Nitrogen and phosphorus fertilization and regrowth age on the fatty acid profile in tropical grasses during the dry and rainy seasons

The concentration of linoleic and [alpha]-linolenic fatty acids (FA) in grasses can be increased through agronomic techniques; however, there is little information in this regard for tropical grasses. Therefore, the objective of the study was to evaluate the effect of N fertilization and regrowth age management on the FA profile of three tropical grasses. To do this, three plots of 100 [m.sup.2] each were used for the species: Urochloa decumbens (Stapf) R.D. Webster, U. humidicola (Rendle) Morrone & Zuloaga, and Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. Jacobs. The study was conducted during dry and rainy seasons. In each period, they were divided into two subplots and only one of them was fertilized. Grass sampling was carried out on days 20, 25, and 30 after fertilization. Five samples were taken from each subplot and an agronomic evaluation was made before each cut. The chemical composition and lipid profile were then determined. An ANOVA of a randomized complete block design with split plot arrangement was performed. The concentration of linoleic and [alpha]-linolenic acid was higher in M. maximus during drought (48.5 g 100 [g.sup.-1] FA) and in U. humidicola during the rainy season (55.1 g 100 [g.sup.-1] FA). This proportion was higher on day 20 (50.5 g 100 [g.sup.-1] FA) compared to that recorded on days 25 and 30 (50.1 and 45.7 g 100 [g.sup.-1] FA, respectively). The fertilized grasses had a higher concentration of linoleic and [alpha]-linolenic acid compared to the unfertilized ones (53.4 vs. 44.3 g 100 [g.sup.-1] FA). It is concluded that fertilization with 50-20-00 (N-P-K) increases the proportion of linoleic and [alpha]-linolenic acids in grasses; however, such concentration decreases with increasing plant maturity. Key words: Conjugated linoleic acid, forage production, lipid metabolism, ruminant nutrition.