Milk fat depression and suspected subclinical acidosis in a crossbred dairy herd
A 24-cow herd of Friesian × local crossbreds averages 16 L/day. The ration is 9 kg/day of an 18% CP concentrate (maize, wheat bran, coconut poonac) fed in two meals, with 12 kg fresh hybrid Napier and 3 kg rice straw. Over three weeks milk fat has fallen from 3.8% to 2.9%, milk yield has risen by 1.2 L/day and the processor has started applying a fat penalty. Six cows sampled had a rumen pH of 5.55 four hours after the morning concentrate. Faeces are loose and contain whole maize grains, cud chewing has dropped from about 60 to 35 chews per bolus and two cows are lame. The farmer's plan is to add 100 g of sodium bicarbonate per cow per day and to raise concentrate to 11 kg/day 'to protect the yield'. Dry matter contents are concentrate 87%, Napier 18% and straw 90%; the extension target is 40–45% concentrate in total dry matter.
1. Calculate the concentrate proportion on a dry matter basis and explain the mechanism behind the drop in milk fat.
Read the worked answer
Dry matter calculation
- Concentrate: 9 × 0.87 = 7.83 kg DM
- Napier: 12 × 0.18 = 2.16 kg DM
- Straw: 3 × 0.90 = 2.70 kg DM
- Total intake: 12.69 kg DM
- Concentrate share: 7.83 ÷ 12.69 = 61.7% - far above the 40–45% target.
The rumen pH of 5.55 at four hours after feeding confirms subclinical acidosis (the usual threshold is below 5.8 for a sustained period).
Mechanism of the fat depression
A high-starch, low-fibre diet is fermented rapidly to propionate and lactate, which drops rumen pH and shifts the microbial population. Fibre-digesting bacteria are inhibited, so acetate, the main precursor of milk fat, falls. At the same time, the more acidic rumen environment changes the biohydrogenation pathway of unsaturated fatty acids, producing trans-10, cis-12 conjugated linoleic acid instead of the normal intermediates. That isomer acts directly on the mammary gland and down-regulates the enzymes of milk fat synthesis. The result is the classic pattern seen here: milk volume rises because more propionate is available for lactose and energy, while fat percentage falls.
The loose manure with whole maize grains, reduced cud chewing and lameness all fit: incomplete starch digestion, loss of the rumen fibre mat, and laminitis from the acidosis and endotoxin release. The problem is a ration composition problem, not a cow problem.
2. Evaluate the farmer's two proposals and give a revised ration for the 24 cows, with figures.
Read the worked answer
The bicarbonate proposal is a partial symptomatic measure. A rate of 0.75–1% of dry matter for a 13–14 kg intake is 100–140 g, so 100 g is defensible as a buffer and will take some of the edge off the pH drop. But a buffer does not restore the fibre mat, does not restore cud chewing and does not stop the trans-10, cis-12 shift. On its own it would mask symptoms while the underlying problem continues.
Raising concentrate to 11 kg/day is wrong. It would take concentrate DM to 9.57 kg in about 14.4 kg total, roughly 66% of intake, which worsens the acidosis, the fat depression and the lameness. Chasing volume at the expense of fat percentage and cow health is also financially poor: the fat penalty plus the cost of the extra feed and of treating two lame cows will outweigh the extra 1.2 L.
Revised ration (per cow per day)
- Concentrate: 7 kg (6.09 kg DM), fed in three or four meals rather than two, so no single meal exceeds about 2 kg
- Hybrid Napier: 30 kg fresh (5.4 kg DM), chopped to 3–5 cm and fed after milking
- Rice straw: 3 kg (2.7 kg DM), which supplies effective long fibre
- Total: 14.19 kg DM, concentrate share 43%, inside the 40–45% target
- Sodium bicarbonate 150 g/cow/day during the transition, then review
- Free access to clean water; a mineral mixture covering calcium, phosphorus and salt
Transition: reduce concentrate by about 1 kg every three days while increasing forage, over 10–14 days, so the rumen population adapts. Watch dry matter intake; if it falls, slow the change. Yield may dip by about 1 L initially, but fat percentage, lameness and the penalty should improve within two to three weeks, which more than compensates.
3. What would you measure over the next three weeks to confirm the correction has worked, and what thresholds would you use?
Read the worked answer
Monitoring has to cover the rumen, the product and the cow, because fat percentage alone lags behind. Set these indicators and targets:
Rumen and feeding behaviour
- Rumen pH in the same six cows, same time of day: target above 5.8 and no more than about 30% of animals below 5.8. Measuring at four to five hours after the concentrate meal is the critical window.
- Cud chewing: target 55–65 chews per bolus and 60% or more of resting cows ruminating; this is the cheapest, most direct indicator of effective fibre.
- Feed refusals: should be under 5%; a sudden rise suggests the forage change is too fast.
Manure
- Manure scoring to a simple 1–5 scale: target the middle scores, with no more than 10% in the watery category.
- Sieve or wash samples weekly: whole maize grains should largely disappear as starch digestion improves.
Milk
- Daily bulk fat and protein percentage: target fat back to at least 3.5% within three weeks, with the fat-to-protein ratio returning towards 1.2.
- Milk yield: accept a small, temporary dip of up to 1 L; a sustained drop of more than 2 L means the energy supply is too low and the concentrate should be restored to 8 kg.
- Bulk milk urea, if available, to check that protein is not being over-fed.
Cow level
- Lameness score the two affected cows weekly and record locomotion; treat them early.
- Body condition score fortnightly: the aim is to hold condition, not lose it.
Decision rule: if fat percentage has not risen after two weeks and rumen pH is still below 5.8, the forage intake or chop length is not delivering enough physical fibre, and the concentrate should come down another 1 kg rather than adding more buffer. If fat recovers but yield has dropped by more than 2 L, rebalance energy with a digestible fibre by-product rather than starch.