Invited Review: Glucosinolates Might Result in Low Methane Emissions From Ruminants Fed Brassica Forages

Abstract

Methane is formed from the microbial degradation of feeds in the digestive tract in ruminants. Methane emissions from ruminants not only result in a loss of feed energy but also contribute to global warming. Previous studies showed that brassica forages, such as forage rape, lead to less methane emitted per unit of dry matter intake than grass-based forages. Differences in rumen pH are proposed to partly explain these low emissions. Rumen microbial community differences are also observed, but the causes of these are unknown, although altered digesta flow has been proposed. This paper proposes a new mechanism underlying the lower methane emissions from sheep fed brassica forages. It is reported that feeding brassica forages to sheep can increase the concentration of free triiodothyronine (FT 3) in serum, while the intramuscular injection of FT 3 into sheep can reduce the mean retention time of digesta in the rumen. The short retention time of digesta is associated with low methane production. Glucosinolates (GSLs) are chemical components widely present in plants of the genus Brassica. After ruminants consume brassica forages, GSLs are broken down in the rumen. We hypothesize that GSLs or their breakdown products are absorbed into the blood and then may stimulate the secretion of thyroid hormone FT 3 in ruminants, and the altered thyroid hormone concentration may change rumen physiology. As a consequence, the mean retention time of digesta in the rumen would be altered, resulting in a decrease in methane emissions. This hypothesis on mitigation mechanism is based on the manipulation of animal physiological parameters, which, if proven, will then support the expansion of this research area.

A Review: Plant Carbohydrate Types—The Potential Impact on Ruminant Methane Emissions

“…Therefore, the full mechanisms for the CH 4 mitigation properties of forage brassicas remain unclear to date. Sun (48) proposed a new hypothesis based on a literature review to explain the low enteric CH 4 emission observed from ruminants fed forage brassica. According to the hypothesis, glucosinolates, a secondary metabolite widely present in forage brassica or their breakdown products, can stimulate the secretion of free triiodothyronine in ruminants.…”

Hydrogen and formate production and utilisation in the rumen and the human colon

“…Brassica forages have also been shown to result in lower CH 4 yields in lambs than perennial ryegrass [ 66 ]. The reason for this is not understood but an altered rumen microbiota or the presence of bioactive glucosinolates in brassicas have been suggested as possible causes [ 67 ]. Generally, however, it takes large changes in diet to bring about significant changes in enteric methane emissions in ruminants.…”

Plant chemistry and food web health

“…Systemic effects are manifested by iodine deficiency, hypertrophy of kidney, liver and the thyroid gland followed by an elevated secretion of thyroid hormones (Tripathi & Mishra, 2007), which is associated with an increased digesta efflux and consequently reduced rumen retention time (Barnett et al ., 2012). Despite the overly negative effects on animal physiology, the use of glucosinolates‐containing fodder can improve environmental health by mitigating methane emissions by ruminants (Dillard et al ., 2018; Sun, 2020). Mechanistically, glucosinolates do not directly inhibit the methanogenic microbes, but are supposed to alter the microbial community and rumen fermentation parameters (Kumar et al ., 2018; Sun, 2020).…”

“…Despite the overly negative effects on animal physiology, the use of glucosinolates‐containing fodder can improve environmental health by mitigating methane emissions by ruminants (Dillard et al ., 2018; Sun, 2020). Mechanistically, glucosinolates do not directly inhibit the methanogenic microbes, but are supposed to alter the microbial community and rumen fermentation parameters (Kumar et al ., 2018; Sun, 2020). Hence, together with the aforementioned indirect effect of reduced rumen retention time, methane production decreases and interestingly, seems to persist when dairy cows continue to feed on pastures (Storlien et al ., 2017).…”