In Brazil, the livestock systems are mainly forage‐based, and nitrogen inputs are an uncommon practice, despite its great benefits. The objective was to evaluate the effect of nitrogen input on the physiological responses of Marandu palisadegrass (Urochloa brizantha [Hochst. ex A. Rich.] R. D. Webster) and Mombaça guineagrass (Megathyrsus maximus [Jacq.] B. K. Simon & S. W. L. Jacobs), and its impacts on the forage accumulation rate (FAR). The experiment was carried out in Sinop, MT, Brazil, in a randomized block design with a 2 × 2 × 5 factorial arrangement (two cultivars: Mombaça and Marandu; unfertilized and fertilized [50 kg N ha−1 cycle−1]; and five seasons: autumn 2015, winter 2015, spring 2015, summer 2015 and autumn 2016) with three replicates. The experimental period was from March 2015 to June 2016, when plots were harvested to mimic intermittent stocking. Fertilized Mombaça presented greater FAR pastures (74.0 kg DM ha−1 day−1) than unfertilized (26.6 kg DM ha−1 day−1) and Marandu fertilized (52.5 kg DM ha−1 day−1), with the greatest values during the summer (147.6 kg DM ha−1 day−1). The greatest leaf (A) and canopy photosynthesis (CP) rates occurred on fertilized pastures during the summer 2015 and autumn 2015. Marandu presented 19% greater A than Mombaça. During the winter, under water stress, Marandu had greater water use efficiency (WUE). However, during the summer 2015 and autumn 16, Mombaça had the greatest WUE (12.76 μmol CO2/mol H2O). In all seasons, Marandu presented the greatest chlorophyll index, which may support the greatest photosynthetic rates. It was concluded that Mombaça was highly responsive to nitrogen input during the rainy season with accumulation over 140 kg DM ha−1 day−1 and is recommended to intensified systems. Marandu had the greatest WUE in the lower precipitation seasons and is an alternative to a year‐round strategy in forage‐based systems.

To Christ, Lord of salvation, to my parents Maurinho and Maria, to my sisters Maraisa (Tata) and Marilia (Lila) and my brother-in-law Jurandi (Jura). I dedicate this work to you all. I love you! I also dedicate this work to all Brazilian graduate students. I know some of the challenges you faced or are going through. ACKNOWLEDGMENTSFirst of all, I would like to thank God for the gift of life, for empowering me and for making all things have cooperated to bring me here. I would like to thank my family for all love, caring and financial support. Mom, Dad, sisters and brother-in-law, thanks for rooting for me, for celebrating my achievements and for consoling me in my defeats. For believing and investing so much in my career, in my English classes, in the improvement of my physical and mental health, that helped me follow my dreams. I know we have not been physically close for a long time, but without your support I certainly would not be here now. Thank you! To my advisor: I thank my advisor and EMBRAPA Pecuária Sudeste researcher, Prof. Patricia M. Santos, for the unique and exceptional training throughout the course. I am grateful for the job opportunity, for having had her as my advisor, teacher and in several moments as a counselor, in addition to being an inspiration to me. I thank for freedom and trust accompanied by discipline, for the advice inside and outside the academic world, which taught me to think, breathe and react differently in the face of the adversities encountered during all these years. I am grateful for all the opportunities offered, for the financial support and encouragement to do my sandwich doctorate program at University of Florida (UFL) and for always instigating me to get out of my comfort zone. I will never forget that! To the Professors: Thanks to all Professors from PPGCAP-ESALQ, especially to Prof. Sila C. da Silva and Prof. Carlos G.S. Pedreira for the opportunity to carry out the teaching internship in the discipline "Forage plants and pastures" and for the lessons learned. I thank Prof. Carla M.M. Bittar, current PPGCAP coordinator, who welcomed me and my colleagues always willing to help us, in addition to the teachings on "Techniques of Scientific Communication in Animal Science and Pastures II". To Professors of the other graduate programs from ESALQ, especially Prof. Fábio R. Marin for the opportunity to conduct a teaching internship in the discipline of "Agricultural Meteorology", for the lessons I have learned and for providing me physical space within his research group, besides the partnership with the thematic project in which this dissertation is inserted. Thanks to Prof. Fernando C. Mendonça and Prof. Miguel Cooper for the consultations and doubts

O objetivo com este estudo foi compreender o efeito da disponibilidade hídrica sobre a produtividade no capim-mombaça [Megathyrsus maximus (Jacq.) B. K. Simon & S. W. L. Jacobs] e capim-marandu [Brachiaria brizantha (Hochst. ex A. Rich.) R. D. Webster], determinando o coeficiente de resposta ao déficit hídrico (ky). O experimento foi avaliado em delineamento de blocos ao acaso, com duas forragens e três repetições. O período experimental correspondeu a doze ciclos de crescimento com intervalos fixos de colheita (28 dias para a estação chuvosa e 42 dias para a estação seca). Os ky foram determinados com base nos dados de produção real e potencial das forrageiras, bem como nos dados de evapotranspiração real e máxima. Para ambas as forragens, o acúmulo de forragem (AF) foi maior no outono e menor no inverno (5310 e 626 kg de matéria seca ha-1, respectivamente). Na média das quatro estações, o AF para capim-marandu foi 21,3% maior que o capim-mombaça (3344 vs. 2756 kg matéria seca ha-1, respectivamente). Para o capim-mombaça, o ky foi de 1,05, indicando sensibilidade média a alta ao déficit hídrico. No entanto, o ky foi de 0,63 para o capim-marandu, o que representa baixa sensibilidade ao déficit de água do solo.Palavras-chave: Brachiaria; coeficiente de sensibilidade; Megathyrsus; tolerância a seca. YIELD RESPONSE FACTOR OF MOMBAÇA GUINEA GRASS AND MARANDU PALISADE GRASS TO WATER DEFICIT ABSTRACT: The objective with this research was to understand the effect of water availability on Mombaça guinea grass [Megathyrsus maximus (Jacq.) B. K. Simon & S. W. L. Jacobs] and Marandu palisade grass [Brachiaria brizantha (Hochst. ex A. Rich.) R. D. Webster] productivity, determining the yield response factor to water deficit (ky). The experiment was evaluated in a randomized block design with two forages and three replicates. The experimental period corresponded to twelve growth cycles with fixed harvest intervals (28 days for the rainy season and 42 days for the dry season). The ky were determinate based on the data of real and potential yield of grasses, as well as the data of real and maximum evapotranspiration. To both grasses, herbage accumulation (HA) was greatest in the autumn and leats in the winter (5310 and 626 kg dry matter ha-1, respectively). On average of the four seasons, the HA for Marandu palisade grass was 21.3% higher than Mombaça guinea grass (3344 vs 2756 kg dry matter ha-1, respectively). For Mombaça guinea grass, ky was 1.05, indicating medium to high sensitivity to water deficit. However, the ky was 0.63 for Marandu palisade grass, which represents low sensitivity to soil water deficit.Keywords: Brachiaria; drought tolerance; Megathyrsus; sensitivity coefficient.

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