A meta-analysis examining effects of particle size of total mixed rations on intake, rumen digestion and milk production in high-yielding dairy cows in early lactation

The primary objective of this meta-analysis was to quantitatively analyze and discuss recent (1997-2005) data evaluating effects of forage particle size of total mixed rations (TMR) on digestion, feed intake and milk production in early lactating dairy cows. For this, a data file containing 59-93 treatment means was generated from 25 published experiments investigating effects of particle size of grass or maize silage included separately or combined in a TMR. Another aim of the study was to focus on interactions of forage particle size with other dietary factors [i.e., neutral detergent fibre (NDF), forage NDF, non-fibre carbohydrates (NFC), NFC:NDF ratio], which may modify animal response. Animal response parameters included feed intake, ruminal fermentation, chewing activity, digestibility, digestion kinetics, milk production and composition. Results of the study showed that dietary forage particle size alone did not affect feed intake, milk production or milk fat content in early lactating cows (median 81 days in milk). In contrast, rumen pH positively correlated with particle size of forage of TMR (P<0.05). The acetate to propionate ratio correlated positively to NDF content (P<0.05), but not to dietary particle size. Furthermore, a positive linear relationship occurred between forage particle size and chewing time, ruminating time and the NDF digestibility (P<0.05). However, important differences were found when these effects were separately investigated in each type of forage composing the TMR. Thus, observed effects of particle size were more evident when arass sila e-based TMR were fed. In contrast, the role of particle size was less important when maize silage was included in TMR, whereas that of NDF, forage NDF and NFC:NDF ratio became more important. Effects of particle size of forage in TMR on intake, digestion and milk production of early lactation cows interacted with fibre content (NDF, forage NDF) and NFC:NDF ratio of both silage or complete ration to modify animal responses. We suggest that to better understand effects of forage particle size of TMR, all feeding conditions (i.e., high feed and NFC intake, low fibre intake) of early lactation cows should be considered. (C) 2007 Elsevier B.V. All rights reserved.