Humic substances and amino acids can promote positive effects on plant development. In addition, if they are associated with nitrogen fertilizations, they can potentiate plant growth, altering physiological characteristics and nutrient absorption. Thus, due to the importance of forages for Brazilian cattle raising and the potential of biostimulants and nitrogen in covering for plants, the objective of this work was to evaluate the effect of humic substances and amino acids associated or not with nitrogen cover fertilization in the production, physiology and protein content of the Mombasa grass. For this, two experiments were carried out, the first conducted in a completely randomized design with four replicates. The 21 treatments were obtained in the factorial scheme 4x5+1, being the first factor five sources of biostimulants (Humic Substance, Glycine, Proline, Humic Substance + Glycine and Humic Substance + Proline) and the second four doses (2; 4; 6 and 8 L ha-1), plus an additional treatment (control), consisting of the absence of application of biostimulant. It was observed that the application of humic substance and amino acids altered the morphology and physiology of plants, mainly plant height, leaf area, stomatal conductance, transpiratory rate, instantaneous carboxylation efficiency and CO2 assimilation rate. In addition, there was an average increment of 33% in the dry matter production and 21% in the protein content of the Mombasa grass, with the use of the Humic Substance and Proline sources. The second experiment was also conducted in a completely randomized design with four replicates, in the 4x5+1 factorial scheme. However, all 21 treatments received nitrogen fertilization (50 kg ha-1 of N) after the first two cuts in the grass, at 48 and 78 days after emergence. The first factor five sources of biostimulants (Humic Substance, Glycine, Proline, Humic Substance + Glycine and Humic Substance + Proline) and the second four doses (2; 4; 6 and 8 L ha-1), plus an additional treatment (control), consisting of the nitrogen cover and absence of biostimulant. The use of humic substances and amino acids associated with nitrogen cover fertilization were effective in promoting the development of Mombasa grass, increasing plant height, shoot biomass production, protein content, as well as the physiological aspects of stomatal conductance and CO2 assimilation rate. For mass production the most efficient sources were Humic Substance and Substance + Proline, increasing on average 13% natural matter and 4% the dry mass of the Mombasa grass. For the protein content, the best sources were Humic Substance and Humic Substance + Glycine, increasing the total protein by 8%. Thus, it is evidenced that biostimulants increase production and improve forage protein content, mainly due to changes in plant morphology and physiology, with the most efficient use of Humic Substance, Substance + Proline and Humic Substance + Glycine. In addition, it is notorious that the gains are more significant in the condition of absence of cover fertilization (33% in dry mass) when compared to those promoted by the association of nitrogen cover and biostimulants (4% in dry mass).
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