Forage fragility impacts on ration fiber
Forages are the foundation of any dairy ration. Thus, the quality and characteristics of forages can have direct impact on milk production and profits. We typically focus on characteristics such as starch content, protein content and NDF digestibility as we include forages in rations. But as the attributes of fiber and its fractions have continued to be explored, forage fragility has emerged as an area of interest.
What is forage fragility? It can be defined as the rate in which forage is reduced in particle size through chewing. It is often referred to as “scratch factor.” To best visualize this, imagine you are holding grass hay in one hand and alfalfa hay in the other. When you squeeze the grass hay, it is pliable and easily compactable, thus having a lower fragility. But, when you squeeze the alfalfa hay, it is stiff and probably breaks, giving it a higher forage fragility.
Forages with lower fragility contribute more to physically effective NDF (peNDF), which is the portion of NDF that stimulates rumination, or chewing activity, and contributes to the rumen mat. Because of how fragile they are, legumes easily and quickly can be physically broken down to a smaller particle size and, as a result, stimulate less chewing.
In contrast, grasses have a higher hemicellulose content, which cross-links with other plant components (most notably lignin), making it less fragile. This requires more physical breakdown to reach a smaller particle size. When chewing activity is increased, cows produce more saliva, which, in turn, helps with buffering and maintaining the rumen environment.
As previously stated, forage fragility also can have an impact on the rumen mat and thus gut fill. Forages with a higher fragility can be broken down to a smaller particle size. This can lead to a higher passage rate as well as increased digestibility due to damage to the cell wall and increased surface area. Thus, fragility can affect the extent of digestion and the retention of particles in the rumen, all of which have an effect on dry matter intake.
Because of the relationship between fragility and the structural components of plants, the ratio of ADF to NDF has been used as an indirect measure of fragility. The NDF fraction of fiber contains cellulose, hemicellulose and lignin while the ADF fraction doesn’t account for hemicellulose. Because of the stabilizing nature of hemicellulose, forages that have a higher ADF-to-NDF ratio will have a lower fragility while a lower ADF-to-NDF ratio will indicate a higher fragility.
A handful of other methods have been developed and explored to estimate fragility as well. One proposed method to measure forage fragility uses the Penn State Particle Separator with the assumption that less-fragile forages will retain more particles on the 1.18-mm sieve. In a process called ball milling, a dried sample is tumbled in a ball mill loaded with ceramic balls to mimic chewing. Fragility is estimated by measuring the difference between the sample’s peNDF prior to and after milling. However, very few commercial laboratories offer ball milling analysis. There is still a need for a greater understanding of factors that relate to forage fragility as well as effective measures or estimates of forage fragility that can be used in ration balancing.
Forage fragility is something to keep in mind when you are debating how to use available forages in rations. Diets containing forages with a lower fragility, such as grasses or any late-maturity hay, present the opportunity to include more concentrates, nonforage fiber sources, or other rapidly digested feedstuffs due to the increased buffering capacity. On the other hand, when higher-quality, more fragile forages are used, additional ingredients will need to be brought into the ration to help maintain the rumen environment and slow the passage rate.
Table 1. Comparison of the chemical composition and effect on chewing activity of a high and low fragility forage cut at the same maturity consumed at similar intakes.1
|DM, % as fed
|Rumination time, min/d
1Adapted from Beauchemin and Iwaasa, 1993.
Feed quality and nutrition
Milk production and components