Abstract:This study aims to model the impacts of the frequency of cuttings of Tifton 85 bermudagrass on the dynamics of evapotranspiration (ET c ) and to derive crop coefficients appropriate for grass water management. Two seasons of experimentation were used with four different cutting treatments which provided field data for calibration and validation of the soil water balance model SIMDualKc for all treatments. Cuttings were performed after the cumulative growth degree days (CGDD) attained 124 • C, 248 • C and 372 • C, thus from short to very long intervals between cuttings. SIMDualKc adopts the Food and Agriculture Organization (FAO) dual K c approach for partitioning ET into crop transpiration and soil evaporation, thus providing for an assessment of their dynamics. All treatments were irrigated to avoid water stress. Grass ET c was modelled adopting a K cb curve to describe the ET variation for each cutting cycle, that is, using the FAO K c curve that consists of a series of K cb curves relative to each cutting cycle. Each individual K cb curve consisted of three segments constructed when knowing the K cb values at the initial, at the end of rapid growth, and at cutting, respectively K cb ini , K cb gro and K cb cut . These K cb values were first estimated using the equation relating K cb to the density coefficient (K d ), which is computed from the fraction of ground cover (f c ) and canopy height (h) at the same dates. The goodness of fit indicators relative to the calibration and validation of the SIMDualKc model were rather good, with the normalized root mean square error (RMSE) ranging from 4.0% to 6.7% of the mean available soil water. As an example, the standard K cb values obtained after model calibration relative to the cuttings treatment with CGDD of 248 • C are: K cb ini = 0.86, K cb gro = 0.91 and K cb cut = 0.96. K cb values were smaller when the frequency of cuts was larger because h and f c were smaller, and were larger for reduced cuttings frequency since h and f c were then larger. Because the soil was wet most of the time, the soil evaporation K e varied little but its value was small due to the combined effects of the fraction of crop cover and plant litter covering the soil. The values of K c = K cb +K e also varied little due to the influence of K e and the K c curve obtained a form different from the K cb curves, and a single K c value was adopted for each cutting frequency, e.g., K c = 0.99 for the treatment with CGDD of 248 • C. Results of the soil water balance have shown that, during the experimental periods, likely due to the effects of the El Niño Southern Oscillation (ENSO), runoff and deep percolation exceeded ET c . Moreover, the soil evaporation ratio was small: 14% in case of frequent cuttings and less for more spaced cuttings, thus with a transpiration ratio close to 90%, which indicates a very high beneficial consumptive water use, mainly when cuttings are not very frequent.

RESUMO O trabalho objetivou determinar a infl uência de intervalos de pastoreio rotativo sobre a morfogênese de cinco gramíneas nativas da região central do Rio Grande do Sul (RS), agrupadas segundo uma tipologia funcional, nas estações

Leaf tissue fluxes of five Pampa biome grasses (Andropogon lateralis, Aristida laevis, Axonopus affinis, Paspalum notatum and Paspalum plicatulum) were evaluated in a natural pasture under rotational stocking with heifers in the spring and summer from October 2010 to April 2011. The experimental design was completely randomized, with two grazing intervals, defined by thermal sums of 375 and 750 degree‐days, and the five grass species. The evaluated species had a higher growth flux and balance between the fluxes when subjected to a grazing interval of 750 degree‐days and a higher real efficiency of utilization when subjected to grazing interval of 375 degree‐days. Andropogon lateralis showed the highest growth, intake and senescence fluxes: 3·61, 2·37 and 3·04 kg DM ha−1 day−1 respectively. Aristida laevis had the lowest real efficiency of utilization.

The study was carried out to evaluate the forage intake and performance of beef heifers from 15 to 18 months of age, grazing on Alexandergrass (Urochloaplantaginea (Link) Hitch) fed rice bran (RB; 0, 0.5 and 1.0% body weight (BW)). The experiment consisted of a completely randomized design with repeated measurement arrangement. Rice bran increased the total dry matter (DM) intake, total crude protein intake and total digestible nutrients intake. Heifers that received 1.0% BW of RB reduced forage DM intake, increasing by 18% stocking rate (SR) and by 27% BW gain per area in comparison with the system exclusively on pasture. In the system in which the heifers were fed 0.5% BW of RB, SR and gain per area were similar to other systems. In this feeding system, we observed average daily gain 18% higher than when heifers were exclusively on pasture and gain similar to that observed when using 1.0% BW of RB. In Alexandergrass pasture it is recommended to supply 0.5% BW of RB for beef heifers from 15 to 18 months of age, to promote better productive responses compared to heifers exclusively on pasture and similar to those observed when supplying 1.0% BW of RB.

The canopy structure and morphogenesis of ryegrass (Lolium multiflorum Lam.) was investigated after it was over seeded with Tifton 85 (Cynodon spp.) and intercropped with red clover (Trifolium pratense L.). Time intervals between cuts were determined by different thermal sums (125, 250, 375, and 500 degree-days [DD]). Cuts were conducted at a height equivalent to 50% of the average canopy height. Morphogenic and structural data were collected from July to October of 2016 and 2017, from a completely randomized block experimental design. Canopy height, extended tiller height, pseudostem length, and forage mass were fitted to linear equations according to the thermal sums. Interaction of ryegrass with red clover was the highest (12.1%) for longer cut intervals (375 and 500 DD). The leaf appearance rate (0.0080 leaf/DD), phyllochron value (128.8 DD), and senescence rate (0.10 cm/DD) of ryegrass were comparable between the different intervals. The slowest elongation rate (0.06 cm/DD) with the shortest interval duration between cuts (171.6 DD) was observed in the 125 DD interval. According to the findings from this 2-year experiment, the use of a 250 DD cut interval was recommended between grazing events, taking into account the efficiency of forage use, ryegrass leaf, stem ratio, and interactions with red clover in the canopy under rotational stocking.