It’s getting hotter in here, and that’s bad for forage quality and cattle nutrition.

Elevated levels of carbon dioxide (CO2) and increasing temperatures are expected to affect the productivity of herbivores in rangeland ecosystems. Both of these factors can negatively affect plant health, which raises the question: how will we maintain cattle efficiency with decreased forage quality?

A group of USDA Agricultural Research Service (ARS) scientists conducted a seven-year study in Wyoming, examining the effects of simulated CO2 levels and warmth on mixed-grass prairie, and ways to potentially mitigate effects. The study was released in January 2018 in the journal Ecological Applications, a publication by the Ecological Society of America.

Effects on rangeland

The earth’s land consists of approximately 30 percent rangeland and is used for the majority of the world’s livestock production. The combination of direct effects from CO2 and warming temperatures are expected to be key drivers of change in semiarid ecosystems.

Elevated CO2 levels have been shown to have positive effects on forage quantity, but the ARS study also analyzed the effects on quality, which have a direct effect on animal digestibility and weight gain. Earlier studies have shown increased levels of CO2 reduce forage protein content and digestibility, leading to decreased forage quality.

Few studies have studied the interaction of rising temperatures and CO2levels, but available evidence suggests declining forage quality could be an important factor for livestock production under future climate change.

Three dominant mixed-grass prairie grasses comprise most of the growing-season diet in the Wyoming region, and the researchers evaluated the forage quality of the three grasses from weather conditions in the study period. The grasses were measured in terms of in vitro dry matter digestibility (IVDMD), cellulose and lignin content, and nitrogen content.

Measuring forage quality

The National Academies of Science, Engineering and Medicine model for beef cattle nutrition was utilized to evaluate potential effects of reduced-quality forage on growing cattle.

Changes in IVDMD or cellulose and lignin content from an interaction between warming and CO2 were not found to be significant. Nitrogen fluctuations for two of the grass species were also insignificant, but one species was found to have reduced levels of nitrogen from CO2 levels.

IVDMD positively correlates with nitrogen levels and negatively correlates with cellulose and lignin content, and both factors play a role in level of digestibility.

Forage impacts

Effects of CO2 and warming on forage production varied by year, but together significantly increased forage production by 38 percent under the simulated environment of future climate. Even though forage production was increased by the effects of CO2 and warming, forage quality and digestibility was substantially reduced, which directly affects weight gains at a time that is critical for rapid animal growth.

Warming alone is not predicted to reduce animal performance, but with the combination of increased CO2, is expected to reduce forage quality and reduce cattle weight gain by 12.5 percent. Reduced gains could also have an effect on future performance through reduced conception rates.

Mitigating impacts

Understanding the impacts of future climate change will influence livestock and forage production and is critical to long-term sustainability in the agricultural industries, according to the study.

Livestock managers already react to year-to-year weather variation, but most strategies are based on past experience and do not incorporate future long-term shifts that could result from climate change.

CO2 levels have already increased since the 1960s and increasing levels in the next 30-70 years are projected as the most certain aspect of climate change, the study said.

Rising CO2 levels, temperatures reduce forage quality

USDA Forest Service Thunder Basin National Grassland, in Wyoming, on July 29, 2019.  Elevation on the national grassland ranges from 3,600 to 5,200 feet, and the climate is semi-arid. The national grassland abounds with wildlife year-round, provides forage for livestock, and is underlain with vast mineral resources. 

With forage quality almost certainly going to be affected in the future, what can livestock managers do to reduce or offset weight gain losses from lesser-quality forage?

Grazed grasses regrow leaves with increased protein and reduced cellulose and lignin levels compared to ungrazed grasses, according to the study, so a clear solution is to increase hooved animal stocking rates.

Increasing the stocking rate to match the increasing forage quantity would maintain the grazing efficiency of current levels. However, due to weather uncertainty and future climate changes, caution is needed when adjusting stocking rates.

Other management strategies include increasing protein supplementation, which will also increase cost and labor. Fertilizing rangeland with nitrogen at rates low enough to avoid invasion of undesirable plants and interseeding legumes, which are favored under CO2, may also help mitigate the effects of decreased forage quality.

Adjusting the seasonality of grazing and strategic prescribed burning can also assist with livestock grazing distribution and forage quality.

One thing is for certain: tactical monitoring of forage quantity and quality is going to become increasingly important in livestock production in order to adapt to a changing future. — Anna Miller, WLJ editor

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