Increasing starch in dairy diets are beneficial to milk yield and efficiency of production, but the practice has limitations due to lactate accumulation in the rumen and inflammation-based negativity to immune function. Although the effects can be minimized by forage and TMR particle size, decreased sorting behaviour, increased passage rate, and manipulating starch fermentability as affected by grain processing, these practices have limitations. To go further, rumen modifiers have come on the market to optimize fibre degradation, shift microbial composition, improve efficiency of microbial function, use nutrients more efficiently, limit enteric methane production etc. The authors cited reviewed the feed additives that have a potential ruminal mechanism of action when fed to dairy cows.
Yeast products: Live yeast and yeast extract are included in diets, primarily to optimise fibre digestion and improve DMI. The action is associated with boosting of cellulolytic bacteria numbers by stimulating growth of Selenomonas, Ruminococcus and Fibrobacter strains, and probably also by pH control via increased lactate uptake. Other reported benefits include oxygen quenching in the obligatory anaerobic rumen environment since the popular yeast Saccharomyces cerevisiae contains antioxidants, stimulate transporters in the rumen epithelium that could increase rate of VFA absorption, and improve feeding frequency; the latter presumably should lessen ruminal pH swings and thereby stabilizing fermentation. Less studied responses are stronger barrier function in the rumen epithelium, thereby lessening leakage of bacterial endotoxin into the blood and the systemic immune response, and dampened inflammatory responses under heat stress and mastitic infections. Overall, the net effect should be improved feed efficiency if yeast products are supplemented.
Methane inhibitors: Numerous additives have been tested to support strategies to limit methane production in the rumen. Many products, such as plant bio-actives, however may be less effective because of variability in composition, or have negative side effects such as at the same time decreasing feed intake and therefore milk production. Decreased methane production should coincide with maintained or improved feed efficiency if the methodology is to be accepted at farm level. Some products are discussed below.
Lactate and Succinate as fermentation intermediates: There is a strong inverse relationship between the abundance of the microbial family Succinivibrionaceae in the rumen and methane production. Members of this group do not use hydrogen but instead use carbon dioxide to capture hydrogen from upstream fermentation to sink to succinate, which is released and subsequently fermented by other microbes primarily to propionate, which is the primary precursor for milk production. Similarly, animals with lower methane production appear more positively associated with Megashaera and Coprococcus to convert lactate to propionate via the acrylate pathway. These observations imply microbial manipulation in the rumen which is in its infancy and not easy to implement at this stage.
Nitrate as suppressor of methanogenesis: Nitrate has potential to lower methanogenesis both as an alternative electron sink and by direct inhibition of methanogens by its intermediate, nitrite. At the same time lactilytic (lactate-consuming) strains such as Selenomonas can assist in preventing nitrate and nitrite accumulation to detrimental levels. Often the impact of a modifier is temporary, but limited evidence suggests that nitrate abatement efficacy may last over extended feeding periods with methane production suppressed up to 20%. Feed intake, however, should not be reduced which does happen sometimes, which led to researchers experimenting with slow-release nitrate sources. The final product of nitrate reduction is ammonia which is built into microbial protein, thereby preventing excess nitrogen excretion in urine. The most promising approach is to use additives to stimulate lactilytic nitrate and nitrite reducers, especially those related to Selenomonas. To that effect a strain of Paenibacillus has shown promise as a nitrite-using probiotic to be included when nitrate is fed.
3-Nitrooxypropanol: The efficacy, persistency and safety of 3-nitroxypropanol (3-NOP) have been largely established. Its efficacy to suppress methanogenesis is less when fibre is increased such as when concentrate in the diet is decreased from 50% to 30%, but persistency has been shown over 15 weeks. A reason for the observation with fibre is that increased forage increases hydrogen production which implies more 3-NOP must be supplied. Also methyltrophic methanogens are less inhibited by 3-NOP, so forage sources with more pectins or other precursors for methanol and methylamines also might need a higher dose. The product appears to have minimal negative effects on palatability, feed intake (DMI) and milk quality and may suppress methanogenesis by 25-30%. Its effect on efficiency as measured by ECM/DMI however is divergent, apparently depending on feed composition.
