cm without affecting the stem population density throughout the year, likewise, this intensity has been related to obtaining yields of up to 8.8 t ha^-1 of dry matter accumulated during the season of maximum precipitation in Güémez, Tamaulipas (^{Garay-Martínez et al., 2018}).

The hybrid Mavuno showed variations in stem population density in the months of July, August and December, favoring the intensity of 15 cm, while in September the intensity of 25 cm was 11% higher, Figure 2, it suggests a response to the precipitation accumulated in September, given that having lower density per area in the previous sampling and when favorable conditions are propitiated, the density of stems increases, the same response occurred in the grass Mulato (36061), when evaluated in the northerly wind, dry and rainy seasons, the highest density of stems was present in the rainy season regardless of the cutting intensity (^{Cruz-Hernández et al., 2017}).

When evaluating the Urochloa hybrid Mulato at intensities of 10 and 14 cm in height, ^{Cruz-Hernández et al. (2017)} found that the intensity of 10 cm generates higher density of stems (between 3 280 and 5 729 stems m²); nevertheless, the precipitation in this place occurred in greater quantity from June to October with more than 1 400 mm, while in the place of this study there is an average annual precipitation of 750 mm, coupled with extreme temperatures, therefore, due to the ideal conditions for the emergence of tillers, it generates a higher density of stems through the regrowth stage (^{Ramírez et al., 2011}). During this stage, the activation of the active meristematic zone located at the base of the stems occurs, thanks to the presence of reserve carbohydrates (^{Rincón et al., 2008}).

Tillering and stem death rate

The highest tillering rates occurred at the intensity of 15 cm on average, compared to that of 25 cm, in addition, during the month of September there were the highest tillering rates at both intensities and for the three hybrids, because it is the month of maximum accumulation of precipitation, which favors the tillering of new stems (Table 2).

In this sense, during the month of August, where the phenomenon known as the dog days of summer occurs, accompanied by high temperatures, which cause greater thermal accumulation for grasses (^{Lucio et al., 2023}) and the absence of precipitation (^{Pacheco-Hernández et al., 2020}), the hybrids Cayman and Mavuno presented the highest tillering rate at both intensities (p< 0.05), possibly due to the use of carbohydrate reserves present at the base of the stems, crown and roots of plants, as a measure of perpetuation (^{Rincón et al., 2008}). The opposite happened with the hybrid cultivar Cobra, which, as there was moisture restriction and temperature increase, the tillering rate decreased by 12% on average (^{Cámara-Acosta et al., 2022}).

The death rate showed variations across the sampling dates (p< 0.05), and February was the month which presented the highest death rate in all cultivars and cutting intensities. Table 2, as a result of the minimum temperatures present prior to the sampling date, Figure 1, it also generated lower survival with values less than 65% in the six treatments on the same date. In the effect of the intensity for each cultivar, the intensity of 25 cm caused a 3% decrease on average compared to the intensity of 15 cm only in the survival of stems of the hybrid Mavuno, so exposing this cultivar to the intensity of 25 cm will affect the survival of the population of stems from one cutting date to another (Table 3).

Likewise, it has been reported that the intensity of 5 cm in the cultivar Mavuno has negative effects on the total population density, therefore, it is reflected in a lower survival rate and this behavior generates changes in the morphological composition, since, at intensities of 5 cm, it generates stems of greater weight, while, at intensities of 15 cm, the weight per stem decreases, however, in the total weight of dry matter, there are no differences between intensities (^{Rodrigues et al., 2021}). In this sense, ^{Cámara-Acosta et al. (2022)} mention that to achieve the persistence of a meadow, a balance between the tillering rate and the survival of the stems is necessary to maintain the density of stems in certain situations.

On the other hand, other dates differ in the behavior of the death rate in each cultivar by cutting intensity, this behavior is attributed to competition for space, to the little or no availability of moisture and stress to exposure to temperatures below 15 °C, determined as the base temperature of tropical grasses (^{Lucio et al., 2023}). In addition, it has been found that, in cuts with fixed intervals, there is a greater pressure on the death rate since the stems weaken more (^{Cámara-Acosta et al., 2022}).

Survival rate and stability index of the stem population

The stability index allows establishing a relationship between survival processes and the tillering rate, since, by itself, it does not allow clarifying whether it is sufficient to replace the death rate and maintain the stem population density (^{Ramírez et al., 2011}). Therefore, the cutting intensity had an effect on the stability of stems (p< 0.05) and the intensity of 15 cm was the one that presented the highest stability index with 9, 4 and 5% compared to the intensity of 25 cm in the three cultivars (Table 3).

However, the hybrid Cayman presented values lower than 0.95; that is, in the evaluation period, a negative stability was obtained in the meadow, so the intensities for this hybrid do not favor the permanence of the meadow; while the hybrids Mulato II and Mavuno remained in equilibrium with values of 0.98 and 0.96. In this regard, ^{Lucio-Ruiz et al. (2021)} when evaluating two cutting intensities defined by a fixed intensity of 15 cm and one at 50% considering the height before cutting in the hybrid Cayman, the latter intensity led to greater stability of the stem population, since the intensities were less than 15 cm of residue, therefore, it is established that for this cultivar in the established environment, greater intensity is required to increase the stability index.

In August, the six treatments presented a positive stability of stems, despite being the month where there is little or no precipitation and temperatures are maximum, due to the relationship that exists between the higher rate of tillering compared to the death rate on this date (Table 2). When evaluating Panicum maximum cv. Mombaza, ^{Ramírez et al. (2011}) found greater stability during the dry season than in the rainy season, while, when evaluating two cutting heights, ^{Difante et al. (2008)} found that at the 15 cm height, stem population remains more stable, with a decrease in variation in stem population density when conditions are limiting for stability.

The stability index showed the contrasts that exist between climatic conditions, given that with water deficit and high temperatures (October), there was less activity in the death rate and an increase in the tillering rate, while, during the month of September, with the ideal conditions to carry out tissue replacement, there was a higher death rate compared to the tillering rate, possibly due to plant growth and competition that exists between available resources such as radiation, soil and water (^{Rueda et al., 2018}).