Dry matter yield (DMY)

Differences in DMY by genera of grasses is shown in Table 1, with highest (P<0.05) differences between Brachiaria y Penisetum in the first (P=0.0021) and in the second (P=0.0109) cycle. Panicum y Pennisetum were different only in the first cycle (P=0.0331). During the first cycle, orthogonal contrasts for grasses among genera indicated that King grass (5,582 kg ha^-1) was different (P<0.05) from Taiwan and Purple King Grass (4,606 y 4,494 kg ha^-1). During the second and third cycles, DMY was similar (P>0.05). DMY varied in the second cycle from 1,788 until 2,136, and in the third cycle, from 893 until 1,889 kg ha^-1. During the three periods grasses of genus Brachiaria showed the lowest DMY.

Parameters of the growth exponential model are shown in Table 2. For genus Pennisetum the parameter "a" had the highest variation, with values ranging from 17.5 to 695.9 kg ha^-1 (cycles 3 to 1, respectively). The relative growth rates (parameter "b") were less variable, excepting Pennisetum spp (cycle 3) whose value was 2.5 times higher than other values. Variabilities in "a" and "b" suggest an interaction according to the week and the growth cycle. The time to duplicate yield (DMY) varied from 1.5 wk (Pennisetum spp, cycle 3) to 4.6 wk (Brachiaria spp, cycle 2). Growth rates (TC₇.₅) at 7.5 wk were highly variable: from 127 kg ha^-1 wk^-1 (18.1 kg ha^-1 d^-1) for Panicum spp (cycle 3), until 873 kg ha^-1 wk^-1 (124.7 kg ha^-1 d^-1) for Pennisetum spp (cycle 1). Determination coefficient R² varied from 0.46 to 0.85, and F Test was highly significant (P<0.0001) in all the cases.

Chemical composition

Table 3 shows means for the evaluated quality variables. Purple King grass had the lowest values (P<0.01) of NDF and hemicellulose, although Chetumal and Guinea had the highest values for NDF; and Chetumal had the highest value for hemicellulose. Panicum and Pennisetum showed the highest CP concentrations (>10 %). Even though in situ dry matter degradability was similar among genera, grasses of genus Panicum showed the lowest values, in average 67.1 %.

Table 4 shows that Brachiaria had the lowest value for ADF (cycle 1). Among ages of regrowth, this value increased according to the time. Cycles 2 and 3 also showed a genus x age of regrowth interaction (P<0.05). In the first cycle, lignin concentration was similar among genera and among ages of regrowth, but this was different in the other two ones. Cycles 2 and 3 showed a genus x age interaction (P<0.05). Regarding hemicellulose, its content in Brachiaria (cycles 1 and 2) was higher than in Pennisetum (P<0.05), but in cycle 3 there was a genus x age interaction (P<0.05). At 12 wk, the content of hemicellulose was similar for all three genera. The concentration of cellulose was higher for Panicum (cycles 1 and 2) than for the other genera; whereas regarding ages, the concentration of cellulose was higher (P<0.05) at 12 wk in the cycles 1 and 2, but in cycle 3, the average value of cellulose at 6 wk was higher (P<0.05). Content of CP varied among genera from 7.6 to 11.9 %; with significant differences in cycles 2 and 3 (P<0.001). For ages, the concentration of CP was also statistically significant (P<0.001) in all the three cycles, with a reduction in the values from wk 3 to wk 12. Low values of CP were registered in the second cycle, by genera and by ages.

In situ degradation of dry matter and energy

In cycles 1 and 2, the in situ degradability of dry matter of grasses of genus Panicum was less (P<0.0001) than those of grasses Brachiaria and Pennisetum (Table 5). This variable decreased (P<0.0001), in all the three cycles, along with an increase in age at harvest. The degradability of samples harvested at 3 wk was higher (81.0 %; P<0.05) than when grasses were cut at 12 wk (67.0 %). For second and third cycle, there was a significant genus x age interaction.

