Factors that can reduce the nutritional quality of forage include elevated temperatures, reduced precipitation, rising atmospheric carbon-dioxide concentrations and sustained nutrient export. Joseph Craine of Jonah Ventures and colleagues examined more than 36,000 measurements from dung of the diet quality of US cattle taken over a 22 year period. They discovered that grazing livestock have become increasingly stressed for protein over the past two decades.

"Right now, we cannot identify with enough certainty the mechanisms causing the decline in protein, although elevated carbon-dioxide levels in the atmosphere seem to be the most likely factor," Craine told environmentalresearchweb. "At the same time, more work needs to be done to assess the contribution that removal of nutrients from animal harvests might have on the quality of forage."

According to Craine, without replacement, soil nitrogen availability is depleted through the export of animal biomass from grasslands, reducing plant productivity.

"Ranchers need to embrace new technologies to help monitor the diet and weight gain of cattle in the field," he said. "Then they can target practices that increase the protein concentrations in plants, such as promoting nitrogen-fixing legumes in their grasslands."

Unravelling the detail can be time-consuming, but the group's work is building a more accurate picture of changes in the nutrient profile of US grasslands and the impact on grazing livestock.

Drought might sound like bad news for plant protein. However, Craine and colleagues Andrew Elmore of the University of Maryland and Jay Angerer of Texas A&M Agrilife Research showed that the general trends in forage quality over time are independent of periods of drier weather. One explanation is that drought might first limit the production of indigestible plant fractions such as lignin, which helps to preserve protein availability. Another hypothesis is that drought encourages livestock to eat higher-quality plant species.

Enriching grassland regions could deliver results in several ways. The ability of cattle to gain weight is linked to the crude protein concentration of their diet, which depends on the tissue nitrogen concentrations in the plants they eat. Another consideration is the metabolizable energy derived from digestible organic matter.

Craine and colleagues reported their findings in Environmental Research Letters (ERL).

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