A feeding trial at the Tully Progeny Test Centre in Co. Kildare, has been exploring how diet influences methane emissions, feed efficiency, rumen microbiome, and growth performance in finishing beef cattle.

The research was undertaken as part of the Department of Agriculture, Food, and the Marine-funded project Methane Abatement in Grazing Systems (MAGS), of which the Irish Cattle Breeding Federation (ICBF) is a partner.

The study collected data on 43 continental suckler-bred bulls, selected from a range of artificial insemination sires and breeds preselected as high or low in methane output based on the ICBF genomic evaluation.

The aim was to quantify methane emissions under zero-grazed and total mixed ration (TMR) conditions indoors, and to understand how diet may affect rumen fermentation and the microbial populations responsible for methane production.

Trial period and performance

Methane output, feed intake and rumen samples were recorded throughout the trial. Rumen samples were taken during both phases to examine the rumen microbiome.

Rumen sampling involves using an oral stomach tube to access the rumen through the oesophagus, allowing the collection of rumen fluid for analysis of the proportions of different microbes present in the sample.

The trial took place throughout the second half of 2025 and Agriland visited the centre in October for an update on the trial.

Phase 1 of trial

The first phase involved a zero-grazed grass diet offered indoors.

Bulls spent 70 days on this diet and consumed, on average, 7.95kg dry matter intake (DMI) per day.

DMI was measured using Insentec feed boxes, which automatically calculate the daily feed intake of all cattle.

Average daily liveweight gain (DLWG) during this period was 1kg/day.

Second phase

In the second phase, the bulls were transitioned to a TMR finishing diet.

All cattle were offered a ration of:

10kg of meal;
9kg of water;
3kg of hay.

The bulls consumed, on average, approximately 13.67kg DMI per day.

Animals were again rumen-sampled to assess microbial changes following the dietary shift.

Growth rate increased compared to the zero-grazed grass phase, with bulls averaging 2.05kg DLWG on the TMR diet.

Animals were slaughtered in early December, with an average carcass weight of 390kg.

At the beginning of the trial, animals averaged 498kg liveweight, increasing to an average of 638kg by the end of the trial. Cattle averaged 530 days-of-age at slaughter.

Methane output and efficiency

Average methane (CH₄) production on the grass diet was 270g/head/day, while average methane production on the TMR diet was 308g/head/day.

However, when compared to performance, the TMR diet substantially reduced enteric methane emissions/kg of liveweight gain.

The bulls had a feed conversion ratio of 7.95kg DMI/kg DLWG on the zero-grazed diet compared to the 6.67kg DMI/kg DLWG on a TMR diet.

According to the ICBF: "This trial highlights the combined importance of higher growth rates and improved feed efficiency in reducing methane output per kg product in beef production systems."

Further analysis is ongoing to examine how the rumen microbiome changes in response to diet, genetics, and management using the data already collected.

The next phase of the project in Tully will take place this spring, where the trial will be repeated with a new cohort of beef steers and heifers.