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Pasture quality may worsen because of climate change

10 September 2020

The increase in average temperatures expected for the next few decades, of at least 2ยบ C, may have an unexpected impact on the pocket of ranchers.

New studies suggest that one of the effects of climate change will be the reduction in the quality of the pasture, which will become less protein, more fibrous and, therefore, of longer digestion.

As a consequence, the researchers said, cattle will need to consume more food to reach slaughter weight and will start to produce more methane, a potent greenhouse gas.

The conclusions are based on experiments carried out by the team of Carlos Alberto Martinez y Huaman, professor at the Department of Biology at the Faculty of Philosophy, Sciences and Letters of Ribeirão Preto (FFCLRP) at USP. Researchers from the São Paulo Institute of Botany, the University of Illinois, in the United States, the Universidade Estadual Paulista (Unesp) in Jaboticabal and the Federal Institute of Goiás, Rio Verde campus, participated in the study.

"We seek to understand how forage pastures will respond physiologically and productively to future climate conditions, which involve an increase in the average temperature and in the concentration of carbon dioxide (CO2), in addition to reducing the availability of water," Martinez told Agência Fapesp.

The main cultivated plant species are classified in C3 and C4, nomenclature related to the path used by the plant to fix carbon in photosynthesis. Soy and beans, for example, use the C3 route. Tropical grasses, such as sugar cane, corn and forage, developed a complementary system to C3 called the C4 pathway.

In an attempt to accurately determine the physiological changes that forages are expected to undergo in the future, Martinez avoided carrying out experiments in greenhouses - places considered limited to carry out the necessary simulations.

As the researcher explained, greenhouse plants are grown in pots and thus have limited root growth. Consequently, they grow less than in the open. Other variables that are impossible to reproduce in the greenhouse are the intensity and variation of light and temperature, caused by the action of the wind on the leaves, in addition to the depth of the soil, into which the roots can penetrate in search of water.

“For some experiments, the vessel model is valid, but for future climate simulations, field experiments are also necessary. We were able to heat the plants outdoors with infrared heaters. In addition, we enriched the air with CO2 in an open environment, thanks to an infrastructure called Trop-T-FACE, installed in the field with support from the Fapesp Global Climate Change Research Program ”, said Martinez.

The experiments were carried out in an open field, where the plants are subjected to normal conditions of temperature, light, wind and humidity and the soil is deep, with the roots being able to extend in search of water.

The species used was mombaça grass ( Panicum maximum ), a tropical forage of African origin that performs photosynthesis through the C4 pathway. Widely used in Brazil as a pasture, due to its high nutritional quality, mombaça grass is common in São Paulo and other states.

“We placed infrared heaters in 16 beds, heating the plants 2ºC above room temperature. The equipment is capable of detecting the ambient temperature every 15 seconds, adjusting the values ??according to the need ”, said Eduardo Habermann, Fapesp grantee and first author of the works published in the magazines Physiologia Plantarum and Plos One .

“The experiment was carried out in November 2016, a period of great heat. The ambient temperature was 38º C and, in the beds, it reached 40º C ”, said Habermann.

Throughout the experiment, the researchers measured the conditions of gas exchange of plants with the atmosphere, the conditions of photosynthesis, the fluorescence of chlorophyll, the production of foliage (biomass) and the nutritional quality of the pasture.

“We saw that, in drought conditions, plants try to conserve soil water. The control is done by the stomata, small structures present in the leaves, which open up to absorb CO2. But in doing so, they lose water. With little water in the soil, the root resents. The plant closes the stomata and transpires less. The effect of saving water is the reduction of photosynthesis, with the consequent deterioration in the quality of the plant ”, said Habermann.

In addition to the support of Fapesp, the work also received funding from the National Council for Scientific and Technological Development (CNPq) and the National Water Agency (ANA).

More fibrous leaves


Other responses of mombaça grass to water stress, detected by the study, were the increase in the amount of leaf fiber and the reduction in the crude protein content - factors that represent loss of nutritional quality.

The researchers estimate that, in future temperature conditions, the increase in the amount of fiber will result in more difficult and time-consuming digestion for livestock. The direct consequence will be the production of more methane by the animals.

“Cattle will need to consume more pasture until they reach slaughter weight. In order to maintain the same level of production, ranchers will need to supplement the breeding stock and water the pastures, with a significant impact on production costs, ”said Martinez.

Another alternative, not always possible, is the expansion of pasture areas, which may favor deforestation or cause the producer to give up other crops.

The team also carried out experiments with C3 plants, such as the styling legume campo grande (a mixture of the species Stylosanthes capitata and Stylosanthes macrocephala ), a forage rich in protein and which performs the function of capturing nitrogen from the atmosphere and fixing it biologically in the soil, reducing investments in agricultural inputs, contributing to the reduction of environmental impacts and enabling greater weight gain for animals.

“The climate change experiments carried out with the C3 legume gave the same result. Nutritional quality is reduced, ”said Martinez.

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