(Die artikel is voorsien deur James Blignaut)
Herkouers is weidende herbivore wat hul voedingstowwe vanuit plant-gebaseerde voedsel verkry. Hulle doen dit deur onder meer hul voer in ‘n gespesialiseerde maag te fermenteer voordat vertering plaasvind.
Hierdie fermentasieproses word hoofsaaklik deur mikrobes gedoen. As gevolg van hul unieke spysverteringstelsel, verskil die spysverteringsproses van herkouers baie van dié van mense of omnivore soos honde. Tabel 1 verskaf ‘n kort illustrasie van hierdie verskille. As gevolg van hierdie fermentasie-gebaseerde spysverteringstelsel stel herkouers groot hoeveelhede metaan (CH4) oraal vry, en CH4 is ‘n kweekhuisgas wat met aardverwarming verbind word. Hierdie vrystelling van CH4, ook genoem enteriese fermentasie, is verantwoordelik vir tussen 80% en 90% van alle kweekhuisgasvrystellings wat met herkouers verbind word.
Tabel 1: Verskille in die spysverteringstelsels van mense, honde en herkouers
Mense | Honde | Herkouers/ skape | |
Tydperk wat maag leeg is | 3 ure | 3 ure | Nooit leeg nie |
Inter-verteringsrus | Ja | Ja | Nooit |
Bakterieë teenwoordig | Nie in die maag nie, maar in die ingewande | Nie in die maag nie, maar in die ingewande | Ja, kardinaal in die rument (eerste maag) |
Spysverteringsdoeltreffendheid | 100% | 100% | 60% of minder |
Grootte van kolon | Kort & klein | Kort & klein | Lank en ruim |
Spysverteringsaktiwiteit van die kolon | Geen | Geen | Noodsaaklike funksie |
Bakteriese flora in kolon | Verrotting | Verrotting | Fermentatief |
Bruto voedsel in ontlasting | Skaars | Skaars | Groot hoeveelhede |
Voeding gewoonte | Onderbroke | Onderbroke | Deurlopend |
Oorlewing sonder maag | Moontlik | Moontlik | onmoontlik |
Lengte van spysverteringstelsel tot liggaamslengte | 1:5 | 1:7 | 1:27 |
Dit is enteriese fermentasie wat die narratief dryf dat mak herkouers, veral skape en beeste, nadelig is vir die klimaat en die omgewing in die algemeen. Daar word dus voorgestel dat ‘n omgewingsbewuste persoon, en die samelewing, die aantal skape en beeste moet verminder en toenemend meer staatmaak op byvoorbeeld alternatiewe plant-gebaseerde voedsel. In dieselfde asem word daar dikwels ironies aangevoer dat die wêreld weer wilde diere moet vestig. So ‘n hervestigingsproses sluit wilde herkouers soos takbokke en bokke in. Alhoewel ‘n mens nie teen die grasie en skoonheid van die wilde herkouers kan argumenteer nie, het hulle dieselfde spysverteringstelsel as dié van beeste en skape. Is herkouers dus werklik sleg vir die omgewing? Is hulle die vloek van die natuur? ‘n Ontwerpfout van een of ander aard?
(Let wel: Op ‘n suiwer liggaamsmassa-basis is daar vandag minder soogdiere (insluitend herkouers) as ooit tevore in die opgetekende geskiedenis, en enteriese fermentasie is direk gekoppel aan liggaamsmassa. As die globale gewig van herkouers vandag minder is as byvoorbeeld ‘n honderd jaar gelede, hoekom is hulle vandag so sleg vir die omgewing?)
Deel van die antwoord lê by die manier waarop konvensionele koolstofrekeningkunde gedoen word volgens wat ‘n Lewensiklusanalise (LCA) genoem word, gebaseer op, onder andere, ISO 14040:2006 en 14044:2006. Volgens die LCA is ‘n plaas soortgelyk aan die produksielyn van ‘n fabriek, en die interaksie van herkouers met die weiding is soos dié tussen ‘n motorkar en die asfaltpad waarop dit ry. Hierdie lineêre benadering is grootliks gefokus op vrystellings, terwyl min klem op versagting- en sekwestrasie-opsies geplaas word. Die meer onlangse standaard van koolstofrekeningkunde (ISO 14067:2018) skets egter ‘n biogeniese benadering. ‘n Biogeniese benadering, per definisie, is ‘n sisteemgebaseerde benadering waardeur die enteriese fermentasie van herkouers geweeg word relatief tot hul interaksie met die weiding of veld, dit wil sê die plaaslike konteks waarbinne hulle wei. Die basiese rekeningkundige identiteit om hierdie interaksie vas te vang word deur die volgende vergelyking bereken:
Die net (sink) of bron =
- Minus CO2 ingebed in die droë materiaal van die beweide biomassa, plus
- (Die vrygestelde emissies, insluitend asemhaling, alle kweekhuisgasse, en vervlugte mis, minder
- Die CO2 wat in die rommel ingebed is as gevolg van die weiding en voerverkope, plus
- Die CO2 wat in die produk ingebed is, hetsy melk-, wol- of veeverkope, plus
- Die CO2 ingebed in eksterne insette soos brandstof, elektrisiteit, plaagdoders en onkruiddoders).
