Milk Fat Collapse After Maize Top-Dress in a Crossbred Dairy Herd
A 40-cow crossbred dairy unit near Ludhiana milks 22 cows twice daily. Eight high-yielding cows averaging 22 kg/day were fed 3 kg crushed maize at each milking for the last 6 days because maize was cheap. The total ration is wheat straw 4 kg, maize silage 12 kg, and a 9 kg concentrate mixture (maize 45%, mustard cake 20%, wheat bran 20%, rice polish 10%, mineral mixture 5%) per cow per day. Within 5 days, herd milk fat fell from 4.1% to 3.2%, three cows became lame, and dung is loose with visible grain. The rumen pH of two cows is 5.6 and 5.7; rumen motility is 1 per minute. Body temperature is normal. The owner wants to add sodium bicarbonate and continue the same concentrate. The farm has wheat straw, maize silage, green berseem, crushed maize, mustard cake, wheat bran, mineral mixture, and sodium bicarbonate. No forage analysis is available.
1. What is the most likely diagnosis, and which feeding change precipitated it? Explain the rumen mechanism and why milk fat fell.
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The herd picture is subacute ruminal acidosis, not a simple fibre deficiency. The trigger is the sudden top-dress of 6 kg crushed maize per cow per day on top of an already starchy 9 kg concentrate. Rapidly fermented starch increases volatile fatty acid production faster than the rumen can absorb or buffer it, so pH falls to 5.6-5.7. At that pH, fibre-digesting bacteria and protozoa are depressed, rumen motility falls, and acetate production drops relative to propionate. A lower acetate-to-propionate ratio, together with altered rumen biohydrogenation of fatty acids, explains the fall in milk fat from 4.1% to 3.2%. Loose dung with visible grain shows that starch escaped rumen fermentation and reached the hindgut, while laminitis-type lameness is consistent with historical or repeated acidosis. The normal temperature argues against an infectious cause. Adding sodium bicarbonate while continuing the same starch load would only mask the problem; the first step must be to remove the extra maize and restore effective fibre.
2. Prepare an immediate 7-day correction and a longer ration strategy using only the feeds on hand. Give practical amounts and explain your priorities.
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Immediate 24-48 hours: stop all crushed maize top-dress; do not feed more than 2 kg concentrate at one time; offer wheat straw and green berseem first; give sodium bicarbonate 100-150 g/cow/day divided between feeds; keep water clean and available. For a 22 kg cow, start with 5 kg concentrate, 20-25 kg green berseem, 10-12 kg maize silage, and wheat straw ad libitum, split concentrate into three feeds. Increase concentrate by 0.5 kg every third day only if dung consistency and rumen motility improve, aiming for about 7 kg by day 14; do not return to 9 kg until rumen pH and milk fat have been stable for at least a week. The priorities are: restore rumen pH, slow starch intake, maintain fibre intake, and avoid a second abrupt change. Maize silage is acidic and should not be increased sharply; berseem supplies protein and buffering precursors. Mineral mixture should continue at 50-100 g/day. Because no forage analysis is available, judge the response by dung, rumen motility, milk fat, and lameness rather than by a calculated ration alone.
3. Which measurements over the next 2 weeks would tell you whether the correction is working, and when would you request veterinary examination?
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Monitor the whole herd and the eight high yielders daily: dung score and visible grain, rumen motility, milk yield, and milk fat at each bulk collection. Record lameness scores twice weekly and check rumen pH in a representative sample of 5-8 cows using rumenocentesis or a reliable oral sample, not just one cow. A rising milk fat trend, firmer dung, rumen motility above 2 per minute, and fewer lame cows over 7-10 days indicate correction. Request veterinary examination if any cow has rumen pH below 5.5 on repeat sampling, fever, complete rumen stasis, blood in dung, severe lameness, or a continuing fall in milk yield. Also ask for examination if milk fat does not improve after 10-14 days, because that may indicate a separate problem such as poor fibre length, fat supplementation, or subclinical mastitis. The aim is to catch the transition from subacute acidosis to acute acidosis or laminitis before it becomes irreversible.