Review/Oorsig Volume 22, Issue 01 - Page 5

Volume 22 • Issue 01 • 2018 in the water holding capacity. Soil aggregate is mainly the result of a carbon-rich substance called Glomalin, which is produced by the mycorrhizal fungi. These have a close symbiotic relationship with plant roots. Capillary water in porous soils is available for uptake by the grass plant. In compacted soils, the Ethylene-Oxygen Cycle cannot take place and hence nutrient availability for plant root uptake becomes limited – leading to a lower grass yield per Ha. The healthy rumen The capacity of the rumen of a normal adult beef cow is about 150 liters, containing 107 – 1012 bacteria and about 105 protozoa and many species of fungi per one milliliter of rumen fluid. These microorganisms thrive in an anaerobic environment. The rumen is a large fermentation vessel in which about 600 liters of gas (mainly methane and carbon dioxide) is produced daily by the 3 to 6 kg bacteria it contains. Apart from approximate water intake of 50-80 liters per day, 100-190 liters of bicarbonate- and phosphorous containing saliva contributes to the fluid influx in the rumen. This is important to buffer the volatile fatty acids produced from bacterial fermentation. Rumen bacteria can be classified according to the type of carbohydrates they ferment: • Fiber or structural carbohydrate (FC or SC) fermenters: They ferment cellulose and hemicellulose and grow slowly. They use ammonia as their primary nitrogen (N) source and utilize 0,15g carbohydrate per gram of bacteria per hour. The N may be from natural degradable protein or from non-protein nitrogen (NPN) sources like urea. • Non-fiber carbohydrate (NFC) fermenters: They ferment starch, pectin and sugars and grow at a faster rate. They use amino acids (from natural protein feed sources) and ammonia as their primary N source and utilize 0,05g carbohydrate per gram of bacteria per hour. The microbial population in the rumen could also be classified according to the site in the rumen where they occur predominantly: 1. Bacteria attached to fibre particles: They have a relatively slow growth rate as well as a low maintenance requirement. They are responsible for the breakdown of plant cell walls. They remain in the rumen for a relatively long period. The more these bacteria “colonize” fibre particles, the faster degradation is – leading to a faster rumen passage rate and ultimately a higher dry matter intake (DMI). Their main end-product is acetic acid, the precursor of milk fat which makes up for the bulkiness of milk and which serves as the main energy source for the suckling calf during its first 2-3 months of life. 2. Bacteria in the rumen fluid: They live off soluble substrate (starches, sugars (neutral detergent solubles, NDS) and rumen degradable protein). They are mainly amylolytic bacteria. They grow relatively fast and have a high maintenance requirement. Their retention time in the rumen is short because they leave the rumen in the fluid phase. 3. Protozoa in the rumen fluid: They live off the rumen bacteria, particularly those (the starch and sugar fermenters) in the fluid part. The large protozoa are the first to disappear during insufficient roughage intake when rumen pH falls below 5,8 - 6,0. They release ‘plant root-available’ nitrogen and phosphorous. 4. A small group of bacteria adhering to the papillae of the rumen wall: These are in particular responsible for the hydrolysis of dietary and salivary urea to ammonia. They play an indirect role in fibre digestion by making ammonia available to cellulolytic bacteria. Rumen pH pH is the scale by which we measure the degree of acidity of alkalinity of a fluid or substance. A 5