1. a Institute for Hygiene and Ergonomics , Swiss Federal Institute of Technology , Clausiusstrasse 21, 8092, Zurich, Switzerland
2. b Institute for Animal Hygiene and Animal Protection , Hannover School of Veterinary Medicine , Bünteweg 17p, 3000, Hannover 71, F.R. of Germany
3. c Verein Deutscher Ingenieure , Graf‐Recke‐Strasse 84, 4000, Düsseldorf 1, F.R. of Germany
4. d AFRC Institute for Grassland and Animal Production , Hurley, Maidenhead, Berkshire, SL6 5LR, UK
5. e Institute of Agricultural Engineering (IMAG) , Mansholtlaan 10–12, Wageningen, The Netherlands