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Abstract

This project will initially evaluate different methods for characterising forage particle size for UK grass and maize silages across a range of dry matter contents and stages of maturity. This will be followed by an on-farm study that will characterise the particle size and functional fibre distribution of grass and maize silages with a range of maturity and dry matter contents, and evaluate the effect of mixing prior to feeding on particle size and distribution, as well as the effect on cow selection. Finally, two controlled feeding studies to evaluate the effect of grass and maize silage particle size on rumen fermentation, animal performance and milk fat composition and whole tract digestibility will be conducted, one each at HAU and University of Reading.

Description

To sustain the increased milk production of dairy cows in the UK has required an increase in the level of concentrate supplementation and production of high quality forages, with a trend towards lower dietary fibre levels (Beauchemin et al., 2003). The consequences of these dietary changes are an increased risk of metabolic disorders including subclinical ruminal acidosis (SARA), milk fat depression, displaced abomasum, laminitis, reduced fibre digestion and fat cow syndrome. Adequate forage particle size is necessary to stimulate chewing activity and as a consequence saliva production, which is required to neutralise acid production in the rumen and result in a ruminal pH above pH 5.8 (Zebeli et al. 2012). Adequate forage particle size is also required to produce a ruminal fibre mat, which acts to retain smaller forage particles, thus increasing their digestion (Zebeli et al. 2006). Fibre that is too short adversely affects rumination, decreases ruminal pH and the level and activity of cellulolytic bacteria in the rumen. In contrast, fibre that is too long lowers the passage rate of digesta and reduces the rate of fibre degradation due to a lower surface area. Additionally, too long a forage particle size promotes sorting in the feed passage, resulting in some cows receiving excess concentrates and others insufficient (Kononoff and Heinrichs, 2003).

Accurate assessment of forage particle size (PS) in dairy cow diets is difficult, and current feeding tables and nutritional programs do not include PS, despite its importance. To include information on PS in dietary formulations, the concept of physically effective fibre (peNDF) has been introduced, which integrates information on the fibre (NDF) content and structural features (e.g. PS; Mertens, 1997). The particle size of diet NDF has also been recommended as a key factor in terms of diet concentration of NDF with adequate size relative to nonforage carbohydrate concentration (Zebelli et al., 2012).There are several methods currently available to assess PS and peNDF in the diet, although there is no accepted standard, particularly for the range of DM of grass and maize silages commonly encountered in the UK.