et al. (2018), it would appear that in many circumstances providing a well-mixed ration that reduces the opportunity for diet selection and is fed ad libitum, may in many circumstances be as important as the composition of the diet or the particle length of the forage.


Conclusions There are many factors that influence rumen pH, including forage to concentrate ratio, forage particle length, dietary starch content, rate of starch release in the rumen and concentrate particle size, making an accurate prediction under commercial conditions problematic. This is further compounded by difficulties in accurately measuring rumen pH throughout the day on-farm. Daily saliva production is important in buffering the release of VFA, although dietary effects on saliva production are often smaller than anticipated. The particle size distribution of grass silage and grass silage based mixed rations in UK dairy herds is considerably longer than current North American guidelines for forages or rations. This suggests that the particle size of UK dairy rations is either too long, or that new guidelines or methods of determining particle size distribution are required. Feeding a short particle size grass silage or high levels of starchy concentrates may often have a smaller effect on rumen pH than anticipated, unless the diet contains high amounts of very finely ground starch or sugars sources, with a well-mixed ration that reduces the opportunity for selection being as important in many circumstances. However, on many UK dairy farms diets are poorly mixed and promote selection, which in combination with the restricted feeding that occurs on a high proportion, may be contributing to SARA.


Acknowledgements Much of the data presented in this paper was derived from the PhD thesis of Usama Tayyab. The author is also grateful to Prof. Chris Reynolds at Reading University, and Dr. Robert Wilkinson at Harper Adams University for their input into the studies, and AHDB Dairy for funding the work.


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This article is based on the text of a paper presented by Liam Sinclair at the recent Ruminants Meeting of the Society of Feed Technologists


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