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).