RESUMO -Objetivou-se avaliar o comportamento de espécies forrageiras (Brachiaria decumbens cv. Basilisk, Brachiaria brizantha cv. Marandu, Panicum maximum cvs. Tanzânia, Aruana e Mombaça, Hemarthria altissima cv.Florida; Paspalum notatum cv. Pensacola, Axonopus catharinensis, Cynodon sp. híbrido Tifton-85; Arachis pintoi cvs.Alqueire e Amarillo) submetidas a diferentes níveis de luminosidade produzidos por árvores de Pinus taeda (céu aberto; 9 m entre linhas e 3 m entre árvores; e 15 m entre linhas e 3 m entre árvores). Avaliaram-se a produção de MS, a relação lâmina foliar:colmo (L:C), o nível de florescimento das plantas, os teores de PB e FDN e as variáveis meteorológicas e do solo, na projeção da copa e no meio da entrelinha de árvores, de cada parcela. A produção de MS foi afetada negativamente pelo sombreamento, por outro lado, o teor de PB foi maior nas parcelas sombreadas em relação ao pleno sol. Além da menor radiação, a velocidade do vento e a temperatura do solo nas parcelas sombreadas foram menores naquelas a sol pleno. O teor de FDN não diferiu significativamente entre os níveis de luminosidade, embora a relação L:C tenha sido maior na entrelinha do 9 × 3 em relação àquela a céu aberto. As espécies forrageiras mais produtivas e com maior potencial para utilização em ambiente Key Words: forage production, shading, silvipastoral system IntroduçãoOs sistemas silvipastoris têm sido amplamente estudados e aplicados na agropecuária tropical e subtropical visando otimizar a utilização das áreas. Há possibilidade de adaptação de várias espécies forrageiras ao cultivo em consórcio com espécies arbóreas e diferenças no comportamento das pastagens, especialmente em virtude das condições microclimáticas e da competição entre os componentes vegetais pelos recursos naturais disponíveis.

A simple multiplicative model using temperature, foliage nitrogen (N) concentration and water status was developed to predict the maximum photosynthetic rate (Pmax) of field‐grown cocksfoot (Dactylis glomerata L.) leaves when none, one, two or all the factors were limiting. The highest Pmax was 27·4 μmol CO2 m–2 s−1 in non‐limited conditions, which was defined as the standardized Pmax value dimensionless (Pmaxs=1). Pmaxs increased 0·058 units per °C from 10°C to the optimum range (19–23°C) (Pmaxs=1) and then declined 0·077 units of Pmaxs per °C from 23 to 31°C. Pmaxs=1 was also measured from 59 to 52 g N kg−1 dry matter (DM) foliage N. Pmaxs then decreased at the rate of 0·115 units per 10 g N kg−1 DM from 52 to 26 g N kg−1 DM, and 0·409 units of Pmaxs per 10 g N kg−1 DM from 26 to 15 g N kg−1 DM. For predawn leaf water potential (ψlp), Pmaxs=1 was measured from −0·1 to −1·2 bar but declined linearly at a rate of 0·078 units per bar of ψlp from −1·2 to −14·0 bar because of a linear decrease in stomatal conductance. An interaction between low N content (≤20 g N kg−1 DM) and high temperature (>23°C) was also detected. Together, this multiplicative model accounted for 0·82 of the variation in Pmaxs.

Maximum light‐saturated photosynthetic rate (Pmax) and stomatal conductance (gs) of field‐grown cocksfoot (Dactylis glomerata L.) leaves in a silvopastoral system were measured at different times under moderate (850–950 µmol m−2 s−1 photosynthetic photon flux density, PPFD) and severe shade (85–95 µmol m−2 s−1 PPFD). Also Pmax and gs were measured after 30, 60 and 180 min of severe shade to determine the lag in the rise of photosynthesis rate from low to high irradiance levels (induction state). The highest Pmax and gs values obtained were 26·5 µmol CO2 m−2 s−1 and 0·41 mol H2O m−2 s−1 in non‐limiting conditions with full sunlight (1900 µmol m−2 s−1 PPFD). These values were defined as standardized dimensionless Pmaxs=1 and gss=1 for comparison of treatment effects. The Pmaxs under severe shade decreased by 0·004 units per minute from 1 to 180 min and reached a steady‐state of 0·37 units after 140 min. Under moderate shade, Pmaxs decreased by 0·002 units per minute from 1 to 120 min and reached a steady‐state of 0·76 units. The time required to reach full induction on return to full sun (Pmaxs=1) was 15 min after 30 min of severe shade and 37 min after 180 min of shade. Mathematical equations were derived to describe the changes in Pmaxs and gss under severe and moderate shade and during induction. The rate of change of gss was slower than for Pmaxs on entering shade and also slower during the subsequent induction process. This indicated other factors in addition to gs were operating in the reduction and increment of Pmax and a two‐step model to explain this is proposed. The defined photosynthetic responses of cocksfoot leaves to fluctuating light regimes could be used to develop quantitative predictions of Pmax for inclusion in a canopy photosynthesis model of silvopastoral systems.

