Nutritive value of wheat for ruminants: An index for ranking wheat varieties
About this project
1. Traditionally, the nutritive value of wheat for ruminants has been described almost entirely in terms of an energy and protein value and the former in particular has been regarded as more or less constant. In wheat, the greatest proportion of its dry matter is starch. The role of starch in the diets of ruminants has become the subject of considerable debate because it appears to have a role in enhancing milk protein content. It is vital to know not only how much starch a feed provides but also the proportion of that starch which is likely to be fermented in the rumen.
2. The extent to which wheat varies in starch content and nature, together with the factors which affect these is not known with any certainty. The objective of this study was therefore to examine the effect of year, site and variety on chemical composition, endosperm texture and nutritive value of wheat for ruminants.
3. A total of 66 wheats harvested in 1997, 1998 and 1999 from six RL sites (Cockle Park, Cambridge, Norfolk, Bridgets, Harper Adams and Cornwall) with six different varieties (Hereward, Reaper, Rialto, Consort, Madrigal and Riband) were obtained and studied. An in vitro gas production (GP) system was used to simulate rumen fermentation to assess the proportion of the starch in the wheat fermented in the rumen (RDS). Near infrared reflectance spectroscopy (NIRS) was examined as a possible rapid means of predicting starch quality and other factors.
4. The wheats showed a wide range of nitrogen and starch contents and notably, these two fractions were negatively related. Since it is known that nitrogen content can be influenced by nitrogen fertiliser this offers the possibility that starch content may be manipulated by agronomic management.
5. RDS disappearance for wheat grains is an important characteristic of the nutritive value of wheat for ruminants. This value is influenced by year of harvest, site of growth and variety. There was no site x variety interaction. The varieties were ranked in 1997 as Hereward<Consort<Reaper, in 1998 there were no variety effects and in 1999 as Riband<Rialto< Hereward<Madrigal<Consort<Reaper.
6. There were no significant correlations between any of the GP parameters and either the chemical composition or the grain quality parameters. Nitrogen content was highly negatively correlated with starch content (r = -0.55). NIRS was able to predict grain hardness, N content and endosperm texture, but was unable to reliably predict any of the in vitro parameters.
7. This work has highlighted the fact that the ability to supply starch to the rumen or post-ruminally is a key aspect of the nutritional quality of wheat and that it can vary substantially with variety and site of growth. It was not possible to develop a rapid method of assessment for the nutritive value of wheat. It is recommended that RDS determined using the in vitro GP technique is carried out for samples from the RL variety testing programme and this parameter along with starch content and endosperm texture be incorporated into the Recommended Variety lists.
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