Forage availability is sometimes limited because of poor growing conditions or because of insufficient land base on individual farms. In addition, forage fiber can limit feed intake, particularly for high producing cows, and forage quality is highly variable. Therefore, there is continual interest in alternatives to forages in diets for dairy cows. Decreasing the forage content of diets requires careful selection of ingredients for use in the dietary space created. Because cereal grains must be limited in diets to avoid ruminal acidosis, byproduct feeds with high fiber content are the most commonly discussed alternatives to forages. These can replace some, but not all, forage in diets of lactating cows. These byproduct feeds are referred to as non-forage fiber sources (NFFS) to distinguish them from other byproduct feeds. However, cereal grains also can be used in place of forages to some extent if fermentability of starch is reduced. Forage alternatives should be considered based on their ability to substitute functionally for forage. This paper discusses how forages function in diets of dairy cows and factors to consider when selecting forage alternatives.
Forages are unique compared with other dietary ingredients because they provide long fibrous particles that are retained in the rumen longer and tend to ferment more slowly than smaller feed particles. This provides a consistent source of fuels to microbes in the rumen as well as a basal supply of fuels to the liver and mammary gland over time, allowing greater milk yield. Some long fibrous particles are necessary for formation of the rumen mat, which entraps small particles, increasing their ruminal digestibility. An adequate mass of digesta in the rumen is required to stimulate chewing, which increases secretion of salivary buffers.
A high supply of fuel from fiber fermentation in the rumen is highly desirable and dependent upon the digestion characteristics of the fiber source. Forage fiber should have a high turnover rate from both digestion and passage to maximize fuel supply while minimizing the filling effect of fiber over time. Increased passage of particulate matter is also expected to increase efficiency of microbial protein production by increasing passage of attached microbes before they die and are redigested in the rumen. However, forages vary considerably in their digestion characteristics, and it is often desirable to find alternatives for poor-quality forages.
Forages also dilute rapidly fermentable feeds such as cereal grains in diets, thus preventing excessive ruminal fermentation. However, this function is not unique to forages, and the ability of feeds to slow ruminal fermentation while providing high total tract digestibility determines their value as a forage alternative. The concentration of forage diets can often be decreased while maintaining or enhancing feed intake, milk yield, and health. Thus, it is possible to provide cows low-fill, highly fermentable diets that also result in consistent fermentation over time.