Neutral Detergent Fibre (NDF), the fibrous fraction of feed, intakes and milk yield from a ration, along with being a vital factor in rumen health and maintaining constituent quality of milk.
To better quantify the role of fibre in rumen function, work has been done to develop physically effective NDF (peNDF) values for feeds. This helps to formulate rations to promote production without the expense of rumen health. Work by Zebeli et al (2006), published by the University of Hohenheim into the best measurement of peNDF found that the best response in rumen pH was seen when multiplying diet NDF by the percentage of material retained on a 1.18mm sieve, compared to the total NDF of the diet retained on the 19 and 8 mm screen of a Penn State Particle Separator. This suggests that fibre is still ‘effective’ in stimulating rumination down to 1.18mm in length. Furthermore, neural control of rumination is done predominantly by stimulation of low and high tension receptors that have the most significance in stimulating rumination.
With this in mind, to optimise feed efficiency and milk yield from NDF, it may not be worth looking at fibre length, but digestibility. If we can better measure the digestibility of fibre sources, we can be more accurate in supply of NDF and better supply energy and protein to the rumen microflora. Work by Van Amburgh et al, and Cornell University look at more accurate measures of NDF to help achieve this by looking further in to divisions of NDF
NDFom is NDF corrected for ash and sulphite, which inflate NDF levels, that do not have a function in rumination, leading to under supply in formulations. Calibration considers low soil contamination, but if there is a lot of soil contamination, this can overlook the addition of ash and sulphites. In a trial of 308 tests on Sorghum x Sudan, the difference between NDFom and NDF was 10 units of NDF, when it typically is 1-3 units.
uNDF is undigested NDF which is unavailable for microbial digestion after a long incubation period. With high yielding ruminants, rumen through flow is such that it will be unlikely for a rumen to digest the NDF fraction completely. NRC Dairy NRC (2001) and forage labs based on Weiss et al., 1992 use (lignin/NDF)0.67 to estimate uNDF levels, but Nousiainen et al. (2003; 2004) and Rinne et al. 2006 demonstrated in grasses that the relationship between lignin and digestibility was highly variable.
With improvements in forage analysis, there is opportunity to improve the accuracy of NDF supply by using uNDF measurements. This will lead to improved predictions of energy from forages as it is a more accurate representation of what is supplied to the rumen, intake potential and the resulting rumen function characteristics
Non Forage Fibre
Due to the varying origins of non-forage fibre, there is a wide range in degradation rates that can have profound effects on digestible NDF (dNDF) supply. This coupled with the consideration of timeframe of degradation in the rumen, provides opportunity to improve nutritional models. The disappearance of the fast and slow pools of dNDF may determine uNDF in the rumen and capacity for intake. With rumen passage as fat as 30hrs for many high yielding cows, we should consider the value of NDF digestibility post 30hrs with caution
A lot of research has been done into the true value of NDF and its various sources. Variation in feedstuffs, particle size and maturity provide a huge challenge in accurately modelling their true value in the rumen. Providing a short, consistent particle size aids the consistency of supply to the rumen, without jeopardising its function. NDFom helps to more accurately predict the true amount of NDF supplied, whist looking at uNDF and dNDF further improves the ability to predict the feed value and passage rates of feedstuffs, helping us better capture nutrients from feed stocks, together with accurately valuing them in a ration.
For more information see article ‘Rethinking the role of fibre’ on the Wynnsaty Dairy Blog, or contact a Wynnstay Dairy Specialist.