From Dan Griscom
You report a research finding that giving cows a genetically engineered hormone called recombinant bovine somatotropin (rBST) “cuts their emissions of the potent greenhouse gas methane by 7 per cent per litre of milk” (5 July, p 6).
I understand that the average Holstein cow produces 21 litres of milk and 280 litres of methane each day. Such a cow, when given rBST, would appear to start absorbing methane at tea-time on the first day of milk production. After only a year it would be absorbing more than 150,000 litres of methane per day, though the barns would get windy. Fewer than 8 million such cows could absorb the total global production of methane from all sources.
The editor writes:
• Yes, it would have been more precise to say “by 7 per cent for a given daily milk production”. But it didn’t fit.
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Melrose, Massachusetts, US
