Understanding the contribution of ingestion to feed digestion
The animal article of the month for June is ‘Characteristics of boli formed by dairy cows upon ingestion of fresh ryegrass, lucerne or chicory’
Most measurements of feed in ruminant nutrition studies primarily concern the chemical composition. While feed chemistry can tell us the total amount of any given nutrient contained in a feed, it cannot tell us how much of, and when, that nutrient is made available to rumen microflora.
There is a reasonable amount of knowledge concerning the breakdown of and nutrient release from feed in during ruminal degradation, but less is known about the contribution of ingestion to total feed digestion. Including how and why this differs between forage species, and how this might influence nutrient utilisation.
In grazing ruminants ingestion involves biting off forages material, masticating (chewing) and adding saliva to reduce feed fragment size and manipulate the feed into a bolus that can be swallowed, and rupturing the plants largely undegradable outer layer and rupturing cells to release nutrients that the rumen microbial can then use. This study examined ingested boli collected from dairy cows (Bos Taurus) with rumen fistula, when fed three contrasting forages (ryegrass, chicory or lucerne), to assess the extent of comminution (feed breakdown to particles) and nutrient release. The study showed that the animal determines the size of the bolus swallowed, but forage physical characteristics influence how much work is required to create the bolus, indicated by the extent of comminution, the amount of saliva added to form the bolus, and swallowing rate.
Nutrient release differed between forages, but interestingly it was not strongly correlated with the extent of comminution, suggesting that damage during the formation of the boli is sufficient to release soluble nutrients from some forages. A higher proportion of soluble carbohydrate than N was released by chewing during eating and the amount of N released was correlated with N content. However, when intake rate was considered, it was shown that while some forages may release less nutrient compared to others, they do so at a more rapid rate. Meaning a greater delivery of nutrient to the rumen microflora during ingestion, which may have implications for utilisation.
The differences in nutrient release between forages illustrate the complex animal × forage interactions that occur during ingestion, and suggest it is not possible to predict nutrient release from herbage based on its chemical composition. Understanding the contribution of ingestion to feed digestion and nutrient availability to the rumen will aid understanding of feed utilisation by ruminant animals.
This article is freely available for one month: ‘Characteristics of boli formed by dairy cows upon ingestion of fresh ryegrass, lucerne or chicory’
Authors: E. M. K. Minnee, G. C. Waghorn, P. Gregorini, R. H. Bryant, D. F. Chapman