16 Jul 2020
Fat in ruminant diets and its effect on rumen microbiota (part 1)
Fat can be added to the diets of dairy cows and feedlot animals to increase their energy content, although it has potentially negative effects on the rumen microbiota. This first article, of a series of two, summarises the metabolism of fat in the rumen, discusses relevant scientific works, and evaluates the extrapolation of experimental data to the everyday practice.
Most of the digestive processes in ruminants, especially carbohydrate fermentation and protein degradation, are the result of the activity of ruminal microflora: bacteria, protozoa, fungi, and archaea. This symbiotic microbiota supplies nutrients to the host, especially volatile fatty acids (VFA), microbial protein, and vitamins. At the same time, there are energy losses, for example through the production of methane.
Despite the high ruminal stability, given by its functional constancy and the resilience of the microbial ecosystem (Weimer et al. 2015), ruminal microbiota verifies important individual variations (Jami and Mizrahi, 2012). It can also be altered by sudden or important dietary changes in, for example, the starch and fat contents of the ration.
In most ruminant diets, fat represents less than 5% of the total dry matter. Oilseeds such as linseed, rape, soya, and sunflower are the main source of dietary fat for ruminants. They are rich in unsaturated fatty acids (UFA), including:
- Oleic acid (OLA, cis-9-C18:1)
- Linoleic acid (LA)
- Alpha-linoleic acid (ALA)
Fat supplements were initially used to increase the energy value of diets, aiming to fulfil the energy requirements of dairy cows or of animals in intensive fattening systems. They can also be used to modify the fatty acid (FA) profile of meat (Wood et al., 2008) or milk, modulating dietary, organoleptic, and technological properties.
Adding fats to ruminant diets can:
- Modulate the ruminal function, reducing methane emissions (Martin et al., 2016)
- Reduce intake and fat content of milk (Rabiee et al., 2012)
The limit for fat supplementation in ruminant diets is given by the negative effect that fats have on ruminal degradability, especially when they have high content of UFA (Brooks et al., 1954)
This article is centred on the influence that fat has on the ruminal microbiota.
Metabolism of dietary fat by rumen microbiota
Most dietary FA are glycerol esters:
- Triacyclglycerols, mainly in concentrates
- Galactolipids y Phospholipids, in forage, except in silages, where the vegetal lipases release fatty acids.
The metabolism of dietary fat by rumen microbiota consists of two steps, namely lipolysis and biohydrogenation.
Step 1: Lipolysis
Lipolysis is the first step in ruminal metabolism of acylglycerols, which results in the release of FA.
Lipolysis: Lipase-mediated hydrolysis of the ester bonds between fatty acids (FAs) and alcohols present in fats. As a result, FAs are released (Fig. 1). This process is mostly carried on by microbial lipases, although there is also some lipolytic activity proper of the vegetal material.
Step 2: Detoxifying adaptation – biohydrogenation
The microbiota saturates completely the UFA. This process is considered to be a detoxifying adaptation (Kemp et al., 1984). It marginally contributes to the elimination of reducing equivalents produced in ruminal fermentation (Lourenço et al., 2010).
The process of biohydrogenation (BH) comprises several steps, depending on the UFAs. It can also use several pathways, depending on diet and ruminal environment (Griinari, 1998).
- Phospholipids and galactolipids can be hydrolysed by some strains of Bacillus fibrisolvens (Hazlewood and Dawson, 1979).
- Triacyclglycerols are also hydrolysed by different species of the genus Butyrivibrio (Latham et al., 1972), but Anaereovibro lipolyticus is the most known bacteria with lipolytic properties.
➢ The lipase of Anaereovibrio lipolyticus was studied for the first time by Henderson (1971). Its genome consists of three genes coding for three lipases (Prive et al., 2013).
➢ The three lipases were more active on lauric and myristic acids than on palmitic and stearic, whilst dietary fat mainly contains FA of 16 and 18 carbons.
Protozoa engulf bacteria, and the bacterial biohydrogenation can still take place inside protozoa (Jenkins et al., 2008). This could explain the high concentration of intermediate products inside protozoa (Devillard et al., 2006).
Figure 2. Megasphera elsdenii YJ-4
Kim et al. (2002) isolated a bacterium identified as Megasphera elsdenii YJ-4. It produces trans-10,cis-12-CLA in the rumen with diets rich in starch. They found that strain T81 also produces this isomer.
Harfoot (1978) demonstrated that two bacteria from the genus Fosocillus spp. reduce C18:1 FA into stearic acid.
Van de Vossenberg y Joblin (2003) isolated a strain of Butyrivibrio capable of completing hiohydrogenation of AL and ALA to stearic acid.
Wallace (2007) concluded that bacteria that form stearic acid, previously identified as Fosocilus spp. or Bacillus hungatei occupy a specific branch of the Butyrivibtio spp. tree.
Figure 3. Butyrivibrio spp.
In vitro studies
Besides studies based on selected microorganism isolates, there have been attempts to evaluate, in vivo or in vitro, the relationship between ruminal bacteria and biohydrogenation (BH). This has been performed by adding bacteria and measuring the products, or by adding diet supplements known for affecting BH, and measuring later the abundance of bacteria.
Figure 4. Laboartory fermentation tests
Inoculating B. fibrisolvens in the rumen on goats fed a diet enriched with oil high in linoleic acid (LA), Shivani et al. (2016) observed an increase in total conjugated linoleic acid (CLA) in the ruminal fluid. This confirmed that this bacterium is involved in in vivo biodegradation.
Since most of dietary fats are glycerides and need to suffer lipolysis before BH, a reduced FA metabolism in the rumen could limit the occurrence of BH in the rumen.
➢ A preliminary study with esterase inhibitors demonstrated promising in vitro results (Sargolzehi et al., 2015).
➢ Apás et al. (2015) showed a higher proportion of cis-9,trans-11-CLA in milk of goats supplemented with a mix of Lactobacillus spp., Bifidobacterium spp., and Enterococcus spp.
➢ Dietary fat shapes ruminal microbiota. Brooks et al. (1954) demonstrated that maize oil, both in vivo and in vitro, reduce ruminal degradation of cellulose and VFA production, affecting the microbiota. They also found that pig fat, more saturated than maize oil, produce a less pronounced reduction in degradation of cellulose.
➢ Similarly, Ikwuegbu y Sutton (1982) found a reduction in degradability of fibre, drop in the percentage of acetate and butyrate, and an increase in propionate when using linseed oil.
Plant extracts also could modulate the activity of biohydrogenating bacteria.
➢ Essential oils decreased or increased the abundance of B. fibrisolvens in vitro (Ishlak et al., 2015). This could explain the changes in the profile of ruminal products of biohydrogenation (BH) (Lourenço et al., 2010).
➢ Similarly, in vitro, tannins reduced the abundance of B. proteoclasticus and increased B. fibrisolvens (Ishlak et al., 2015).
It is convenient to mention that most experiments were performed with oils, included in quantities that exceeded common practice values.
Consequently, experimental results on the effect of added fat on rumen microbiota and its activity, should be extrapolated to field conditions with precaution.
In the second part of this work, we will analyse the results obtained in in vivo trials. We will also revisit ruminal biohydrogenation of fats in more detail.