A typical lactation curve of a dairy cow:

Question

BlackTom AI · Accepted Answer

A dairy cow’s milk yield typically increases after calving, reaches peak production at about 4–8 weeks (commonly cited as ≈6 weeks), and then gradually declines for the remainder of the lactation.

1) Relevant knowledge points and concepts
- Lactation curve: pattern of milk yield over time after calving (fresh period → peak → decline/persistency).
- Physiology: rising prolactin, growth hormone effects and mammary secretory cell activity drive the early increase; negative energy balance around calving and gradual reduction in secretory activity contribute to the post-peak decline.
- Peak timing: most dairy cows peak in milk yield within the first 4–8 weeks; 6 weeks is a typical average.

2) Step-by-step reasoning
- Immediately after calving, mammary gland is highly active; milk synthesis capacity and hormonal support cause a rapid rise in yield.
- Secretory cell activity and numbers, together with feed intake and metabolic state, reach maximal output in the early lactation window (around 6 weeks).
- After peak, natural physiologic changes (reduced synthetic activity per cell, changing hormonal milieu, and management factors) cause a steady decrease in daily yield — the curve “declines” but at a rate described as persistency.

3) Calculations/intermediate steps
- No numeric calculation required; timing is descriptive: rise (weeks 0–6) → peak (≈week 6) → decline (weeks 6+ through lactation).

4) Why the correct answer is correct
- It matches standard lactation biology and empirical herd production data showing an early post-calving peak followed by a progressive decline in yield.

5) Why the other options are incorrect
- “Rises and falls at monthly intervals”: inconsistent with physiology; yield does not oscillate monthly.
- “Declines after calving, then rises steadily”: opposite of observed pattern (initial rise is expected).
- “Rises after calving to the maximum … and remains”: milk yield does not remain at peak; secretory activity and management lead to decline.

6) Conclusion
The correct choice is: “Rises after calving, then peaks around 6 weeks after calving, then declines steadily over the rest of the lactation,” which matches the provided answer.

A typical lactation curve of a dairy cow:单项选择题

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