In the first evaluation, the variable plant height was adjusted to the quadratic regression model when grown in Oxisol, with the largest height of marandu grass being 47.93 cm in the wood ash dose of 9.12 g dm-3. For the Ultisol, the height of plants was adjusted to the linear regression model with an increase of 44.96% comparing the highest dose of the experimental interval (15 g dm-3) with the absence of fertilization with wood ash (Figure 1A). The height of marandu grass plants, in the second evaluation, showed no significant difference between the doses of wood ash and in the last cut of the forage grass cultivated in Ultisol, the plant height was adjusted to the linear regression model, with a decrease of 18.31% relating the absence of the application with the dose of 15 g dm-3 wood ash (Figure 1B).
In the first cut, the height of the marandu grass grown in both soils from the dose of 9 g dm-3 exceeded 40 cm. In a study aimed at reconciling greater leaf production and lower production of stems with dead material of B. brizantha cv. Marandu irrigated in the field; the greatest heights ranged from 35 to 40 cm, thus justifying the fertilization with wood ash to obtain significant growth of the forage (Rodrigues, 2004). On the contrary, in the third cut, the forage grass did not reach this minimum margin of plant height, since there was no reapplication of the residue from the second cut and the nutrients began to run out.
The Ultisol, even in the absence of fertilization with wood ash, provided greater plant height, standing out about the Oxisol, in the first and third cut. This may be due to higher natural fertility through the nutrient concentration in Ultisol, especially phosphorus, an important nutrient for plant growth. Nonetheless, in the second cut, there was no significant difference between the two soils, indicating that the Oxisol fertilized with wood ash raises the height of marandu grass, equating up to the Ultisol. Gonçalves and Moro (1995) observed that the use of forest biomass ash, under laboratory conditions, substantially increased soil fertility, responsible for the rise in productivity of Eucalyptus grandis. In the third evaluation, in particular, there was a reduction in plant height with the increase of wood ash doses, in virtue of the ash solubilization in the soil and the availability of nutrients (Demeyer et al., 2001) for the marandu grass in the first cut, and as the nutrients were extracted at the time of the cuts, they were restricted to plants, since there was no re-application of the wood ash.
According to Garay et al. (1997), plant height is an important structural feature for the adoption of appropriate management, indicating the time of entry of the animals in the pasture. The consumption increases with the elevation of the pasture height to a level in which it remains constant, depending on the species and animal category (Hodgson, 1990).
In the first evaluation of the forage grass, the dry mass of leaves was adjusted to the linear regression model, where the highest wood ash dose increased by 78.41 and 66.25% in the dry matter production of marandu grass leaves grown in Ultisol and Oxisol, respectively, when compared to the treatment without fertilization (Figure 2A). In the second evaluation of the plants, the dry mass of leaves of the marandu grass grown in Oxisol was adjusted to the linear regression model increasing by 17.49% in the production when comparing the dose of 15 g dm-3 with the absence of application of wood ash. The marandu grass grown in Ultisol showed dry mass of leaves with adjustment to the quadratic regression model, in which the least amount of leaves was 4.4 g pot-1 using 7.09 g dm-3 of wood ash (Figure 2B).
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In the third cut, the dry mass of leaves was adjusted to the quadratic regression model in which the maximum production (2.63 g pot-1) was observed when the marandu grass was grown in Oxisol using the wood ash dose of 10.87 g dm-3 (Figure 2C). It was found, as in this study, the positive effect of wood ash doses on Brachiaria brizantha grown in Oxisol, in the first two cuts, with higher results for the dry mass of leaves with increasing doses of this waste (Santos, 2012). In this way, it is emphasized the potential of using fertilization with wood ash in forage, since the dry mass of leaves represents the portion of highest nutritional value, once the leaves are more digestible than the stems and the senescent material, and also have higher crude protein levels (Paciullo, 2002), being therefore preferred by the animal.
In the third cut, the significant difference in dry mass of leaves of marandu grass only when grown in Oxisol indicates that despite being a soil with low natural fertility, the application of the residue can exceed the production potential of this soil about that of greater natural fertility. The agricultural production in low fertility soils requires nutritional supplementation, which can be made with organic fertilizers, such as wood ash (Bonfim-Silva et al., 2014). The dry matter yield of stems of marandu grass in the first cut was adjusted to the linear regression model with an increase of 87.9% for Ultisol and of 77.24% for Oxisol, from the higher dose of wood ash when compared to the absence of the same (Figure 3A).
In the second cut of the forage grass, there was a 33.17% increase in dry matter yield of stems of marandu grass in Oxisol comparing the higher wood ash dose with the absence of application of this waste, while the lowest dry matter of stems (1.37 g pot-1) was found at a dose of 6.96 g dm-3, applied in Ultisol (Figure 3B).
The dry mass of stems in the last cut was adjusted to the quadratic regression model in which the maximum production was equal to 3.51 g pot-1, observed at a dose of 12.38 g dm-3 when the marandu grass was grown in Oxisol. For the Ultisol, the adjustment of the results of this variable was given by the quadratic regression model with minimum point (3.20 g pot-1) in the wood ash dose of 4.22 g dm-3 (Figure 3C).
As in the present study, the dry mass of stems of marandu grass in the first cut, in a study conducted by Bonfim-Silva et al. (2013), showed linear effect with increased doses of wood ash. There was a greater increase in dry mass of stems about the dry mass of leaves (Figure 2A) in both soils since forage grasses have a critical time when there is the greater targeting of their photoassimilates to the production of stems (Bianco et al., 2005). From the second cut, the rate of leaf growth slowed the success of the development of stems. This is a strategy of the plant to accumulate mass, since the excess of leaves causes self-shading, reducing the overall plant photosynthesis (Negreiros Neto, 2007). In the first cut of the marandu grass, the leaf/stem ratio were adjusted to the quadratic regression model, and the lowest score, of 2.32, was obtained in the wood ash dose of 10.79 g dm-3, applied in Oxisol. As for the Ultisol, there was a decrease of 41.58% in the leaf/stem ratio when comparing the absence of ash application with the maximum dose of the experiment by 15 g dm-3 (Figure 4A).
The wood ash doses decreased the leaf/stem ratio by 21.68% in the second evaluation of the marandu grass when grown in Oxisol, compared with the absence with the maximum dose of this waste (Figure 4B). In the third cut of the marandu grass, there was no significant effect of wood ash doses on both soil classes. The reduction in the leaf/stem ratio of the marandu grass was due to the increase in the production of stems from the application of wood ash doses. As the nutrients have been extracted at the time of the cuts, they have become insufficient because there was no re-application of wood ash, which can interfere with the grass structure, compromising the grazing efficiency due to the decrease in the leaf/stem ratio (Rodrigues et al., 2008). The variable leaves/stems, according to Euclides et al. (2000), is directly related to the performance of grazing animals, important from the point of view of the nutritional value and of the management of forage species (Alden and Whitaker, 1970; Pinto et al., 1994).
Despite this decrease in the leaf/stem ratio, the results of this study are above the ideal critical threshold, which is equal to 1.00. This critical level regards the quantity and the quality of the produced forage (Pinto et al., 1994; Andrade, 1997). The marandu grass, in general, had higher growth and yield when grown in Ultisol fertilized with the wood ash dose of 15 g dm-3. The maximum yields of marandu grass grown in Oxisol were obtained in the wood ash doses range of 9 to 15 g dm-3.
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