GE content of the samples before ruminal incubation showed significant differences among genera in the three evaluated cycles (P<0.003). Genus Pennisetum had the lowest values, averaging 3,608 kcal kg^-1 DM, compared to Brachiaria and Panicum, which averaged 3,802 y 3,757 kcal kg^-1 DM, respectively. By ages, only in the second cycle there were significant differences (P<0.016). The GE values of samples after the ruminal incubation was statistically significant only in the second cycle, for genera, ages and its interaction.

Ruminal energy degradability (kcal kg^-1 DM) was higher for Panicum in the second cycle, and was different by ages of harvesting during the second and the third cycles (P<0.0001). In the second cycle, grass harvested at 3 wk had a higher content of ruminal degradable energy (2,658 kcal kg^-1 DM) than in other ages (P=0.001). In the third cycle, grass harvested at 9 wk had a higher ruminal degradable energy content (2,276 kcal kg^-1 DM; P<0.001), than in others harvest ages.

Dry matter yield

The seasonality of forage production, a common fact under tropical conditions, leads to fluctuations in the production²³ and quality of forage throughout the year. In the present study, grasses of Genus Pennisetum, (erect growth habit), were different compared with species of Brachiaria and Panicum. Similar facts were observed by Valerio et al²⁴. They found that erect-type growing grass species Merkeron grass, Tanzania and Guinea grass, performed better than decumbent ones (B. humidicola, Cynodon spp and Digitaria spp), since erect-type grasses accumulate more forage, as a function of a higher size and leaves area.

In the state of Guerrero, Mexico, Peralta et al²⁵ found significant grass species x age of regrowth interactions in six cultivars of Brachiaria spp (Toledo, Insurgente, Señal, HBA-4062, HBA-2094, Mulato) and two of Panicum (Tanzania and Mombasa), and in regrowth ages of 3 to 12 wk (P<0.01). They mentioned that, cultivars Tanzania and Mombaza had higher yields than those achieved by Brachiaria. In the present study, in the first cycle (dry season), Pennisetum yielded more DM than the other grass.

The exponential growth model described properly the quantitative increase in aerial biomass throughout the time. The positive intersection determined by the model is an estimator of the biomass residue left after the standardization cut (week= 0) at the beginning of each cycle. Parameter "c" estimates the efficiency by which the present dry matter produces more dry matter. This parameter predicts that for every kilo of DM present produced between 149 and 473 g of new biomass per week. This means that between 1.5 and 4.6 wk are needed in order to duplicate the amount of grass harvested. Growth rate (kg ha^-1 d^-1) is the tangent of the curve in a determined point, which resulted in values varying from 127 to 873 kg ha^-1 wk^-1, equivalent to 18.1 and 124.7 kg ha^-1 d^-1 (Table 2). These estimates of growth rates are higher than those measured in the same Experimental Station in several grasses cultivated in small plots, whose values (average of 4 yr) varied from 24.6 to 52.1 kg ha^-1 d^{-1 26}, and are clearly higher than those obtained in the same place for native grasses in intensive rotational grazing, with yearly average of 13.7 ± 12.8 kg(²⁷X It is considered that growth differences between plants in small plots and grasslands are due to microenvironments originated by the different managements that are applied to the plants as a consequence of the grazing activity of the animals. In the present work, the genus Pennisetum spp, showed the highest growth during cycle 3, with growth rate at 7.5 wk, of 873 kg ha^-1 wk^-1, 2.5 times superior than the average of all other genera. The capacity of the species of this genus to produce huge quantities of forage has been verified in studies realized in the tropics of Mexico²⁸ and in the world²⁹.

