Animal Nutrition, Digestion & Lactation – Condensed Exam Notes

Nutritional Needs & Energy

• 3 diet functions: Fuel (ATP), Raw Materials (biosynthesis), Essential Nutrients (co-factors, vitamins, trace elements)

• $ATP$ produced via oxidation of dietary CHO, fat, protein ➔ drives basal metabolism, thermoregulation, nerve impulses

• ATP is not a storage molecule; surplus energy stored as glycogen, fat, or body tissues

Digestion – Key Stages

• Ingestion ➔ Digestion (chemical & mechanical) ➔ Absorption ➔ Elimination

• Digestive tract roles: transport, digestion, absorption, synthesis (microbial), excretion

Digestive Tract Types

• Monogastric (simple stomach); limited fibre digestion; fermentation (if any) in hind-gut

• Ruminant (four-compartment fore-gut): rumen, reticulum, omasum, abomasum; extensive pre-gastric fermentation

• Fermentation site: pre-gastric (ruminant, kangaroo) vs post-gastric (horse, pig)

• Gut capacity trends: Carnivore stomach > SI = LI; Herbivore LI >> SI >> stomach; Ruminant stomach $\approx 70\%$ of capacity

Ruminant Compartments

• Rumen – microbial vat; VFA & $NH_3$ absorption via papillae
• Reticulum – bolus formation, hardware trap
• Omasum – water & VFA absorption, particle size <$1{-}2\,\text{mm}$
• Abomasum – true acid/enzyme stomach secretes HCI/enzymes
• Young ruminant: reticular groove shunts milk to abomasum; functionally monogastric

Major Digestive Enzymes / Secretions

Hormonal Control (Monogastric)

• Gastrin (stomach) ➔ gastric juice

• Secretin (duodenum acid) ➔ pancreatic $HCO_3^-$

• CCK (fat/AA in duodenum) ➔ pancreatic enzymes & bile

• Enterogastrone (fat-rich chyme) ➔ ↓ gastric motility/acid

Digestion of Major Nutrients

Carbohydrate

• Monogastric: mouth & SI enzymes ➔ monosaccharides; hind-gut fibre fermentation ➔ VFA (minor)

• Ruminant: rumen microbes convert CHO ➔ VFA (acetate, propionate, butyrate) + gas; only residual starch reaches SI
  Protein

• Monogastric: pepsin (stomach) ➔ pancreatic & brush-border proteases ➔ AA absorbed

• Ruminant: dietary protein split to ammonia & peptides; microbes synthesise microbial protein; undegraded intake protein (UIP) bypasses rumen; true digestion in abomasum/SI
  Lipids

• Monogastric: bile emulsifies; lipase ➔ FA + monoglyceride; absorbed as micelles, re-esterified ➔ chylomicrons (lymph)

• Ruminant: lipolysis & biohydrogenation in rumen; FAs absorbed as in monogastric; glycerol ➔ VFA in rumen

Absorption Mechanisms

• Simple diffusion (VFA, $NH_3$)

• Facilitated diffusion (e.g. $GLUT$ transporters)

• Active transport (AA, glucose + $Na^+$)

• Lipid: micelle diffusion ➔ chylomicron ➔ lymph; <12-C FA enter portal blood directly

Post-Absorptive Fate

Simple sugars
• Oxidation (ATP) \; • Glycogen synthesis \; • Fat & cholesterol synthesis \; • Pentose phosphate pathway (NADPH)
VFA (ruminant)
• Acetate/Butyrate ➔ fat or TCA cycle \; • Propionate ➔ hepatic gluconeogenesis
Amino acids
• Tissue & plasma proteins \; • Nucleotides \; • Catabolism (gluco- vs keto-genic) \; • Ruminant $NH_3 \rightarrow$ urea (recycle/excrete)
Fatty acids
• TAG storage (adipose) \; • Milk fat \; • $\beta$-oxidation (ATP) \; • Ketone, cholesterol, bile, lipoproteins

Feed & Diet Evaluation (Proximate Analysis)

• Dry Matter = weigh, oven $100^{\circ}C$

• Ash = muffle $600^{\circ}C$ ➔ $\%OM = 100 - \%Ash$

• Kjeldahl $N \times 6.25$ ➔ Crude Protein

• Ether Extract = crude fat

• Crude Fibre = residue after acid/alkali boil

• $NFE = 100 - (\text{Ash+CP+EE+CF})$ (carbohydrate)

• Van Soest: $NDF$, $ADF$, lignin fractions for fibre quality

Mammary Gland Development (Mammogenesis)

Fetal
• Ectoderm (secretory) & mesoderm (stroma) form mammary bud → ducts/cisterns; teat canal keratinises pre-birth
Birth ➔ Puberty
• Isometric growth; mainly ducts, stroma, fat pad development
Puberty
• Allometric (4:1) duct growth driven by $GH$, oestrogen, progesterone
Pregnancy
• Exponential alveolar proliferation; late gestation differentiation for milk synthesis; greatest growth in 1st pregnancy
Lactation
• Lactogenesis at/near parturition; requires milk removal to maintain secretion
Involution
• Gradual (late lactation), initiated (weaning/dry-off), or senile; apoptosis, immune factors; dry period $\approx 40{-}60\,\text{d}$ for tissue renewal

Mammary Anatomy & Function

• Secretory alveolus: epithelial cells produce milk; surrounded by myoepithelial cells (oxytocin sensitive); vascular supply delivers precursors $\approx 400{-}500\,\text{L blood}$ per $1\,\text{L milk}$

• Ducts ➔ gland cistern ➔ teat cistern ➔ streak canal (keratin plug, primary defence)

• Support: median & lateral suspensory ligaments; weakness leads to pendulous udder & mastitis risk

• Lymphatic drainage prevents oedema; moved by muscle action & one-way valves

• Neural: afferent teat receptors trigger oxytocin via CNS; efferent control blood flow

Mastitis Essentials

• Inflammation due to bacteria: $Strep\,uberis$ (environment), $Staph\,aureus$ (contagious), etc.

• Signs: ↑ somatic cell count (>150\,000\,\text{cells}\,/\,mL), clots, swollen quarters, ↓ yield (≈ $1{-}2\%$ per $+100\,000$ SCC)

• Defences: keratin plug, leukocytes, teat spray; management – hygiene, dry-cow therapy, cull chronics

Key Numbers to Memorise

• Blood:milk flow $\approx 500:1$ (e.g. $30\,\text{L milk} \rightarrow 15{,}000\,\text{L blood day}^{-1} \approx 10.4\,\text{L min}^{-1}$)

• Rumen capacity: cow $160{-}200\,\text{L}$, sheep $5{-}10\,\text{L}$

• Critical particle size to exit rumen: cattle <2\,\text{mm}; sheep/goat <1\,\text{mm}

• Protein $N$ conversion: $\%CP = \%N \times 6.25$ (based on $16\%$ N in protein)