Effect of cutting interval and cutting height on morphogenesis and forage accumulation of guinea grass (Panicum maximum).

Abstract

Response patterns of guinea grass to defoliation were assessed over a 4-month period at the Federal University of Vicosa using a randomised complete block design with 3 replications. The treatments were combinations of cutting height (25 and 50 cm) and cutting interval (2, 3 and 4 new leaves per tiller) and were arranged in a 2 x 3 factorial design. Morphological and structural characteristics, regrowth period and forage accumulation were assessed. The average regrowth period was similar for the cutting intensities assessed. Stem elongation, final leaf length and number of live leaves per tiller were greater with the 50 cm cutting height. Increasing the number of leaves/tiller at harvest increased period between harvests, canopy height at harvest and stem relative to leaf elongation, causing reduction of the leaf:stem ratio. It appears that cutting at the 3-leaf stage at 25 cm height would be the most appropriate for Mombaca guinea grass. However, a more practical standard would be cutting at a canopy height of about 90 cm. Longer-term studies are needed to confirm these preliminary findings, along with feeding studies to determine production benefits from this regime. Introduction Most pastures that support Brazilian beef herds consist of monocultures of C4 grass forages characterised by high levels of dry matter production (Da Silva and Nascimento Junior 2007). However, when producers base management actions on simplistic and empirical criteria and do not fertilise, pasture degradation occurs after a few years. In an endeavour to correct this situation and increase the efficiency of livestock production, new technologies, based on factors that regulate the productivity of tropical forage plants, must be generated and diffused. Important issues to be considered include: regrowth following cutting or grazing; morphological characteristics; and the environmental conditions (Barbosa et al. 2002). In this context, significant improvements in the efficiency and productivity of pastureanimal production systems could be obtained by starting with simple actions, such as those related to grazing or defoliation management, that would result in highly efficient use of the forage produced. Identification of cutting intervals and cutting heights that optimise the net forage accumulation and delay senescence and stem accumulation would favour the efficient accumulation of large quantities of highly nutritious forage (Pinto et al. 2001; Carnevalli et al. 2006; Barbosa et al. 2007; Difante et al. 2009; Da Silva et al. 2009). This study assesses the responses of Mombaca guinea grass (Panicum maximum cv. Mombaca) to various cutting strategies in terms of regrowth and forage accumulation in order to generate data for planning efficient management practices for this forage plant. Material and methods The experiment was carried out in the Department of Animal Science at the Federal University of Vicosa (UFV) (20o 45’S, 42o 51’W; 651 masl) in Vicosa, MG, from October 2003 to May 2004. Correspondence: D. do Nascimento Junior, Universidade Federal de Vicosa, Departamento de Zootecnia, Avenue PH Rolfs, s/no, Campus Universitario, CEP 36570-000, VicosaMG, Brazil. E-mail: domicio@ufv.br 104 M.C.T. Da Silveira, D. Nascimento Junior, B.A.L. Da Cunha, G.S. Difante, K.S. Pena et al. According to the Koppen classification, the climate of the region is subtropical (subtype Cwa) with mild, dry winters and well defined dry and wet seasons. The average annual temperature is 19°C, oscillating from 22 to 15°C for the maximum and minimum means. The average relative humidity is 80% and the mean annual rainfall is 1340 mm. Climatic conditions during the experimental period were monitored at the meteorological station of the Department of Agricultural Engineering, about 1 km from the experimental area. Monthly maximum, mean and minimum temperatures and rainfall during the experimental period are shown in Figure 1. The soil type is classified as Red–Yellow Argissol (EMBRAPA 1999) with a sandy–clay texture. Soil samples were collected to a depth of 20 cm and chemically analysed to calculate the requirements for liming and phosphate and potassium fertilisation, to raise the base saturation to 50%, the phosphorus content to 8–12 mg/dm3 (Mehlich I extraction) and the potassium content to 80–100 mg/dm3 (EMBRAPA 1999). Guinea grass was