Grass height in cow nutrition: size does matter

Grass height in cow nutrition: size does matter

Unlike humans, cows have the ability to convert grass into foods of high nutritional value such as meat and milk. This is because one of their stomachs, rumen, is responsible for the fermentation process that converts pasture into milk precursors. However, other by-products such as methane (CH4) gas is also generated, emitted into the atmosphere primarily through the cows’ belching, and contributes to climate change.

This is one of the reasons for which livestock is currently at the center of increasingly heated debate and criticism. Thus the question remains: is it possible to produce those foods, which are also necessary to meet the nutritional needs of an increasing population, and still be environment friendly?

The answer is yes, it is indeed possible. In the last 20 years, ecologists, zootechnicians and agronomists have joined forces to better understand the relationships between plants, animals and the environment. The understanding of grazing, a process that originates from an act that seems simple but is not, that of the cow catching a bite of grass, has important implications in animal daily productivity and consequently in greenhouse gas (GHG) emissions.

In one day, a cow can eat between 25,000 and 30,000 morsels of grass. What do the differences in these amounts depend on? They will depend on how accessible the grass is to them. For instance, the height of the grass could make a difference. Accordingly, a doctoral thesis conducted by Alejandra Marín, from the Universidad Nacional de Colombia and with support of the CIAT Tropical Forages team, looks at whether there is an optimal height of grass that would allow cows to consume better quality grass in less time while producing less CH4.

To do this analysis, they evaluated five different heights of Kikuyo grass (Cenchrus clandestinus, Hochst. Ex Chiov), which is commonly used pasture in Colombia’s dairy production. They looked at heights of 10, 15, 20, 25, and 30 centimeters by observing dairy cows that grazed in short grazing sessions repeated at different times of the day within a set time period.

They measured how quickly the cows consumed grass at those different heights, the quality of the grass at those different heights, and modeled the production of CH4 and propionate (also called propionic acid, one of the precursors of milk production).

The in vitro gas production technique is a lab technique that recreates the ruminal fermentation process or bovines. It is a solid tool to estimate the methane output and other end products of (bovine in vitro) ruminal fermentation for a large range of forages and diets.

Herbage sampling of the top stratum of kikuyu grass. Kikuyu grass (Cenchrus clandestinus - Hochst. ex Chiov) is a highly productive pasture species that is well adapted to the forage-based dairy systems and widely used in some countries of Latin America, Australia, and Africa.
Herbage sampling of the top stratum of kikuyu grass, a highly productive pasture species that is well adapted to the forage-based dairy systems and widely used in Latin America, Australia, and Africa.
Gas measurement using a pressure transducer connected to digital wide-range manometer and a 60 mL syringe through three-way valve.

Laboratory facilities

In vitro incubations were conducted at the Forage Quality and Animal Nutrition Laboratory (certified by the FAO-IAG proficiency test of feed constituents 2017 including in vitro gas production) and Greenhouse Gas Laboratory facilities at the International Center for Tropical Agriculture (CIAT) located in the Valle del Cauca department, Colombia.

What did they measure?

The gas production (GP) and the in vitro dry matter digestibility (IVDMD), as well as the end products of in vitro ruminal fermentation like CH4 output, expressed per unit of dry matter digested (ml /g IVDMD) and VFA (the acetate, propionate, and butyrate proportions) were measured at 24 and 48 hours.

What were the results?

Gas production (GP) and in vitro dry matter digestibility (IVDMD) were high and positively correlated and the IVDMD was negative correlated with sward height (SH) of kikuyu grass. It is widely accepted that the higher GP, the higher the IVDMD. Although, variations in the chemical composition are closely related to the sward height It is worth highlighting the relationship between digestibility and SH.

In other words, we found that in 20 centimeters the cows consumed larger bites, and therefore of better quality in less time. That is to say, they were more efficient consuming grass comparatively with the cows they ate at the height of 10 and 30 centimeters. At 20 centimeters, also produced more propionate and less CH4 at that height compared to the height of 30 centimeters. These findings are important to aid decision making on the optimal sward height of the kikuyu grass for being grazing by animals in order to reduce in vitro CH4 production.

The research outcomes are gaining attention

Final round of the 3MT Contest, who Alejandra won the first place. Source: YouTube.

On December 2, 2019, Alejandra presented her thesis at the 3MT National Contest, a contest designed to present the doctoral thesis in three minutes. This competition celebrates the findings and results obtained by students and research graduates, and promotes their skills to communicate the importance of their research to a non-specialized public. In such a short time, Alejandra presented to the jury her research and was selected as the winner of the contest. Read the story on the University’s website (In Spanish).

Preliminary results also were published as an Info Note in the context of the Project LivestockPlus, of the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), both in English and Spanish. Also, Alejandra defended her Ph.D. thesis on December 20 of 2019 at the National University of Colombia in Medellín, and the study will be submitted for peer review to a scientific journal.

This study was undertaken as part of the LivestockPlus project and CLIFF program funded by CGIAR Research Program (CRP) on Climate Change, Agriculture and Food Security (CCAFS), which is a strategic partnership of CGIAR and Future Earth. In addition, this work was also done as part of the Livestock CRP.