Studies in grazing behaviour of dairy cattle: II. Bloat in relation to grazing behaviour

1. Four trials on methods of bloat-prevention vare described: ( a ) A comparison of ‘break’ grazing and normal ‘rotational’ grazing without restriction of feed in either case. Break grazing observations covered two and five breaks per day. ( b ) A comparison of these two systems where ‘break’ grazing involved restriction of area available to the cows, and ‘breaks’ varied from seven to eleven per day. ( c ) Comparison of ‘off and on’ grazing with normal rotational methods in an attempt to force changes in grazing pattern. ( d ) Comparison of the same pasture grazed at different lengths under rotational grazing methods. 2. Grazing behaviour and degree of bloat were noted. 3. In general, the results showed that, irrespective of number of breaks, break grazing with or without restriction gave no effective control of bloat. 4. There were definite indications that a strictly enforced system of ‘off and on’ grazing could afford considerable practical control. 5. Long feed proved definitely safer than feed of the same botanical composition grazed at shorter stages of growth. 6. It was found that cows on potentially dangerous grass had short grazing times with few periods of intense activity. Thus, the oft-stated opinion that bloat is caused by cows eating greedily of dangerous pastures was not substantiated. On the contrary, cows on such pasture showed marked reluctance in their grazing activity. 7. Bloat-producing swards were also associated with reduced rumination time, even in respect to cows showing no symptoms. Bloated cows showed generally very low rumination times. 8. Many of these observations were made on identical twins. From the similarity in their behaviour, it can be concluded that inheritance is an important factor affecting the occurrence of bloat amongst cows grazing the same herbage. 9. In the early stages of bloat, ruminal movement and belching increased in frequency and intensity. This is contrary to the theory that bloat is due to rumen atony. Atony was observed only in the advanced stages of the trouble.