Considering the dry matter yields for the three evaluation cycles (Table 2), mean annual yields ranged from 13.9 to 29.8 DM t ha^-1. Averages by genus were: 15.4 (Brachiaria), 18.7 (Panicum) and 35.5 (Pennisetum) DM t ha^-1. In Brazil, Aparecida et al³⁰ reported a range of 43.2 to 66.4 DM t ha^-1. For three grasses of genus Brachiaria grasses (B. decumbens, cv Basilisk; B. brizantha, cv Marandu; B. brizantha, cv Xaraes) and two Panicum grasses (P. maximum, cv Mombaza; P. maximum, cv Tanzania), harvested monthly during 10 mo, with application of 45 and 80 kg ha^-1 of N and KCl, respectively, after every harvest, and irrigation when needed. In Thailand²⁹ reported significant differences between Pennisetum cultivars, Napier (31.3 DM t ha^-1), Merkeron (28.0 DM t ha^-1) and Tangashima (30.3 DM t ha^-1) on the one side, and Dwarf Napier (26.1 DM t ha^-1) and Taiwan A-25 (26.1 DM t ha^-1). These findings agree with results of the present study for annual dry matter for Taiwan, but were lower for Cuban King grass and for Purple King grass, with values of 29.8, 41.4 and 35.5 DM t ha^-1, respectively.

In situ dry matter degradation (ISDMD)

In the present study, samples of Pennisetum had always higher ISDMD in each evaluation cycle. It decreased as the cutting age increased, in a proportion of 17.3, 21.5 and 3.3 %, at cycle 1 to 3, respectively. In Indonesia (West Sumatra, 2,289 mm year^-1), ISDMD of B. decumbens, P. purpureum and A. compressus, after 72 h of incubation during both, rainy and dry season was 60.5 and 57.4 %, 63.3 and 63.5 %, and 58.3 and 57.6 % respectively³⁹, being these values in general lower than the findings of the present experiment; however, there is not an explanation of this fact.

In Tabasco, México, Jimenez et al⁴⁰ reported that season and age (21, 28 and 35 d) of regrowth affected significantly the DM degradation of B. humidicola after 48 h of incubation. Highest values were detected during the dry season (59.0 %) followed by rain (57.3 %) and winter (55.7 %) period. By regrowth age, they reported largest degradation at 28 d (59.2 %), followed by 21 (55.5 %) and 35 d (56.8 %). These results were lower than the values recorded for 3 to 12 wk, which varied from 81.0 to 53.8 %. Respect to cycle, report of Jiménez et al⁴⁰ agrees with the results of the present experiment, when also highest values of ISDMD were recorded during first and third cycles with low rainfall.

Gross energy (pre and post incubation)

In the present experiment, total gross energy content was higher for Brachiaria and Panicum, than for the Pennisetum grasses; however, not a clear trend was observed considering regrowth ages. In north of Veracruz, Mexico, Juárez-Reyes et al⁴¹ evaluated four representative tropical pastures, P. maximum (Guinea), P. maximum (var Tanzania), Digitaria decumbens (Pangola) and Cynodon dactylon (Bermuda), at beginning of flowering, and reported a gross energy content of 3,940, 3,930, 4,090 and 4,070 kcal kg^-1 DM, respectively.

Ruminal degradable energy

At first and second cycle of evaluation in the present study, Pennisetum, had the highest values of degraded energy, both in terms of percentage and of kcal kg^-1 de DM). During the two first cycles, a decreasing value of degraded energy was observed as age increased. In Indonesia, a value of 2.6 Mcal ED kg^-1 DM was reported for samples of B. brizantha (vegetative stage), evaluated during the rainy season (1,800 mm rain/yr)⁴². This figure is close to the value for Brachiaria genus (2.52 Mcal kg^-1 DM) reported for the second cycle (rainy season). In India other researchers found that digestibility of gross energy of buffelgrass (Cenchrus ciliaris), declined with maturity, from monsoon (rainy season) to summer (54.6 to 37.1 %)⁴³. This performance is similar to the results of the present study for ruminal degradable energy, where the highest value (64.0 %) was recorded during the rainy season, and lower values were registered during the first (49.9 %) and third (50.1 %) cycles, both with dry conditions.