Milk Synthesis & Energy for Lactation

Milk Composition

  • Complete food; only low in Fe & vitamin D
  • Gross composition (cow): 87.3\%\,H_2O, 3.9\%\,fat, 3.2\%\,protein (casein + whey), 4.6\%\,lactose, 0.7\%\,ash
  • Isosmotic with blood → balance between diffusible ions (proteins) & lactose; ↑protein ⇄ ↓lactose
  • Fat content varies widely across species; reflects neonatal needs (insulation, energy, brain growth, etc.)

Precursors Delivered by Blood

  • Glucose, AA, NEFA/TG, acetate, butyrate, minerals, water
  • Determined by blood flow rate & nutrient concentration (diet, tissue mobilisation)

Secretory Cell Organelles & Roles

  • Cell membrane (GLUT-1 glucose transport)
  • Cytosol (glycolysis, FA chain elongation)
  • RER (protein translation; de-novo FA)
  • Golgi (casein micelle assembly, lactose synthesis, packaging)
  • Mitochondria (ATP)
  • Secretion routes: exocytosis (protein/lactose), milk-fat globule budding, transcytosis (Ig), paracellular (pathological)

Lactose Synthesis

  1. Glucose → G6P \xrightarrow{phosphoglucomutase} G1P
  2. G1P+UTP \rightarrow UDP!\text{-}glucose
  3. Epimerase → UDP-galactose
  4. Golgi lactose synthase (UDP-galactose + glucose) → lactose
  • Glucose trapped as G6P; three fates: lactose (galactose), glycolysis/ATP, pentose-P (NADPH)

Protein Synthesis

  • AA from blood/muscle → ribosome (mRNA blueprint) → RER
  • Caseins assembled in Golgi → micelles
  • Whey proteins (α-lactalbumin, β-lactoglobulin) largely synthesised similarly

Milk Fat Synthesis

  • Precursors: acetate, \beta-hydroxybutyrate, glucose (glycerol)
  • Triglycerides formed in ER; globules bud from apical membrane
  • Readily altered by diet lipid profile

Metabolic Interactions

  • Limited glucose pool: ↑lactose ↓fat/protein (and vice-versa)
  • NADPH from pentose-P & isocitrate pathways drives FA synthesis

Energy Requirements (MJ ME)

  • To produce 1\,\text{kg} milksolids (MS): 62!\text{–}!65\,\text{MJ ME} (breed dependent)
  • Daily ME example: maintenance 64 + 2\,\text{kg MS} \times 65 – weight loss 0.25\,\text{kg}\times32 ≈ 186\,\text{MJ ME}

Maintenance Energy

  • Basal heat: 0.32\times BW^{0.75} (MJ d⁻¹)
  • Major costs: ion transport (30–40 %), protein turnover (9–12 %), organ work (heart 9–11 %, kidney 6–7 %, resp. 6–7 %)
  • Activity increments: standing ↑10 kJ/d, walking level 2.6 kJ/km, vertical 28 kJ/km

Lactation Energy

  • Energy for synthesising & retaining fat, protein, lactose
  • Efficiency declines as lactation progresses
  • Factors altering ME cost: stage of lactation & feed energy density

Negative Energy Balance & Ketosis

  • Early lactation: DMI limited → NEB; mobilisation of adipose (NEFA) & muscle (AA)
  • Lipolysis products → acetate, \beta-hydroxybutyrate; excess → ketosis (↓pH, acetone odour, ↓appetite)
  • Prevention: high-energy ration, glucose precursors, optimal BCS pre-calving

Key Review Points

  • List precursors & cell locations for synthesis of lactose (glucose → Golgi), protein (AA → RER/Golgi), fat (acetate/NEFA → ER)
  • GLUT-1 + phosphorylation traps glucose
  • Fat is the milk component most easily altered by diet
  • Energy functions: maintenance, activity, milk, pregnancy, body condition change
  • Maintenance affected by BW, activity, environment
  • NEB highest in early lactation; ketosis = excess ketone bodies due to low glucose