Reproductive Physiology – Control of Birth Weight & Reproductive Consistency

Email correction: Thursday’s lecture is in room S-2, not FR C.
Tutorial / farm‐oriented session scheduled for Wednesday (field-trip day); attendance encouraged.

Birth weight influences:
- Neonatal survival (dystocia if too big, hypothermia if too small).
- Post-natal growth trajectories and lifetime productivity.
- Dam welfare (over-conditioned vs excessively catabolic ewes/cows).
Practical difficulty: fetuses are “efficient parasites” that protect their nutrient supply; extreme maternal under-nutrition is required before birth weight moves noticeably.

Treatments (ME = metabolisable energy):
- Singles: 14,\;10,\;8\;\text{MJ d}^{-1}.
- Twins: 16,\;11,\;10\;\text{MJ d}^{-1}.
Key observations (weeks pre-partum):
- Medium feed (≈10 MJ) → maternal live weight static, yet true body weight ↓ ≈ 8\;\text{kg} (conceptus growth masked loss).
- Low feed (≈8 MJ) → ewe live weight ↓ \approx20\;\text{kg} (≈25\% of original 65\text{–}70\;\text{kg}), birth weight only fell from 5.99\;\text{kg}\rightarrow5.90\;\text{kg}.
- Twins mirrored pattern; need severe restriction before lamb BW drops.
Practical implication: Screwing singles down “to prevent dystocia” demands unsustainably harsh deficits, leaving ewes unfit for lactation.

Single pregnancy: fetus 5\text{–}6\;\text{kg} + placenta/fluids 2\text{–}3\;\text{kg} \Rightarrow \approx8\text{–}10\;\text{kg} total.
Twins: 10\text{–}13\;\text{kg} conceptus mass.

Unequal horn distribution → early embryonic/fetal loss evidenced in post-mortem counts.

Type A: cotyledon sits inside caruncle.
Under nutrient stress, cotyledon grows around & over caruncle → Types B, C, D.
Effect: ↑ surface area → ↑ transfer efficiency, partially buffering fetal nutrition.
Result: Moderate maternal underfeeding barely shifts BW until dam is dangerously catabolic.

Maternal–fetal AA flux highlights arginine as limiting.
Trials: lipid-coated arginine vs control (pasture diet).
- Placenta number: no change (≈100).
- Total cotyledon weight: ↑ with arginine.
- Avg. cotyledon mass: ↑.
- Fetal membrane wt: ↔.
- Birth-weight response: ♀ lambs ↑, ♂ lambs ↔ (sexual dimorphism).
Mechanism: arginine up-regulates genes for AA transport into fetus.
Practicality: rumen-protected AA is expensive; simpler to ensure adequate rumen-degradable protein in late gestation.

Last 4–6 weeks = highest fetal growth rate (placental plasticity largely set).
Avoid low-protein crops (e.g., fodder beet) as sole diet:
- If crop retained, supply \ge2\;\text{g RDP kg}^{-1}\text{DM} via baleage, silage, or concentrate.
Monitor BCS; prevent losses >0.5 units pre-lambing.

Reproductive success: performance this season (conception rate, litter size, live births).
Reproductive consistency: ability to repeat success next season (year-on-year reliability).
- Driven by: Body Condition Score (BCS), disease, genetics, lactational catabolism & repletion.

Beef cows
- Goal: calving interval ≤365 d.
- Post-partum anoestrous interval (PPAI) ↑ sharply when BCS <4/9.
Sows
- Back-fat depth <12\;\text{mm} at weaning → weaning-to-oestrus interval ↑, litter size ↓.
Ewes
- BCS and feeding during lactation jointly affect ability to re-breed.

Two BCS groups at lambing (2 vs 3) × four pasture masses.
Weight of lamb weaned at 79 d:
- Feed level effect (Low→High) ++.
- BCS effect minimal when feed equal; current intake > stored fat.

Categorised >1{,}000 ewes by BCS at: mating, scan, lambing, set-stocking, weaning.
Only BCS at weaning correlated with weight-of-lamb-weaned:
- BCS <3 → 55\;\text{kg lamb ewe}^{-1}.
- BCS >3.5 → 47\;\text{kg}.
Interpretation: the “skinniest at weaning” had mobilised most tissue → best milk yield.

ΔLive-weight (mating→weaning) vs lamb-weight-weaned.
Negative slope for singles, twins, triplets: greater maternal loss → heavier lamb crop.

Desirable genotype/management:
- Mobilise fat during lactation (supports milk).
- Regain ≥1 BCS unit before next mating.
Requires strategic nutrition post-weaning; failure → compromised consistency.

Pathogens (e.g., Campylobacter, Toxoplasma) can erode consistency; vaccines mitigate.
Cross-breeding improves PPAI, conception, and survival owing to heterosis.

Moderate feed restriction rarely lowers birth weight; severe restriction harms dam more than fetus.
Placental plasticity (Types B–D) is fetus’s built-in defence; don’t rely on it.
Supply adequate protein (especially arginine precursors) in last trimester; easiest via mixed pasture or legume supplements.
Monitor BCS at weaning: skinny ewes are often top milkers but need priority re-feeding for next cycle.
Target BCS gains post-weaning: +0.5\text{–}1.0 units before mating to preserve reproductive consistency.
Use hybrid vigour and vaccination to buffer against biological shocks.

Heritability of birth-weight consistency: archival lambing records could reveal genetic variance in dam’s ability to deliver uniform BW year-after-year, but data remain un-mined.