A six-cow dairy with poor conception and 9 litres: where should the money go?
A peri-urban dairy keepers' group owns six Friesian-cross cows averaging 9 litres/day. The cows are hand-milked twice daily and run on natural pasture for four hours a day. For the rest of the day they receive maize stover ad libitum plus 2 kg/day of maize meal, and no mineral supplement. Body condition scores are 2.0–2.5 out of 5. The last three calvings averaged 470 days apart, first-service conception is 30 %, and two cows have shown no heat for 70 days after calving. Water is available from a trough 25 m from the kraal, but the trough is often dry by afternoon and the cows are then walked back to the kraal. The group has a small grant, but only enough to change one thing properly this season.
1. Rank the limiting factors in this herd, with reasons for the order you choose. Say explicitly whether genetics can be blamed.
Read the worked answer
Ranking
- Energy and protein intake - first. Maize stover is only about 4–5 % crude protein and 45–50 % digestible, and it passes slowly through the rumen, so intake is limited by fill and by slow fibre digestion; total dry matter intake may be only 1.8–2.0 % of body weight. Maize meal supplies energy but almost no protein, so the rumen microbes lack the ammonia they need to break down the stover. Two kilograms of maize meal against 9 litres of milk leaves a large deficit, and that deficit explains body condition 2.0–2.5, negative energy balance after calving, the 70-day anoestrus and 30 % conception almost exactly.
- Water - second. A milking cow needs roughly 60–100 litres a day. A trough that is dry by afternoon limits intake, cuts milk and deepens the energy deficit. Fixing it is cheap, which is why it ranks high on return even though it is not the biggest single constraint.
- Minerals - third, but cheapest. No mineral block means no supplementary salt, phosphorus or trace elements; intake, fertility and bone strength suffer. It will not, however, produce milk on its own.
- Heat detection and service timing - third equal. Two cows silent for 70 days may have been cycling and missed. Without records, heats and services cannot be checked.
Genetics cannot be blamed. Nine litres from a Friesian cross on this ration is about what the diet supports; the cows are not the problem.
2. Given enough money for only one proper intervention, choose it and justify the choice with a marginal (break-even) calculation. State the assumptions and how the group would check the result.
Read the worked answer
Choice: a protein-and-energy concentrate (12–14 % CP dairy meal or an oilseed cake), with the water trough made reliable as a low-cost add-on.
The marginal calculation, using the group's own prices:
- 1 kg of 12 % CP dairy meal costs 6.00 currency units.
- A realistic response on this poor base diet, in mid-lactation, is 1.0–1.5 extra litres per kg fed, up to about 3 kg/day.
- Milk is 5.50 units per litre.
- At 1.5 litres response: extra income 1.5 × 5.50 = 8.25 units; feed cost 6.00; margin +2.25 per cow per day. Across six cows that is about 13.50 units a day.
- Break-even response = 6.00 ÷ 5.50 = 1.09 litres per kg. The feed pays as long as each kilogram returns more than about 1.1 litres.
Assumptions to state: the response is not guaranteed; cows close to drying off or already pregnant will not respond; feed prices and milk price are the group's actual figures, not assumed ones.
How to check it: feed 1 kg/day to the three poorest cows for three weeks and keep nothing else changed; record milk per cow per day and condition score fortnightly. If the response is above 1.09 litres per kg, extend to all six cows and consider a second kilogram. If it is below break-even, stop and put the money into the water supply and a mineral block instead. A mineral block (about 1 unit per cow per week) should be bought alongside because it is cheap and complements the ration.
3. A committee member argues that doubling the maize meal to 4 kg/day will fix the conception rate. Explain why this is unlikely to work and what would actually improve fertility.
Read the worked answer
Why more maize meal alone will not fix conception
- Maize meal is mostly starch (about 8–9 % CP) and is rapidly fermented. It supplies energy but no protein, so the rumen microbes still cannot digest the stover efficiently; fibre digestion slows, intake falls and the extra starch is poorly used.
- Excess starch relative to fibre lowers rumen pH, risking sub-acute acidosis, reducing fibre digestion and butterfat, and depressing intake. The cow may look better in condition while still mobilising body reserves to make milk.
- Conception returns only when the cow stops losing weight. A cow at condition 2.0 giving 9 litres is in negative energy balance; the hormone pattern (low LH pulses, poor follicle development) blocks heat and conception. Adding starch without protein does not close that gap.
What would work
- A balanced ration with a genuine protein source (oilseed cake, legume hay, or a properly formulated dairy meal) plus a mineral block and reliable water.
- A planned 60-day dry period and a target condition score of 2.75–3.0 at calving; monitor monthly by condition scoring and keep milk records.
- Deliberate heat detection: observe twice daily for 20–30 minutes, use a tail-paint or chalk, and keep a service calendar; check the bull or inseminator's timing (12 hours after standing heat).
- If conception is still poor after three months of adequate feeding and good detection, then investigate infectious causes such as brucellosis and check the bull's fertility.