Take home message: Fresh hops reduced in vitro methane production when included as an antimicrobial feed additive in TMR at 5 and 10%. This provides evidence for future studies to determine the use of fresh hops in vivo. However, spent hops increased in vitro methane production and further anti-methanogenic experimentation is recommended since recycling spent hops into livestock feed would have a lower environmental impact than fresh hops.

In working towards achieving net zero carbon emissions by 2050, the livestock industry is exploring strategies to reduce enteric methane production, the leading contributor to greenhouse gas emissions from livestock farming. Alongside this, there is a greater expectation from livestock producers to reduce their antibiotic usage to address the growing concern regarding antibiotic-resistant bacteria in humans. This has driven investigations into alternatives for livestock of which novel nutritional approaches have identified antimicrobial properties in hops. However, the results are conflicting when plant materials have been used in total mixed rations (TMR) to mitigate enteric methane.

Hops are the flowers of the hop plant Humulus lupulus, which are used in the brewing industry to add bitterness, aroma, and flavour to beer. Spent hops are produced as waste after brewing and can be used in livestock feed. Hops are of interest because their extracts can inhibit microbial fermentation from methanogens in the rumen and have antimicrobial characteristics due to the production of secondary metabolites including α-acids, β-acids and xanthohumol. These metabolites have shown activity against various gram-positive and gram-negative bacteria by interfering with the bacterial cell wall membrane. However, there is limited evidence on the effectiveness of fresh and spent hops to reduce in vitro methane production which is needed before its use in vivo.

This study used chinook hops to determine the effectiveness of fresh hops and spent hops to reduce in vitro methane production. Four treatments were assigned where fresh hops and spent hops were included in TMR at 5% and 10%, and these were compared against a control treatment of TMR only. Rumen fluid was added to each treatment and the volume of methane produced was recorded using headspace gas chromatography every two hours between two and eight hours of incubation. At each recording 10 mL of gas was injected into a gas chromatograph which separated gas components by extracting volatile material from a heavier sample matrix. Methane was identified using a flame ionisation detector that measured the gas sample's total hydrocarbon concentration. Methane concentration (ppm) was then quantified using a standard curve and total gas volume produced was accounted for in the calculation of total methane volume.

The results of this study found that in vitro total gas volume did not differ between treatments after eight hours of incubation, indicating that hop inclusion did not alter the digestibility of the TMR. At both TMR inclusion rates, fresh hops effectively reduced in vitro methane production after eight hours of incubation, although to a greater extent when provided at 10%. However, further in vitro research is required to determine a precise TMR inclusion rate and this work needs to be repeated in vivo before fresh hops can be implemented as a safe feed additive.

The inverse was found for spent hops as no effect was observed at 5% inclusion and methane production increased when included at 10%. Due to a lack of in vitro research using spent hops, and the conflicting result observed in this study, similar experiments are recommended to verify whether spent hops can effectively reduce enteric methane production. If successful, the benefits of recycling spent hops into livestock feed can support a circular economy that fresh hops otherwise cannot.

Lousie Rayment, University of Reading

This study was carried out by Louise Rayment in fulfilment of her BSc in Animal Science at the University of Reading. Louise is now starting an MSc in Animal Nutrition where she would like to continue her research in nutritional strategies for sustainable agriculture. Well done, Louise!

Lucy Ross, PhD student, University of Nottingham

Lucy Ross has been a member of the BSAS Early Careers Council for four years. She is in the final year of her PhD at Nottingham, researching heifer rearing and whole-farm feed efficiency in UK dairy systems.