Lighter Fare for Cattle and Microbes

by Chris Hubbuch (Great Lakes Bioenergy Research Center) Rebecca Smith uses genetic tools to develop plants that can be digested more easily. The outcomes could be greater efficiency for biofuel production and lower methane emissions from dairy cows.

In the King Hall greenhouse on the UW–Madison campus, Rebecca Smith stands amid a potted jungle of sorghum, clipping stalk segments into a plastic bin to be weighed and cataloged. Seeds, bagged and labeled, are stacked to the side. Tissue samples have been flash-frozen in liquid nitrogen for chemical and genetic analysis.

The plants have an extra piece of DNA that increases their production of a chemical that weakens bonds in the cell walls. The walls are like bank vaults full of carbohydrates; the goal is to make it a little easier to break in and turn those sugars into energy.

With more than 50 plants, it’s tedious but necessary work to figure out which specimens perform best depending on where the extra gene landed and how active it is. “Best” in this case means the plant cell walls are easiest to break down after harvest, but the plants otherwise grow like native sorghum, a hardy crop that withstands arid conditions and produces a lot of organic material, Smith says.

An assistant professor in the Department of Plant and Agroecosystem Sciences at CALS, Smith investigates how forage and bioenergy crops develop at the cellular level. She uses her unique combination of skills to engineer new varieties to help solve some of humanity’s biggest challenges.

As a scientist with the Great Lakes Bioenergy Research Center (GLBRC), Smith spent the past decade researching ways to grow non-food plants as feedstocks for fuels and chemicals traditionally derived from petroleum. Now, as a new member of the Dairy Innovation Hub, she is working to grow crops that are easier for cows to eat. Both projects aim to make farming more sustainable, bolster rural economies, and ultimately reduce greenhouse gas emissions.

“The goal is to improve digestibility,” Smith says. “There are synergies in what makes a good plant for biofuel and for feed.”

The Lignin Problem

Underpinning Smith’s research is a basic problem: With the exception of fruits and grains, plants store chemical energy in the form of carbohydrates that are hard to access.

Plants evolved to grow and survive often hostile conditions — gravity, wind, and predators, to name a few — and create their own food by using sunlight to combine carbon dioxide from the air with water from the earth. A key part of that survival strategy is lignin, a chain of molecules (or polymer) bound to the sugars in the cell wall that helps ward off pests, repel water, and provide structural support.

Just as it protects plants in the field, lignin acts as a barrier to the energy stores — whether in a biorefinery or a cow’s gut. As a result, lignin is one of the biggest obstacles to producing cost effective advanced biofuels, which are made from plant fiber rather than corn kernels or soybeans. And it’s hard for cows to digest, which means they get less energy from their feed, and they burp more methane, a potent greenhouse gas.

The challenge is to grow plants with less lignin, or lignin that’s easier to remove, without affecting overall growth and development.

Bioenergy crops — including grasses and trees — pull carbon from the air and can be grown on land unsuitable for food production, so they could be renewable sources of transportation fuels and petroleum-derived products, such as plastics.

Microbes can ferment the plant sugars into alcohols and convert lignin components into valuable chemicals, but only after the biomass is broken down, which requires a lot of energy and chemicals that can inhibit fermentation.

Cattle and other ruminants break down feed with the help of microbes in the rumen, the first in a series of stomachs. The amount and structure of lignin in the plant determines how much time and energy it takes to free up the carbohydrates. And the longer it takes to digest, the more methane a cow produces.

To confront these challenges, Smith uses multiple methods, including gene editing tools such as CRISPR, to identify the genes involved in lignin formation and dial them up or down to modify the lignin content and structure. In the case of biofuels, that translates to more efficient refining and, ultimately, products that can compete with fossil fuels on price.

“We’re making it easier to get to the products,” Smith says. “The less chemicals and energy we have to put into breaking apart the plant, the easier it is to bring the cost down.”

For cattle, it means better health and fewer burps, which Smith says could make the dairy industry “a little more sustainable.

Smith is also part of a precision agriculture project that integrates plant genetics, animal nutrition, microbiome physiology, and artificial intelligence. The project’s goals are to develop plants that improve feed efficiency, reduce methane emissions from dairy cows, and require less fertilizer. READ MORE