Shade cloth is commonly used in agroforestry research. It produces a continuous, uniform reduced light environment. Shade cloth and a slatted structure were compared in relation to the inability to represent the light regime and plant responses of an agroforestry system. The split-split-plot randomised block experiment had main plots as covering status (with or without radiata pine trees), subplots as artificial shade (none, shade cloth or wooden slats) and sub-subplots as growth rotation, over sown alfalfa, in three replicates. The quantity of light transmittance was 49% under trees, 41% under cloth and 44% under slats. Temporal changes and spectral composition under trees were more accurately reproduced under the slats than shade cloth. The red to far red ratio was 0.64 under tree shade and 0.74 during the shaded period under slats. This compared with 1.31 in open pasture, 1.28 under shade cloth in open and 1.26 under slats during sunny periods. To compensate for low light quantity and quality, alfalfa had elongated stems and internodes. In open pasture and under cloth in the open, it produced short stems. The mean dry matter yield under trees was 68% of the 30.3 t ha -1 in open pasture, 56% under cloth and 57% under slats. The slats induced similar morphological responses in alfalfa to those in the agroforestry system. The magnitude of changes had little effect on growth and yield responses. The artificial slatted structure approximated the intermittent light environment and consequent plant responses observed in an agroforestry system.

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The successful achievement of a highly productive understorey pasture in silvopastoral systems depends on the use of well-adapted forage genotypes, showing good agronomic performance and persistence under shading and grazing. In this study, the herbage dry matter yield (DMY) and nitrogen nutrition dynamics were determined in three native warm-season grasses (Paspalum regnellii, Paspalum dilatatum and Paspalum notatum) and a forage legume (Arachis pintoi) under two shading levels compared with full sun. The experiment was conducted in the Campanha region, Bagé, state of Rio Grande do Sul, Brazil, during two evaluation cycles (2008/2009 and 2009/2010). Three shade cloth levels (0%, 50% and 80% of light restriction) were applied to the forage genotypes in a split plot design, in which shading levels were the main plot and forage genotypes were the subplots, with three replications. P. regnellii showed the highest accumulated DMY (1500 and 1700 g m-2, respectively, for the first and second evaluation cycles) at all shading levels and showed no DMY decreased under the heavy shade (80%). Average DMY over the four genotypes under the 50% shade level was higher or equal compared with full sun. Influence of rainfall was observed on the DMY performance of all genotypes: the positive effect of moderate shading (50%) on P. dilatatum and P. notatum DMY was associated to a low soil water availability status. Increased shading level resulted in high nitrogen nutrition index values on grasses, in comparison with full sun. All genotypes performed well under the moderate shading level, but the DMY of both P. regnellii and P. dilatatum and the herbage N content in P. notatum and A. pintoi of all genotypes stood out, showing that those main genotypes are promising to grow in silvopastoral systems at the Campanha region in southern Brazil

Avaliou-se o efeito do sombreamento provocado por duas densidades arbóreas em uma floresta de Pinus elliottii Engelm. com 10 anos de idade sobre o rendimento e o valor nutritivo da forragem de três gramíneas de ciclo hibernal. Como tratamentos, avaliou-se a combinação de dois fatores (3 x 3) em um delineamento experimental de parcelas subdivididas com três repetições, no qual as parcelas foram as condições luminosas (proporcionadas por duas densidades arbóreas: 555 e 333 árvores/ha e luz solar plena) e as subparcelas as espécies forrageiras azevém-anual (Lolium multiflorum Lam.); aveia-preta (Avena strigosa Schreb.); e aveia-branca (A. sativa L.) cv. Fapa 2. A semeadura foi realizada entre 25/7/2005 e 5/8/2005 e entre 26 e 27/4/2006. O rendimento de matéria seca foi estimado em avaliações durante o estádio vegetativo (aos 104 dias após a semeadura em 2006) e em pleno florescimento (aos 132 e 170 dias, em 2005 e 2006, respectivamente). O valor nutritivo da forragem foi avaliado considerando os teores médios de proteína bruta (PB) e a digestibilidade in vitro da matéria orgânica (DIVMO). O sombreamento moderado reduziu em 57% o rendimento médio de forragem dos três genótipos avaliados, mas aumentou em 2,3% o teor de proteína bruta (PB) e em 5,5% a digestibilidade in vitro (DIVMO) quando as plantas estavam em florescimento pleno. Entre as espécies forrageiras avaliadas, a aveia-branca e a aveia-preta apresentam maior potencial para utilização em sistemas silvipastoris na Região Sul.

Plant and animal production from cocksfoot pasture and lucerne under 10-11 year-old radiata pine trees (200 stems/ha) and from adjacent open pastures (without trees) were measured from the Lincoln University Silvopastoral Experiment. Light intensity under trees was 50-60% of the open pasture. Liveweight gain (LWG) from lucerne was 220 g/hd/d in the open and 158 g/hd/d under trees. On cocksfoot, LWG was 132 g/hd/d in the open and 100 g/hd/d under trees. When converted to LWG/ha, lamb production from open pastures was double that from shaded pastures for lucerne (5.1 and 2.5 kg/ha/d) and cocksfoot (3.4 and 1.7 kg/ha/ d). Lucerne produced 11.2 t DM/ha in the open and 7.9 t DM/ha under trees which was 58% and 76% more than cocksfoot in the open and under trees, respectively. However, from September to November, when soil moisture was non-limiting, the lucerne DM production was 36% lower under tree shade than in the open. In comparison, the reduction was only 20% for cocksfoot, which confirms its greater 'shade tolerance'. The reduced lamb LWG produced from lambs on shaded pastures was attributed to the reduced pre-grazing pasture mass and pasture bulk density, leading to reduced apparent intakes. Crude protein and digestibility values were not influenced by shade, and clover content in cocksfoot pastures were low (