Effects of Diet and its Physical Form on Rumen pH and Dairy Cow Performance
By Liam A. Sinclair, Department of Animal Production, Welfare and Veterinary Sciences, Harper Adams University e-mail:
lsinclair@harper-adams.ac.uk
Annual milk production per cow in the UK has increased steadily over the last 40 years at a rate of approximately 100 kg/cow/year, with a current average of approximately 8000 kg/cow (AHDB, 2019). This increased production has primarily been achieved through a greater use of concentrates and production of high quality forages, with the consequence that the risk of subacute ruminal acidosis (SARA) has increased. Estimated prevalence of SARA across 6 studies and 87 herds Europe and North America ranged from 11 to 24% (Rojo-Gimeno et al., 2018). The implications of SARA are several fold, and include reduced intake, diarrhoea, poor body condition, impaired rumen motility, liver abscesses and decreased milk production, with costs ranging from approximately £90 to £350/cow/year (Rojo-Gimeno et al., 2018). However, SARA is often poorly defined, with several thresholds being proposed across a number of sampling sites and methods. Various means of reducing the impact of SARA have also been proposed including altering the rate of fermentation of non-structural carbohydrate in the rumen, and the supply of adequate amounts of physically effective fibre (Humer et al., 2018). This is an active area of research, yet there are still a number of misconceptions and beliefs on SARA such as the relationship between SARA and lameness, that are not grounded in evidence (Bergsten, 2003). Additionally, most of the recent work in this area has been conducted in North America using maize silage and lucerne haylage based diets that are supplemented with maize grain, and may not therefore be representative of the wetter, grass silage based rations that are supplemented with wheat, barley or by-products in the UK. This paper will briefly review the control of pH in the rumen, before addressing means of characterising and assessing physically effective fibre under UK conditions, the consistency of diet mixing and selection on UK farms, and the effect of chop length and supplement type on rumen pH and cow performance.
Principles of rumen pH The fermentation of structural and non-structural carbohydrates releases organic acids (short chain volatile fatty acids and lactic acid) that readily dissociate, increasing the concentration of protons (Aschenbach et al., 2011). This process can decrease ruminal pH if rumen buffering or absorption rate across the epithelium is insufficient, or outflow from the rumen is slow (Aschenbach et al., 2011). If rumen pH drops for a sufficient period of time then rumen function can be affected, a condition referred to as SARA (Humer et al., 2018). However, the definition of SARA is problematic. The current guideline is that the risk of SARA increases when ruminal pH drops below pH 5.6 for more
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than 3 h/d, or below pH 5.8 for 5 to 6 h of the day (Zebeli et al., 2012). However, recent research has reported that the dorsal area of the rumen (where the rumen mat is formed) can have a VFA concentration that is 50 mmol/L higher compared to the ventral area, be 0.7 pH units lower, and support a higher microbial population (Aschenbach et al., 2011). However, as the ventral area is easier to sample, most measurements have been conducted here (Zebeli et al., 2012). Sampling methods such as the oesophagus, rumenocentesis or pH boluses all have inherent limitations (Humer et al., 2018), whilst the use of rumen fistulated cows is often not possible under commercial conditions. It is also worth bearing in mind that the dissociation (pKa) constant
of the main VFA in the rumen is approximately 4.8. The consequence of this is that as rumen pH increases, the VFA dissociate and release more protons. For example, using the Henderson-Hasslebach equation, approximately 1, 10, 50, 90 and 99% of the VFA have released their protons at pH 2.8, 3.8, 4.8, 5.8, and 6.8 respectively. Passive diffusion of the dissociated form of the VFA across the lipid bilayer of the rumen epithelium is limited (Dijskra et al., 2012), and at pH 5.8 only 10% of the VFA are present in this form, although this proportion increases as pH falls. As a consequence, the absorption of VFA at the most common pH values found in the rumen relies on transport proteins, particularly for acetate and propionate. Acetate in particular utilises a bicarbonate dependent uptake for absorption, releasing bicarbonate buffer into the rumen. This can result in up to 50% of the VFA absorption occurring in a bicarbonate dependent manner, and introducing approximately the same amount of bicarbonate as saliva (Table 1). The effect of diet on ruminal pH is also multifactorial, with aspects such as the pH and buffering capacity of the feed, relative proportions of fatty acids (e.g. the fermentation of 1 mol of hexose produces 2 mol of acetate or propionate, but only one of butyrate), rate of saliva production and rate of absorption
Table 1. Contribution of saliva and the rumen epithelium to bicarbonate flow into the rumen
Roughage diet
Feed intake (kg DM/d) Saliva production (L/d) VFA production (mol/d)
Total bicarbonate flow (g/d) From saliva (g/d)
From rumen epithelium (g/d) Dijkstra et al. (2012)
20
250 100
4010 1875 2135
Concentrate diet
20
200 120
4245 1500 2745
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