Aspectos morfológicos e cinética de produção de gás in vitro de genótipos de capim-elefante de diferentes portes submetidos a duas frequências de colheita

Abstract

Elephant grass [Cenchrus purpureus (Schum.) Morrone sin. Pennisetum purpureum Shum.] has a high potential for dry matter production, however, it presents an early stem elongation that contributes to a rapid decline in the forage nutritive value. The objective of this study was to evaluate and explain variations in morphological characteristics and in vitro gas production kinetics of irrigated Elephant grass genotypes of different sizes under two harvesting frequencies. The experiment was carried out at the Experimental Farm of UFRPE, located in Garanhuns-PE, during a period of two years (2020 and 2021). The randomized complete block design was used with a split-plot arrangement and four replications, with the main plot consisting of harvesting frequencies of 60 and 90 days and the subplots of four elephant grass genotypes, two of which were tall (IRI- 381 and Elephant B) and two small ones (Taiwan A-146 2.37 and Mott). The morphological characteristics were evaluated before each harvest. Representative samples collected in the months of July, September, November and December used for the analyzes of in vitro gas production. Elefante B and IRI-381 genotypes had higher heights. Elefante B, Mott and IRI-381 genotypes presented the largest stem diameter in the rainy season, while in the dry season Mott showed the largest diameter. In the rainy season, the greatest number of nodes was observed for the genotypes IRI-381 (7.8) and Elefante B (7.2), and the genotype IRI-381 was the one with the greatest number of leaves per tiller with 12.9. Mott showed the greatest leaf /stem ratio (1.33), with values, on average, being greater during the rainy season (1.6). The internode length variable was influenced by the time of year, with a greater value in the rainy season (8.67 cm). The leaf angle was greater at 90 days harvest frequency during the rainy season (65.4°). Taiwan A-146 2.37 produced a greater volume of gas during the fermentation of non-fibrous carbohydrates (NFC) when harvested every 90 days (155.9 mL). Mott (0.0603 h-1) showed the greater rate of degradation for NFC. Mott showed the greatest gas production by fermentation of fibrous carbohydrates (FC) when harvested every 90 days (150.4 mL), and higher rate of FC degradation by the genotypes Taiwan A-146 2.37, IRI-381 and Elephant B. There was a higher lag time at 90 days of harvest during the rainy season, and higher total gas production at 60 days in the dry season (302.5 mL) for the entire plant. A higher lag time was observed for Mott (7.4 h) and higher total gas production for Taiwan A-146 2.37 (310.5 mL), at 60 days of harvest. The genotypes Mott (197.5 mL) and Taiwan A-146 2.37 (193.9 mL) showed a higher volume of gas by FC on leaf blades in the dry period, and IRI-381 showed a higher rate of degradation to FC (0. 0522 h-1). A higher total gas production (285.6 mL) was observed for the stem fraction at 60 days. It is concluded that the frequency of harvest at 60 days is indicated for genotypes of small size in the rainy season, and at 90 days in the dry season. Small ones genotypes continue to show better fermentation parameters in terms of harvest frequency at 90 days.