Could warming temperatures and elevated atmospheric CO₂ levels affect the quantity and quality of forage used to raise livestock? That’s the question that David Augustine and his fellow researchers at the Agricultural Research Service’s Rangeland Resources and Systems Research Unit wanted to answer. Their results are a mixture of “good news” and “bad news” along with some recommendations for how graziers might adapt to remain profitable into the future.

The Study

The Prairie Heating and Carbon Dioxide Enrichment (PHACE) experiment, began in 2006 as a look at how future environmental conditions will influence the mixed-grass prairie. This is the largest remaining grassland ecosystem in North America, and is critical for both agricultural productivity and conservation of biological diversity in the western United States.

One of the most well-documented changes in the atmosphere over the past hundred years is the rise in CO₂ levels, which have currently reached 410 ppm. Because global CO₂ levels are predicted to reach 600 ppm in the next 30 to 70 years, Augustine and his colleagues set up their study to simulate that level of atmospheric CO₂ , and the warmer temperatures associated with it (an increase of 1.5° C during the day (2.7° F) and 3° C at night).

While this kind of study can be done in special chambers that elevate CO₂ and temperature, there may be differences from plants growing in natural settings. So the research followed a protocol that allows natural areas to be warmed and treated with elevated concentrations of CO₂ in the field. Doing something like this is no small thing as these photos from the study in progress show. It includes digging trenches and installing flanges to isolate the plots, sensors to continually monitor water, temperature, and CO₂ levels, and tiny cameras to monitor root growth. Then to ensure that vegetation isn’t disturbed, staff did all their monitoring and forage collection while laying or sitting on planks and platforms above the plots.

The Good News – More Forage and More Efficient Water Use

The good news is that, in general, as CO₂ levels in the atmosphere increase, so does plant growth. In fact, previous experiments have shown  CO₂ concentrations of 475–600 ppm increases leaf photosynthetic rates by an average of 40% (Ainsworth & Rogers 2007).

In this case, forage production increased by 38% overall, though the species composition of the forages changed. The three dominant grasses (western wheatgrass, needle and thread grass, and blue grama grass) declined from 83.3% of forage production in 2007 to 67.5% in 2013. Three other forage species filled in the gap (needleleaf sedge, prairie Junegrass, and scarlet globemallow), increasing from 15.2% of forage production in 2007 to 24.8% in 2013.

Plants can also become more efficient water users with elevated CO₂ levels. The reason is structural. Plants open their stomata to allow CO₂ to come in. But open stomata are also a pathway for water to leave the plant. With elevated CO₂ levels, plants can decrease stomatal opening, reducing how much water exits. This in turn can have consequences for the hydrological cycle of entire ecosystems, with soil moisture levels and runoff both increasing under elevated CO₂ (Leakey et al. 2009). The degree of this change is not yet known.

(To complicate this, photosynthesis occurs two different ways, and to differentiate, plants are labeled as either C₃ or C₄. C₃ plants rate of photosynthesis increases more with elevated CO₂. We can cover those details another time.)

The Bad News – Lower Nutritional Quality and Decreased Animal Weight Gains

Augustine and his team looked at three different measurement of forage quality:

• in vitro dry matter digestibility (IVDMD)
• cellulose and lignin content (acid detergent fiber or ADF)
• Nitrogen content as a measure of protein

What they learned was that, “Although forage production was enhanced…this came with a substantial reduction in forage quality, which would negatively affect ruminant weight gains.” (Augustine et al 2018) The combination of warming and elevated CO₂ reduced the digestibility of C₃ grasses from 62.8% to 61.5% and C₄ grasses from 63.3% to 61.2%.

While those changes seem small, it reflects an increase in fiber content, and a decrease in crude protein at a time when young animals are supposed to be gaining weight rapidly. Based on this change, weight gains would be about 12.5% less – or 30 pounds. Loss of nutritional value might also affect cow performance by increasing the time to mating age or reducing conception rates. Finally, rumen microbes require protein to function properly, and the degraded forage is less than what they need to survive, meaning producers may have to provide supplements.

How Do Ranchers of the Future Adapt?

There are a wide variety of solutions described by researchers. One option is to use grazing to improve the forage. “Grazed grasses regrow leaves with increased protein and reduced cellulose and lignin concentrations compared to ungrazed grasses in rangelands worldwide…a clear solution is for producers in the North American Great Plains, where plants are well adapted to ungulate grazing, is to adaptively increase stocking rates to match forage availability. Increased stocking with increase forage production would maintain the grazing pressure index…thereby either maintaining or increasing total mass gain per unit area.” (They note that in areas that do not have the same grazing history, this practice could be detrimental.)

Other options include 1) adjusting the grazing season to account for the change in nutritional value and to graze weeds and invasive plants, 2) feeding cattle nutritional supplements, 3) seeding pastures with legumes to add nitrogen to the soil which in turn improves nutritional value of the plants, 4) applying low levels of fertilizer to accomplish the same thing, 5) stimulating new grass growth with prescribed burning and 6) adjusting stocking rates in response to changing weather and climatic conditions (e.g., wet periods and drought) that affect grass production.

The researchers remain optimistic. “There is certainly an important issue to be addressed regarding forage quality, but there also are a number of potential solutions,” Augustine said.

References

Thanks to Dennis O’Brien, ARS, for sharing information included in this article.

The article was also informed by these two websites:
Integrate – Interdisciplinary Teaching about Earth for a Sustainable Future
The Nature Education Knowledge Project – the citations in the body of this text came from this site and are shown below:

Ainsworth, E. A. & Rogers, A. The response of photosynthesis and stomatal conductance to rising (CO₂): mechanisms and environmental interactions. Plant, Cell and Environment 30, 258-270 (2007).

Leakey, A. D. B., Ainsworth, E. A. et al. Elevated CO₂ effects on plant carbon, nitrogen, and water relations; six important lessons from FACE. Journal of Experimental Botany 60, 2859-2876 (2009).

Thanks also to the Agricultural Research Service, the U.S. Department of Agriculture’s chief scientific in-house research agency. ARS focuses on solutions to agricultural problems affecting America. Each dollar invested in agricultural research results in $20 of economic impact.

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