Red seaweed: Different seaweed (macroalgae) species have been evaluated as methane depressants. Most results are however available on red seaweed of the genus Asparagopsis, particularly the species A. taxiformis. It has multiple potential biologically active compounds, notably bromium compounds such as bromoform which inhibit methanogens, but may also inhibit other bacteria and protozoa. Also, when fed at high doses bromoform may have toxicity issues to the animal, but fed at moderate levels will be favourable. Being a Br compound, elevated levels of Br are found in milk and also I when some seaweed species are fed. This does not necessarily present a problem as the milk of seaweed fed cows can be mixed with other milk, and in addition I in milk in any case varies substantially.
Improved milk and milk fat production without depression in feed intake have been shown in meta-analysis, but there may be interaction depending on animal and dietary variables. Overall, if decay, residue, safety, and marine and land environmental concerns can be addressed, red seaweed offers strong potential for suppressed methanogenesis as response in methane production reduction in the literature varies between 12 and 36%.
Additives which may improve gut health and/or feed efficiency:
Lactate-producing probiotics: There are various strains and therefore also variable efficacies; the function being maintaining a healthy hindgut by inhibiting potential pathogens such as Escherichia coli 0157. Some may also play a role in decreasing methanogenesis, if combined with a lactilytic probiotic such as Selenomonas ruminantium. Some studies did not show a major difference in lactation performance if a lactate producer and consumer was added to the TMR, but it depends on whether sub-acute ruminal acidosis (SARA) is present or not.
Ionophores: Ionophores have been well studied for their improvement in feed efficiency. Among other modes of action, they increase propionate production while inhibiting lactate-producing bacteria and other bacteria that enhance proteolysis and deamination. The hydrogen acceptance to propionate instead of binding into methane also suggests a decreased effect on methanogenesis, also because monensin reduces feed intake slightly, which has the additional advantage of preventing SARA, the latter also being addressed because monensin increases feeding frequency, thereby limiting lactate accumulation in the rumen. Protozoa adapt to monensin which is important as they have benefits related to ruminal N recycling. They do however enhance methanogenesis and consequently additives have been added to suppress protozoa. This, however, often has side effects as feed intake may be depressed and fibre degradability lowered. Therefore, monensin inclusion in dairy diets is not been consistently recommended as a methane abatement strategy, also because of microbial adaptation.
Branched-chain volatile fatty acids (VFA): Some prominent cellulolytic bacteria in the rumen require branched-chain VFAs which originate from branched-chain amino acids and which are built into the membranes of anaerobic bacteria. Their addition to the diet has both a ruminal and a cow metabolic affect, but it depends on actual composition of the acids added, feed composition and intake, and appears to be more effective in cows with lower propensity for milk production and not fed a total mixed ration. The branched-chain VFAs do increase fibre degradability depending on N availability in the rumen. Overall, the addition of these VFAs may increase milk production by 4-8% as has been noted by feeding the commercial product IsoPlus.
Plant components and extracts: Many studies have explored various plant components and extracts that inhibit methanogens and/or decrease protein wastage in the rumen by inhibiting protozoa and the bacteria that rely on amino acids as energy sources (so-called hyper-ammonia producers). Each class of plant bio-active compounds has various chemicals involved in their mode of action. Tannins in some cases have been favoured as they may inhibit undesirable bacteria and protozoa and can reduce protein degradability in the rumen. The benefit may be partial as the undesirable microbes can adapt their cell walls and tannin-bound protein may be lower digestible in the intestines. Saponins may inhibit protozoa but may decrease feed intake. Essential oils offer much in dairy nutrition and hypotheses of combining monensin with an essential oil which should inhibit protozoa and suppress methane have been tested, but with not much success. Combinations of plant bio-actives have also been explored, such as saponins plus tannins, also with mixed success. One experiment though reported better response over time, with feed efficiency improving by 4% and methane production per unit EPCM decreasing by about 10%.
Rumen modifiers cannot be used as substitute for good feeding management. They can, however, reduce variability among animals to improve feed efficiency in the herd and in particular cases reduce enteric methane production with less risk to lowering milk production and efficiency. Before embarking on a journey to include modifiers in the diet, the mode of action and the variability and circumstances of when success can be expected should be well studied as the added cost to the diet could well not be worth the while.