Inside the Cow

Follow starch

1. Ingestion: Starch enters with grains and corn silage. Processing (grinding, flaking, kernel processing) changes how quickly it can be fermented.
2. Rumen: Amylolytic bacteria ferment most dietary starch. Propionate is the main glucogenic product; fast fermentation also raises acid load.
3. Absorption: Propionate is absorbed across the rumen wall and carried to the liver in the portal blood.
4. Liver: The liver converts propionate into glucose. In ruminants this is the main route of glucose supply.
5. Mammary gland: Glucose is used to make lactose. Because lactose draws water into milk, it is an important determinant of milk volume.

Follow fibre

1. Ingestion & chewing: Forage fibre stimulates chewing; saliva contains bicarbonate and phosphate that buffer rumen acids.
2. Rumen: Cellulolytic bacteria and fungi slowly degrade cellulose and hemicellulose; acetate is the main product. Rumination reduces particle size.
3. Absorption: Acetate is absorbed and largely passes the liver unchanged, reaching peripheral tissues.
4. Mammary gland: The udder uses acetate and β-hydroxybutyrate to synthesise short- and medium-chain fatty acids, a major contribution to milk fat.

Follow protein

1. Dietary protein: Feed protein is partly degradable in the rumen (RDP) and partly undegradable (RUP).
2. Rumen: Microbes use RDP-derived nitrogen with fermentable energy to build microbial protein. Excess ammonia is absorbed and converted to urea.
3. Abomasum: Microbial protein and RUP form metabolizable protein; digestion starts in the abomasum.
4. Small intestine: Amino acids such as lysine and methionine are absorbed into the blood.
5. Metabolism & milk protein: The udder uses absorbed amino acids to synthesise milk protein; amino acids are also used by other tissues.

Follow fat

1. Dietary fatty acids: Fat comes from forages, oilseeds, by-products and fat supplements.
2. Rumen: Microbes release fatty acids and saturate unsaturated ones. Altered biohydrogenation produces intermediates linked to milk fat depression.
3. Intestine & absorption: Fatty acids are absorbed in the small intestine and packaged into lipoproteins for transport.
4. Tissues & mammary gland: Preformed long-chain fatty acids are taken up by the udder for milk fat or stored in adipose tissue.

Follow calcium

1. Dietary calcium: Calcium is supplied by forages (especially legumes) and mineral sources.
2. Absorption: Calcium is absorbed mainly from the small intestine; active absorption is regulated by vitamin D metabolites.
3. Bone reserve: When blood calcium falls, parathyroid hormone mobilises bone calcium and increases absorption. Metabolic alkalosis can blunt this response (Goff, 2008).
4. Colostrum & milk: Colostrum and milk suddenly draw large amounts of calcium at calving, the reason hypocalcaemia risk peaks then.