Reconhece-se cada vez mais a crescente falta de água para irrigação e outros usos. Também, está-se consciente de que, por sua irregular disponibilidade, a qual varia marcadamente ao longo do ano, de ano a ano e de região a região, o uso da água de forma contínua e indefinida, torna-se impossível. Portanto, uma das metas estratégicas para a preservação da disponibilidade e da qualidade dos recursos hídricos consiste em se estabelecer critérios de uso adequado em todas as atividades produtivas, razão pela qual este trabalho aborda a escassez de água, a agricultura irrigada e o meio ambiente, com base em informações e estudos que possam contribuir para o direcionamento de uma agricultura planejada, conservando e otimizando os recursos naturais. Os modelos tecnológicos propostos devem considerar um rigoroso equilíbrio entre produção agrícola e preservação dos recursos naturais. O desenvolvimento atual da irrigação depende de procedimentos tecnológicos e econômicos para otimizar o uso da água, melhorar a eficiência de aplicação, proporcionar ganhos de produtividade baseados na resposta da cultura à aplicação de água e outros insumos, sem que comprometa a disponibilidade e a qualidade do recurso.

Seasonality in tropical forage production is common in tropical and subtropical regions and should be identified for decision making on the stocking rate for each period. This study aimed to evaluate the variability of dry matter production of Megathyrsus maximum cv. Mombaça ‘Guinea grass’ over two years. The experiment was carried out from February 2016 to August 2018, under a randomised block design in a 2 × 4 factorial scheme, with four replications. Treatments consisted of (i) two growing years (2016/2017 and 2017/2018) and (ii) four seasons (spring, summer, fall, and winter). The grass was fertilised with 1,000 kg N ha⁻¹ year⁻¹ and sprinkler irrigated, with soil moisture monitored by tensiometry. The evaluated parameters consisted of leaf dry matter, stem dry matter and total dry matter production. The results showed that the irrigated Guinea grass reached an elevated production of dry matter (DM), with the highest yields occurring in the spring/summer period, evidencing the yiel gap in the autumn/winter. The grass showed high daily forage accumulation, expressing high productive potential, demonstrating a good indicator for animal production in intensive systems with irrigated and fertilised pastures for regions of tropical climate. Thus, Guinea grass showed great potential for use in livestock.

The objective of this work was to develop and validate linear regression models to estimate the production of dry matter by Tanzania grass (Megathyrsus maximus, cultivar Tanzania) as a function of agrometeorological variables. For this purpose, data on the growth of this forage grass from 2000 to 2005, under dry‑field conditions in São Carlos, SP, Brazil, were correlated to the following climatic parameters: minimum and mean temperatures, degree‑days, and potential and actual evapotranspiration. Simple linear regressions were performed between agrometeorological variables (independent) and the dry matter accumulation rate (dependent). The estimates were validated with independent data obtained in São Carlos and Piracicaba, SP, Brazil. The best statistical results in the development and validation of the models were obtained with the agrometeorological parameters that consider thermal and water availability effects together, such as actual evapotranspiration, accumulation of degree‑days corrected by water availability, and the climatic growth index, based on average temperature, solar radiation, and water availability. These variables can be used in simulations and models to predict the production of Tanzania grass.

Crop coefficient (Kc) is the ratio between crop evapotranspiration (ETc) and reference evapotranspiration (ETo), representing the phenological effects on crop water consumption. Kc is fundamental to estimating ETc by agrometeorological methods. This research study aimed to determine Kc and ETc values for Guinea grass (Megathyrsus maximus cv. Mombaça) and Bermuda grass (Cynodon spp.), both single cropped for one year and overseeded with black oat (Avena strigosa) and ryegrass (Lolium multiflorum) during fall/winter. The experiment in the field comprised four plots, two for each tropical forage, with and without overseeding. At the center of each plot, there was a weighing lysimeter with an automated system for data collection. ETc was measured daily over four seasons following the lysimetric method; ETo was calculated using the Penman-Monteith equation. ETc and ETo values were used to estimate Kc values. The single cropped Guinea grass showed the highest values for ETc, with mean ETc and Kc of 3.99 mm d^–1 and 1.07, respectively. The single cropped Bermuda grass showed ETc and Kc values of 3.57 mm d^–1 and 0.96, respectively. The results of paired t-testing for Kc showed no significant differences (p = 0.05) between single cropped and intercropped for both Guinea grass and Cynodon spp. During winter, intercropped Guinea grass did not show an ETc significantly higher than single cropped Guinea grass, with mean Kc values 0.98 for intercropped and 1.10 for single cropped. Similarly, Bermuda grass did not show significant differences between mean Kc values for intercropped (1.02) and single cropped (1.00).

Quantification of the drained volume is one of the difficulties involved in using weighing lysimeters. Typically, this volume is measured by accessing a moat at the base of a lysimeter. However, it is not feasible to install the moat in small devices. Thus, the aim of this study involves developing, installing, calibrating, and checking the efficiency of small weighing lysimeters with automated drainage systems to test their functionality in field conditions. Each lysimeter is composed of a round PVC water tank with a diameter of 1.22 m and a depth of 0.58 m that is placed over a metal frame with three electronic load cells with the nominal capacity of each cell corresponding to 500 kg. The drainage system is composed of a small reservoir with a volume of 10 L, a weighing structure composed of a load cell with a nominal capacity of 30 kg, and an automatic solenoid valve driven by a device coupled to a data logger that records the data from the lysimeter and from the drainage system. Two calibrations are performed for the lysimeter as well as the drainage system to obtain equations with significant correlations (R² > 0.9999). The drainage system was activated several times during the tests after receiving approximately 63.4 L of water from rainfall, and this in turn indicated a good performance.