Measuring Feed Efficiency for Cows on Pasture

Background

The number one cost in cow-calf production is feed, meaning that selecting feed efficient cows is important to reducing input costs. Similarly, goals to reduce the environmental impact of cattle by reducing methane emissions from forage-fed cattle are of industry interest. While residual feed intake (RFI) works well in growing animals, forage-fed cows may re-rank when measured over the production cycle, so this type of measurement may not be as effective at identifying cows’ long-term efficiency.

Since cows consume high-fibre, lower quality diets, diet digestibility may be underestimated in previous models developed on growing animals where highly digestible diets are fed. This may also have implications on enteric methane emissions, which also have not been evaluated over the long run. Methane and feed intake are closely related and may be another possible measurement of interest for forage-fed cattle relating to overall efficiency. Furthermore, individual cows may alter feed intake patterns to adapt to these low-quality diets and maintain nutrient absorption.

Objectives

• Identify traits in the cow-calf industry associated with improved feed efficiency and reduced enteric methane emissions in forage-fed animals 
• Determine relationships between traditional feed efficiency measures (e.g., DM intake, RFI), gas exchange (e.g., CH4, O2, CO2) and apparent total tract digestibility measures in beef cows
• See if these measures differ across various production phases and determine if rankings are consistent on variable diet types (i.e., digestible forage diet vs. indigestible forage diet)
• Develop a model for the prediction of DMI and methane data

What they did

A cohort of 70 Angus crossbred heifers were used across multiple trials. During the first measurement period, heifers were fed a forage-based TMR containing straw and rye-triticale haylage ab libitum for eight weeks in late gestation, then were re-evaluated for 28 days in mid-lactation. Animals were weighed throughout the study, and feed intake, feeding behaviour, RFI and performance were recorded. Enteric methane emissions were measured using a C-Lock Greenfeed Trailer, and manure samples were collected to estimate apparent total tract digestibility (aTTD).

Heifers measured during gestation were grouped by RFI, methane emission metrics and dry matter digestibility. In addition, correlations were determined between RFI and methane emissions across different production stages, with follow-up measurements of the same animals as two- and three-year-old cows. In the third experiment, cows were fed either a high- or low-NDF diet with measurements of weight, feed intake and behaviour, performance, methane emissions and manure samples collected for aTTD. A modeling approach was used to identify key traits relating to methane emissions, and a machine learning approach was used to determine if gas emissions and feed parameter information can be used to predict feed intake. This may be useful on pasture systems, where individual feed intake cannot be measured.

what they learned

Low-RFI (most efficient) heifers ate approximately 2.5 kg less (dry matter basis) and emitted about 25 grams per day less methane, with similar results observed when animals were classified by daily methane emissions. While lactating heifers did not differ in daily methane emissions, methane emissions relative to feed intake differed in lactating cows relative to heifer RFI classification.

Feed efficiency (RFI) measured during gestation was not correlated with RFI measures during lactation. However, intake differences between efficiency quartiles remained. Most measurements of apparent total tract digestibility were not related to feed efficiency, intake or methane emissions during late gestation or lactation.

During gestation, more efficient heifers had fewer bunk visits. When grouped by methane emissions, lower emitting heifers also spent less time at the feeder and had smaller feeding events. Feeding behaviour differences were not observed for RFI classification during lactation, although feeding events and meal size were lower for lower-emitting animals.

Modelling identified feed intake, RFI and intake as a percentage of body weight as the strongest predictors of methane emissions, followed by meal characteristics and body weight. Digestibility was not strongly correlated to efficiency. Methane emissions showed moderate repeatability from late gestation through lactation and in three-year-old cows but were less consistent from younger heifers to two-year-old cows.

what it means

Overall, this project demonstrated the close relationship between feed intake, feed efficiency and enteric methane emissions in forage-fed beef animals. This means activities that improve feed efficiency on pasture can also reduce greenhouse gas emissions, meaning what is good for the bottom line can also be good for the environment.

This study also showed methane production may be a heritable trait, so it could potentially be looked at as a way to select for more forage-efficient animals in the future.