Oorsig / Review
pH equal to 7,0 is considered neutral – i . e . neither acid nor alkaline . A pH below 7,0 is considered more acid and one above 7,0 indicates a more alkaline environment .
Rumen pH = -log [ rumen H +]; or [ rumen H +] = 10-rumen pH .
For example : [ H +] of rumen fluid with a pH of 5,0 and 5,5 is 1 x 10-5M and 3,16 x 10-6M , respectively . The above equation illustrates that a relatively small change in rumen pH – for various reasons - constitutes an enormous change in hydrogen ion concentration in the rumen fluid .
Rumen pH is one of the critical factors affecting rumen microbial growth . Physical characteristics of feeds as related to their effectiveness in stimulating chewing , rumination and increased rumen motility based on their total cell wall content and particle size within classes of feeds ( effective NDF ; eNDF ) contribute to rumen pH . The eNDF value is defined by bovine nutritionists as the percent of the NDF retained on a 1.18 mm screen . Factors other than particle size that influence the eNDF value are degree of lignification ( age of plant – the older , the more unfermentable and indigestible lignin it contains ) of the NDF , degree of hydration , and bulk density .
Total microbial yield and SC growth rate rapidly declines below a pH of 6.2 , which relates to a diet eNDF content of 20 %. Microbial yield is reduced by 2.5 percentage units for each percentage drop in diet eNDF below 20 %. Thus the diet eNDF must be accurately predicted to accurately predict microbial amino acid production and cell wall digestion . In beef cows on pasture , dietary eNDF is not used as a ration management tool , but these figures simply serve to illustrate the effect of nutrients in the diet on rumen microbial yield . Unlike cattle fed on diets high in concentrates , the ones on natural extensive pasture do not experience the same degree of fluctuation in rumen pH between meals . The average “ normal ” rumen pH of a grazing bovine is slightly acid - about 6,5-6,8 - with fluctuations depending on the amount and type of supplement eaten .
Fluctuations in rumen pH are usually the result of :
• Level of DMI – a lower DMI usually leading to an increase in rumen pH
• Interval between feed intake determines the magnitude of pH fluctuation between feed intake
• Rate of fermentation which is determined by the starch or sugar concentration of the diet
• Water intake dilutes the rumen content and decreases the osmolality thereof
FC ( cellulolytic ) bacteria do not grow at a pH below 6,0 and their growth rate decreases by 14 % per hour for each 0,1 pH unit drop between 6,5 and 6,0 . Fermentation of cellulose occurs at an optimal rate at a pH > 6,5 .
Why is rumen health important ?
Rumen microbial health determines the efficiency by which grass cell wall material is broken down in the rumen . This has an effect on passage rate of ingesta - which impacts on the daily dry matter intake ( DMI ) from pasture . Should rumen microbial health be negatively affected , neutral detergent fiber degradation ( NDF , a laboratory parameter for plant cell walls which is closely and inversely correlated with DMI ) - and hence passage rate - will be impaired . This will lead to lower intake of pasture DM – ultimately leading to a lower gain per Ha and eroding the potential to maximize profit per Ha .
Rumen microbial health is determined by the balance between rumen degradable protein ( RDP ) and non-fiber carbohydrates ( NFC , starches and sugars ) of the diet ingested by the cow .
Excess RDP , or unutilized RDP through a lack of NFC , leads to a higher ammonia delivery to the liver , which has to be removed through formation of urea . This happens most often in winter when , for example , ad-lib . cheap chicken manure is fed on dry , poor quality standing hay . The additional metabolic function by the liver to “ clean up ” the unutilized ammonia is counterproductive and leads to a less efficient intermediary ( energy ) metabolism . Also , a
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