sown in September on plastic trays containing a commercial organic substrate in a greenhouse with daily irrigation. At transplanting to the experimental area in October 2003, 1.1 t/ha dolomitic limestone was applied to the soil, incorporated to a depth of 20 cm and homogenised manually using a hoe. Single superphosphate was then applied (48 kg/ha P) on the plot rows and incorporated to approximately 15 cm. Nitrogen (50 kg/ha N as urea) and potassium (75 kg/ha K as potassium chloride) fertilisers were applied in 2 applications after the plants were established. A complete randomised block design was used with 3 replications. Each block consisted of 6 plots, with 18 subplots of 0.6 m2 each (1.0 x 0.6 m). Subplots had 4 rows spaced at 20 cm, each with 6 plants spaced at 15 cm, giving a total of 24 plants per subplot. Experimental treatments were combinations of 2 cutting heights (25 and 50 cm) and 3 intercutting intervals, corresponding to the appearance of 2, 3 or 4 leaves per tiller. Four, 3 and 2 subplots were used in a block for intervals of 2, 3 and 4 leaves per tiller, respectively, to allow for different harvest frequencies. In December 2003, when the plants were completely established, a standardising cut was made at a height of 35 cm from the soil. Thirty days later, all subplots were cut to their planned target cutting height (25 or 50 cm), and observations commenced from this point. Two plants were chosen in each subplot to study tiller dynamics according to the methodology reported by Carvalho et al. (2000). Two tillers were marked on these plants with coloured plastic wires at random to monitor and study the morphogenetic and structural characteristics. Twice a week, the following measurements were made: length of leaf blades of expanding and expanded leaves; length of the green parts of senescent leaf blades; and the length of the pseudostem (height from Figure 1. Monthly means of the maximum, average and minimum temperatures and rainfall during the experimental period. 0 5 10 15 20 25 30 35 Oct 03 Nov 03 Dec 03 Jan 04 Feb 04 Mar 04 Apr 04 May 04 0 100 200 300 400 500 Rainfall Maximum temp. Minimum temp. Medium temp. R afall (m m ) Te m pe ra tu re ( ◦ C ) Growth of tropical grasses in the mediterranean 105 the soil to the ligule of the last expanded leaf). New leaves were also recorded. These data were used to calculate: leaf appearance rate (LAR – leaves/tiller/day), the phyllochron (Phyl – days/ leaf tiller), leaf elongation rate (LER – cm/tiller/ day), stem elongation rate (SER – cm/tiller/day), final leaf blade length (FLL – cm/tiller), number of live leaves per tiller (NLL – leaves/tiller), leaf life span (LLS – days) and leaf blade senescence rate (LBS, cm/tiller/day). Cutting dates were determined from the number of new leaves on the tillers being monitored. The mean for a treatment was calculated as a combination of cutting interval and height, regardless of the blocks. When the mean reached a pre-determined number of new leaves (NNL), cutting was carried out and the date was recorded. At cutting, the harvested forage from 4 plants per subplot was collected and used to calculate accumulated biomass and its morphological composition. The data were analysed as a balanced factorial in split plots in time, where the treatments, cutting height x cutting interval combinations, constituted the plots and the cuts constituted the subplots. The GLM procedure was used from the SAS statistical package (SAS Institute 1996). Means were compared using the appropriate contrasts both for the main effects of cutting height and cutting interval and for the effects of cutting height x cutting interval interaction and the cut within the cutting height x cutting interval interaction. A significance level of 5% was adopted. The following model was used for the regrowth period variable: Yijk = μ + Ii + Hj + IHij + Ck + eijk where: Yijk = observed value of the cutting interval i and cutting height j in cut k; μ = general constant (population mean); Ii = effect of the cutting interval i, i = 1, 2, 3; Hj = effect of the cutting height j, j = 1, 2; IHij = interaction between cutting interval i and cutting height j; Ck = effect of cut k, k = 1, 2, 3, 4; and eijk = random error associated with each observation Yijk. The following model was used for the variables dry matter, morphological composition and those regarding the canopy morphogenetic and structural characteristics: Yijkl = μ + Ii + Hj + IHij + Bk + ea + Cl(IHij) + eb