Organs and their role
Rumen
Microbial fermentation chamber. Microbes ferment structural (fibre) and non-structural (starch, sugars) carbohydrates into volatile fatty acids (VFA), the cow’s main energy source.
Reticulum
Sorting and mixing with the rumen. Works together with the rumen: contractions mix digesta and move small, dense particles towards the omasum. Its honeycomb lining traps heavy objects.
Omasum
Water and VFA absorption. Leaf-like folds absorb water, minerals and remaining VFA before digesta enters the abomasum.
Abomasum
True (glandular) stomach. Acid and enzymes start digestion of microbial and undegraded feed protein, as in monogastric animals.
Small intestine
Digestion and absorption of protein, starch and fat that escape the rumen. Metabolizable protein (microbial protein + RUP) is digested here and absorbed as amino acids.
Liver
Metabolic hub: glucose production and fat handling. Ruminants absorb little glucose; the liver makes most glucose from propionate (and some amino acids, lactate, glycerol).
Mammary gland
Synthesis of milk fat, lactose and protein. Acetate and β-hydroxybutyrate are used for de novo fatty acid synthesis; preformed fatty acids come from diet and body fat.
Adipose tissue
Energy store and buffer. Body fat is stored when energy intake exceeds needs and mobilised as NEFA in negative energy balance, mostly around calving.
Hoof
Claw health linked to nutrition and housing. Laminitis has been associated with rumen acidosis; zinc, biotin and other nutrients support horn quality. Housing and standing time also matter.
Physiological modes
- Normal cow: Mid-lactation, balanced ration: fermentation, hepatic glucose production and milk synthesis are in a steady state.
- Close-up: Last 3 weeks before calving: intake tends to fall, the udder prepares for lactation and calcium demand is about to rise sharply.
- Early lactation: Milk rises faster than intake: negative energy balance, fat mobilisation (NEFA) and higher liver load.
- SARA scenario: High rapidly fermentable starch with too little effective fibre: acids accumulate faster than they are removed. Factors worth investigating include milk fat depression, loose manure and lameness.