Hierdie identiteit is afgelei van die biogeniese siklus, wat beskryf kan word as:
- Soos die herkouer CH4 wei en uitasem, verskaf dit die voedsel en energiebron vir metanotrofe, ‘n grondgebaseerde bakterie wat CH4 as energie gebruik en wat metaan in grondgebaseerde suikers omskakel, en sodoende die CH4-lading wat in die atmosfeer voorkom.
- Die oorblywende CH4 beweeg na die top van die troposfeer – ‘n reis van ongeveer 90 dae – waar dit die hidroksiel (HO)-radikale teëkom.
- Die HO-radikale is ‘n groep baie kortstondige molekules wat as die natuur se skoonmakers optree. Hulle skakel onder meer CH4 en koolstofmonoksied (CO) om in koolstofdioksied (CO2) en H2O (reën/water).
- HO reageer vinniger met CO as met CH4. Hoe meer CO vrygestel word as gevolg van industriële prosesse en brand, hoe meer uitkompeteer dit die CH4 – wat meer CH4 laat om in die stratosfeer vrygestel te word. Dit is in die stratosfeer waar CH4 as ‘n kweekhuisgas optree. Die CH4-molekule het egter ‘n baie kort lewensduur, naamlik tussen 10 en 13 jaar, voordat dit afgebreek word en as CO2 en H2O na die troposfeer teruggekeer.
- Die terugkerende CO2 en H2O, in kombinasies met sonlig, stimuleer plantegroei deur fotosintese.
- Dit is die plant wat bewei word, en veral die koolstof binne daardie plant, wat vir kudde-ontwikkeling, melkproduksie, vleis- en proteïenvorming gebruik word en in die vorm van mis en urine in die grond neergesit word. Slegs ‘n fraksie, tussen 3% en 5%, van die koolstof word deur enteriese fermentasie terug in die troposfeer vrygestel, en die siklus begin weer by #1.
Nie net is dit net ‘n gedeelte van die CH4 wat vrygestel word wat in die stratosfeer eindig nie, maar sy verblyf is van korte duur; dit terwyl die teruggekeerde CO2 en water instrumenteel is in plant- en dieregroei. Hierdie insigte het onder andere daartoe gelei dat die interregeringspaneel oor klimaatsverandering in hul sesde evalueringsverslag (2023) die globale temperatuurveranderingspotensiaal (GTP) van metaan verminder tot 4,7 keer dié van CO2, in vergelyking met ‘n vroeër algemeen aanvaarde aardverwarmingspotensiaal van 28 .
Wanneer die koolstofsekwestrasievermoë van plante oorweeg word en die bydrae wat verantwoordelike kuddebestuur kan lewer om sulke sekwestrasie te versnel, kan ‘n plaas wat herkouers huisves as ‘n potensiële netto-sink van koolstof funksioneer, wat die atmosfeer verkoel. Dit kan gedoen word deur regeneratiewe praktyke soos veelvuldige rotasies op ‘n enkele hektaar toe te pas, soos reeds deur vooruitskouende boere beoefen word. Byvoorbeeld, ‘n plaas van 1 000ha met twee rotasies stimuleer doeltreffend plantgroei en koolstofonttrekking op 2 000ha. In die geval van besproeiingstelsels is tot tien of meer rotasies moontlik. Dit vergroot die jaarlikse koolstofonttrekkingsgebied aansienlik. Boonop kan sulke bestuurstelsels onder meer verbeterde waterinfiltrasie, biodiversiteit en verbeterde voedingstofsiklusse bevorder. Daar moet kennis geneem word dat die gebruik van rumenaanvullings sowel as noukeurige genetiese seleksie ook kan help om enteriese fermentasie te verminder.
