Association between grinding energy and in-vitro neutral detergent fibre digestion kinetics in forages

Abstract

Through the measurement of grinding energy, the possibility exists to predict forage fragility as related to the chemical composition of forages. It is also possible to predict a potential relationship between forage fragility and 240 hour in vitro neutral detergent fibre digestibility (uNDF240). These results could lead to improved predictions of particle size reduction, animal chewing activity and energy usage during the process of chewing. Physically effective fibre (peNDF) is a key component of many nutritional models used to predict the effect of forage particle size on cow chewing response. Chewing activity is a response which reflects the chemical and physical properties of feeds, including intrinsic fragility. Forage fragility, or the ease of particle size reduction during chewing, has been said to be similar among different sources of NDF, when attempting to estimate peNDF. However, different NDF sources with similar particle sizes can elicit ariable chewing responses and this variation has serious implications for nutritional models which use peNDF values. This variation has led to numerous inaccuracies in the system; therefore factors affecting peNDF particularly forage fragility, need to be better understood as forage fragility may be closely linked to NDF digestibility. Therefore, in this study, a possible association between forage fragility and short term or long term in vitro NDF digestibility (ivNDFd) was investigated. In order to investigate the possibility of predicting an association between forage fragility and in vitro NDF digestion, a total of 35 forage samples from three forage species were collected from 25 different locations. Forage species included commonly used fibre sources in ruminant nutrition in South Africa, namely Medicago sativa, Maize silage and Eragrostis curvula. The forage samples were analysed for numerous chemical components, as well as 6-, 12-, 18-, 24-, 36-, 48-, 72-, 96-, 120-, 240-h ivNDFd and rate of NDF digestion (kd). The 240-h ivNDFd was used to estimate indigestible NDF (iNDF). Particle size distributions were measured for all forage samples. Dried samples were pre-cut with a knife mill fitted with a 2 cm screen, after which particle size distributions were determined for each sample using a Retsch sieve shaker. For the measurement of grinding energy, 10 g duplicates of the 2 cm milled samples were milled with an ultra-centrifugal mill, fitted with a 1 mm screen. During the grinding process, energy usage of the specific mill was measured using a data logger with corresponding computer software and energy transducer. Energy measurements were reported as J/g sample on dry matter (DM) basis. The 1 mm samples were then used for determining particle size distribution again, in order to analyse change in particle distribution for each forage sample. The results of this study showed, according to the final models, that initial particle size (IPS), final particle size (FPS), cellulose and undigested NDF at 6 hours digestion (uNDF6) explain most of the variation in forage fragility. All of these variables can be associated with a decrease in forage fragility, due to an increase in energy usage during grinding with an increase in any of the aforementioned components. Upon adding species as a variable that could influence forage fragility, it could be seen that an interaction between M. sativa and FPS can be associated with a decrease in forage fragility, whereas an interaction between maize silage and FPS can be associated with an increase in forage fragility, due to a decrease in energy usage during grinding with an increase in this interaction. From the simple associations and correlations, it was evident that kd can be associated with increased forage fragility, as there was a decrease in energy usage during grinding with an increase in this parameter. Further correlations and/or linear associations indicate that NDF, acid detergent fibre (ADF), uNDF18, uNDF24, uNDF36 and uNDF48 can possibly be associated with a decrease in forage fragility, due to an increase in energy usage during grinding with an increase in any one of these variables. It would be expected that acid detergent lignin (ADL) is also associated with decreased forage fragility; however, this can only be assumed as the results for the effect of lignin on forage fragility are non-significant in this study. The use of grinding energy has the potential to be a practical and useful measure to predict forage fragility. However, the relative contribution of physical factors such as original particle size, particle shape, surface area, morphology and a multitude of chemical factors toward the fragility of forages is difficult to predict. Additional research is needed on the prediction of forage fragility and the possible relationship between forage fragility and NDF digestion and which factors influence this concept, before it can be incorporated as a meaningful and accurate input into nutritional models such as the National Research Council (NRC) and the Cornell Net Carbohydrate and Protein System (CNCPS).