Feeding willow to ruminants could reduce greenhouse gas emissions

Trees provide shelter and shade for livestock, and some offer additional forage.  In 2019, we were one of three research sites which contributed to a study of the potential of goat willow, oak and alder leaves as a source of supplementary minerals.  Willow was consistently higher in zinc and cobalt which is often deficient in grass and is important for the synthesis of vitamin B12. If you have not seen it, you can find the summary here.

In 2019, we also carried out an experiment with Nottingham University School of Veterinary Science in which willow leaves were fed to weaned lambs to determine whether the higher cobalt in leaves was reflected in higher concentrations in the animals.  Blood samples taken before and after feeding willow for a two-week period confirmed that blood cobalt concentrations and vitamin B12 were significantly higher in willow-fed lambs.

Lambs feeding on willow leaves at Loddington

Condensed tannins in willow leaves have the potential to supress microbial activity in the rumen, reducing uptake of nitrogen into the blood, and ultimately into urine.  This has the potential to reduce emissions of nitrogenous gases, primarily nitrous oxide and ammonia from urine patches.  Inhibition of microbial activity in the soil could have the same effect.  As nitrous oxide is a major greenhouse gas, and ammonia has negative air quality implications, the use of willow to reduce these gaseous emissions from urine could potentially contribute to climate change and human health targets.

In August 2020, we fed 200g of goat willow leaves per day to two groups of six weaned lambs over a two-week period.  Another two groups of six lambs were not fed willow.   At the end of the experiment, we identified fresh urine patches by direct observation of the lambs (six willow-fed, and six not willow-fed) and used our Gasmet gas analyser to measure emissions of carbon dioxide, as well as nitrous oxide and ammonia.  We did this within 20 minutes of urination, and again one and two weeks later.

There was a consistent trend for urine patches in pens with lambs that were fed willow to have lower emissions than those that were not fed willow for each of the three gases, although this was only statistically significant for nitrous oxide in Week 2, probably because of the small number of urine patches sampled.  Ammonia emissions declined rapidly, nitrous oxide emissions were mainly in Week 2, and carbon dioxide emissions declined gradually over the two-week period.  Lower carbon dioxide emission suggests that microbial activity was supressed in the soil, rather than in the rumen, but we cannot discount a contribution from the latter.  Willow is well suited to mechanical harvesting, or to direct browsing of coppiced trees if livestock access is managed to ensure sustainability, and our results suggest that feeding willow to ruminants could contribute to climate change and air quality targets.

 

Stoate, C., Fox, G., Bussell., J. & Kendall, N.R. 2021. A role for agroforestry in reducing ammonia and greenhouse gas emissions from ruminant livestock systems. Aspects of Applied Biology 146.