Gras se lewensiklus volg een van drie moontlike paaie as dit nie deur herkouers bewei word nie. Eerstens kan dit verbrand word deur deeltjies en kweekhuisgasse in die atmosfeer vry te stel. Dit stel ook CO vry wat die HO se vermoë verminder om die CH4 te verwyder, terwyl die grondbakterieë uitgeput word. Tweedens kan dit met behulp van fossielbrandstowwe gesny word. Dit is egter soortgelyk aan die ontginning van die hulpbron aangesien dit die voedingstowwe wat daarin voorkom verwyder sonder om dit te vervang. Derdens kan gras ook sterf en droog word en ‘n steriele stelsel raak. Dikwels is brand of sny die enigste manier om so ‘n stelsel te regenereer. In alle gevalle vermy weiding die vrystelling van emissies en kweekhuisgasse, terwyl grondgesondheid en biodiversiteit bevorder word en dit nie vernietig word nie.
Samevattend stimuleer fotosintese die groei van gras en ‘n toename in koolstofonttrekking en die afsetting daarvan in óf biomassa óf die grond – en hierdie hele proses word gestimuleer en versnel deur beweiding terwyl die nadelige gevolge van brand en sny vermy word. Hierdie sistemiese en wedersyds voordelige saambestaan van herkouers en gras handhaaf die funksionering van grasdominante ekosisteme. Dit het dit van die begin van tyd af so gewerk. Die enteriese fermentasie stimuleer die metanotrofe verder terwyl die ensieme in die speeksel die hergroei van plante aan die gang sit. Boonop maak die hoefbeweging die grond los en die stikstof in die urine en mis stimuleer plantgroei en grondkoolstofontwikkeling. Dit aktiveer suikers wat lei tot verdere wortel- en plantontwikkeling, wat lei tot ‘n proses waardeur herkouers nie net hul vrygestelde emissies kan verreken nie, maar dit ook doen. Hulle doen dit terwyl hulle laewaarde en ontoeganklike stysel herwin tot hoëwaarde, voedsame en toeganklike proteïene.
Lank lewe die weidende dier, sy skoonheid en sy simbiotiese verhouding met gras.
(Article submitted by James Blignaut)
Ruminants are grazing herbivores that acquire the nutrients for their sustenance from plant-based food. They do so by, among others, fermenting their feedstock in a specialised stomach prior to digestion.
This fermentation process is mainly done by microbes. Because of their unique digestive track, the digestive process of ruminants differs vastly from that of humans or omnivores like dogs. Table 1 provides a brief illustration of these differences. Due to this fermentation-based digestive system, ruminants orally release large quantities of methane (CH4), and CH4 is a greenhouse gas associated with global warming. This release of CH4, also called enteric fermentation, is responsible for between 80% and 90% of all greenhouse gas emissions associated with ruminants.
Table 1: Differences in the digestive tracks of humans, dogs and ruminants
Humans | Dogs | Ruminants/sheep | |
Empty time of stomach | 3 hours | 3 hours | Never empties |
Inter-digestive rest | Yes | Yes | Never |
Bacteria present | Not in stomach, but in gut | Not in stomach, but in gut | Yes, vital, in the rumen (the first stomach) |
Digestive efficiency | 100% | 100% | 60% or less |
Size of colon | Short & small | Short & small | Long & capacious |
Digestive activity of the colon | None | None | Vital function |
Bacterial flora in colon | Putrefactive | Putrefactive | Fermentative |
Gross food in faeces | Rare | Rare | Large amounts |
Feeding habit | Intermittent | Intermittent | Continuous |
Survival without stomach | Possible | Possible | Impossible |
Length of digestive track to body length | 1:5 | 1:7 | 1:27 |
It is enteric fermentation that drives the narrative that domesticated ruminants, notably sheep and cattle, are detrimental for the climate and the environment in general. It is thus suggested that an environmentally conscious person, and society, should therefore reduce the number of sheep and cattle and rely increasingly more on alternative plant-based foods, for example. In the same breath it is often ironically argued for the rewilding of the world. Such rewilding includes non-domesticated ruminants like deer and antelope. While one cannot argue against the grace and beauty of the non-domesticated ruminants, they have the same digestive system than that of cattle and sheep. Thus, are ruminants truly bad for the environment? Are they the curse of nature? A design error of some kind?
(Note: On a pure body-mass basis there are fewer mammals (including ruminants) today than ever before in recorded history, and enteric fermentation is directly linked to body mass. If the global weight of ruminants is less today than, say, a hundred years ago, why are they so bad for the environment today?)
Part of the answer lies with the way conventional carbon accounting is done according to what is called a Life Cycle Analysis (LCA) based on, among others, ISO 14040:2006[1] and 14044:2006[2]. According to the LCA, a farm is akin to the production line of a factory, and the interaction of ruminants with a pasture is like that between a motor car and the asphalt road it travels on. This linear approach is largely focused on emissions while placing little emphasis on mitigation and sequestration options. The more recent standard of carbon accounting (ISO 14067:2018), however, outlines a biogenic approach. A biogenic approach, per definition, is a systems-based approach whereby the enteric fermentation of ruminants is weighed relative to their interaction with the pasture or veld, i.e. the local context within which they graze. The basic accounting identity to capture this interaction is given by the following equation:
The net (sink) or source =
- Minus CO2 embedded in the dry matter of the grazed biomass, plus
- (The released emissions inclusive of respiration, all greenhouse gasses, and volatised manure, less
- The CO2 embedded in the litter because of the grazing and fodder sales, plus
- The CO2 embedded in the product, be that milk, wool or livestock sales, plus
- The CO2 embedded in external inputs such as fuel, electricity, pesticides and herbicides).
This identity is derived from the biogenic cycle, which can be described as:
- As the ruminant grazes and exhales CH4, it provides the food and energy source for methanotrophs[3], a soil-based bacteria that uses CH4 as energy and which converts methane into soil-based sugars, thus reducing the CH4 load that is emitted into the atmosphere.
- The remaining CH4 travels to the top of the troposphere – a journey of about 90 days – where it encounters the hydroxyl (HO) radicals.
- The HO radicals are a group of very short-lived molecules that act as nature’s scrubbers. They convert CH4 and carbon monoxide (CO), among others, into carbon dioxide (CO2) and H2O (rain/water).
- HO reacts faster with CO than with CH4. The more CO is emitted due to industrial processes and fire, the more it outcompetes the CH4 – that leaves more CH4 to be released into the stratosphere. It is in the stratosphere where CH4 acts as a greenhouse gas. The CH4 molecule, however, has a very short lifespan, namely between 10 and 13 years, before being broken down and returned to the troposphere as CO2 and H2
- The returning CO2 and H2O, in combinations with sunlight, stimulate plant growth through photosynthesis.
- It is the plant that is grazed, and notably the carbon within that plant, that is used for herd development, milk production, meat and protein formation, and deposited into the soil in the form of manure and urine. Only a fraction, between 3% and 5%, of the carbon is released back into the troposphere through enteric fermentation, and the cycle starts at #1 again.
Not only is it just a portion of the CH4 released that end in the stratosphere, but its stay is short-lived; that while the returned CO2 and water are instrumental in plant and animal growth. These insights, among others, led the Inter-governmental Panel on Climate Change, in their 6th Assessment Report (2023), to reduce the global temperature change potential (GTP) of methane to 4.7 times that of CO2, compared to an earlier generally accepted global warming potential (GWP) of 28.
When considering the carbon sequestration capability of plants and the contribution that responsible herd management can make to accelerate such sequestration, a farm housing ruminants can function as a potential net sink of carbon, cooling the atmosphere. This can be done by applying regenerative practices such as multiple rotations on a single hectare, as is already being practised by forward-looking farmers. For example, a farm of 1 000ha with two rotations effectively stimulates plant growth and carbon drawdown on 2 000ha. In the case of irrigated systems, up to ten or more rotations are possible. This expands the annual carbon drawdown area significantly. In addition, such management systems can promote improved water infiltration, biodiversity and enhanced nutrient cycling, among others. It should be noted that the use of rumen supplements as well as careful genetic selection can also help to reduce enteric fermentation.
Grass’ life cycle follows one of three possible pathways if not grazed by ruminants. First, it can be burned releasing particulate matter and greenhouse gasses into the atmosphere. It also releases CO which reduce the HO’s ability to remove the CH4, while depleting the soil bacteria. Second, it can be mowed using fossil fuels. This, however, is akin to mining the resource since it removes the nutrients contained therein without replacing it. Third, grass can also become moribund and dry – inert – becoming a sterile system. Often the only way to regenerate such as system is by means of burning or mowing. In all cases, grazing avoids the release of emissions and greenhouse gasses, while promoting soil health and biodiversity and not destroying it.
In summary, photosynthesis stimulates the growth of grass and an increase in carbon drawdown and the deposit thereof in either biomass or the soil – and this entire process is stimulated and accelerated through grazing while avoiding the detrimental consequences of fire and mowing. This systemic and mutually beneficial co-existence of ruminants and grass maintains the functioning of grass-dominant ecosystems. It has done so from the beginning of time. The enteric fermentation further stimulates the methanotrophs while the enzymes in the saliva kick-start the re-growth of plants. In addition, the hoof movement loosens the soil and the nitrogen in the urine and manure stimulates plant growth and soil carbon development. This activates sugars that leads to further root and plant development, resulting in a process whereby ruminants not only can, but do, offset their released emissions. They do so while upcycling low-value and inaccessible starch into high-value, nutritious and accessible protein.
Long live the grazing beast, its beauty and its symbiotic relationship with grass.
[1] https://www.iso.org/standard/37456.html