Alfalfa and corn silage are the most widely used forages for dairy cattle in the Midwest, while grasses and pasture are used more in beef cattle diets. Alfalfa became the forage of choice in dairy cow diets because it was higher in crude protein compared to grass and thus would lower purchased protein costs.
Corn silage provided good yields of consistent forage high in energy. Let’s take a look at the three most common forages used in the dairy ration.
Corn silage in the dairy ration
Corn silage has become the dominant base forage used on many dairy farms. Corn silage offers many advantages to a feeding system. It is high-yielding, very consistent, very palatable and is high in energy. Its main challenge is: It is low in protein. Corn silage brings two components to the diet fed: forage and grain.
Within each component, there can be variations in digestibility, which needs to be determined. Analysis needs to be done in order to determine both a set-time neutral detergent fiber digestibility (NDFd; such as NDFd-30 and uNDF-240) or total tract neutral detergent fiber digestibility. Further, a prediction of the rate and extent of ruminal starch digestion is valuable.
Another limitation to a high-corn silage diet has often been a limitation to effective fiber. Traditionally, corn silage had been chopped to a theoretical length of cut (TLC) of half-an-inch. With the advent of processors, a TLC of three-quarter-inch became standard while improving grain starch digestion.
Now there are differential speed processors, and the TLC has been increased to 1 inch or more and increased the milk production of cows fed the corn silage.
There are three main types of corn varieties used for corn silage. They are usually referred to as silage-specific, dual-purpose and BMR. They differ in NDFd and grain content, usually in inverse order, with BMR varieties typically being highest in NDFd and lowest in grain content, silage-specific varieties are intermediate, and dual-purpose corn is highest in grain content but lower in NDFd.
These rankings will usually remain similar if grown at the same location but can differ between locations and years due to differences in rainfall, temperature, planting dates and other variables.
Alfalfa complements corn silage
Alfalfa, due to its high protein content and palatability, makes an ideal complement to corn silage in diets for dairy cattle. Thirty years ago, forage testing and the formula for relative feed value (RFV) were being introduced to forage producers.
The guideline was 20-30-40; that was based on forage testing, 30 percent acid detergent fiber (ADF) and 40 percent NDF would be 150 RFV with 20 percent crude protein. This was considered a good goal for the forage quality needed for feeding lactating dairy cows.
Thirty years is a long time in terms of research advances in feeding cows and forage analysis. In this time period, we have come to a greater understanding of fiber analysis, incorporating the role lignin plays as an indigestible fiber component.
And we have herds producing 50 percent more milk per cow per year than 30 years ago. RFV was changed to relative forage quality (RFQ). The question in 2016 is: Is 150 RFQ high enough for high-producing dairy cows today?
If you look at the forage summaries in Figures 1 and 2, you will see while we are hitting our original average of 40 percent NDF, a large percentage of the total forage tests at Dairyland Labs are lower than 40 percent NDF.
This would indicate that a growing group of alfalfa producers are consciously cutting their alfalfa at lower maturities in order to lower NDF and increase NDFd.
This shows up in the second bar graph. If we are trying to achieve NDFd values greater than 50 percent, we clearly have to cut earlier. Dairy producers have found that this quality of alfalfa produces more milk and you can feed more of it. It also fits nicely with higher-corn silage diets.
But alfalfa also has limitations in the amount of rumen-soluble protein. In very high-quality (greater than 175 RFQ, greater than 24 percent crude protein) alfalfa haylage, soluble nitrogen, in the form of ammonia, passes across the rumen wall and can be lost in the urine.
The rate of ammonia release in the rumen becomes problematic in maximizing microbial protein synthesis and net metabolizable protein. However, dry matter intakes tend to be higher for cows fed alfalfa-based diets compared to grass-based forage diets.
Alfalfa has two different portions or pools of digestible NDF, which digest at different rates. Leaves digest faster than stems. But alfalfa stems will be chewed into smaller pieces at a faster rate compared to grasses and pass out of the rumen before reaching the greatest extent of NDF digestion.
Grasses in the ration
Grasses have lower lignin content at a similar stage of maturity as alfalfa, which results in higher NDFd for grasses as compared with alfalfa. This also increases the potential energy derived from an equal amount of NDF digested. But digestion of NDF also requires smaller particle size to increase rate and extent of digestion.
Grass is not as brittle as alfalfa forage. This slows the rate of NDF digestion, but it tends to be more constant and increases the extent of NDF digestion. This might suggest that a difference in particle size of grass hay or haylage may be beneficial in increasing the rate of NDF digestion, if not the extent.
Given the high levels of feed intake and resultant short rumen retention time of dairy cows, the more rapid rate of digestion for legumes than grasses and the faster particle size breakdown of legumes, one would predict that feeding legumes should result in greater milk production than will grasses.
However, milk production can be similar or even greater with diets containing grasses rather than alfalfa. Certainly, supplemental ingredients and grain-to-forage ratios strongly influence the outcome, but another factor is the relationship between rumen retention time and digestion kinetics.
Researchers in 1999 provided an analysis based on the model of 1972 research that showed potential extent of digestion had a larger impact than rate of digestion on predicted extent of fiber digestion at rumen retention times beyond 15 hours.
Incorporating rate of passage into such a model found that all three factors (rate and extent of digestion, and rate of passage) contributed equally to predicting extent of digestion.
Therefore, the higher potential extent of grass NDF digestion may also contribute to better-than-expected performance of dairy cows fed grass-based diets. If production of cows fed grass-based diets is lower than predicted, it may be related to the rate of particle size reduction in addition to the rate of NDF digestion.
Can forage be too good?
Can forage be “too good”? Can forage be too high in NDFd? It seems unlikely, since that is likely a major limiting factor to higher production. In a 1997 review of recent research, it was summarized that a 1-unit rise in NDFd content in the diet results in a 0.37-pound-per-day rise in dry matter intake and a concomitant rise in milk production (Table 1).
Observed effects of such forage may be higher milk urea nitrogen levels, lower fat test and possible health problems if not balanced correctly. Some or all of these effects were often seen in cows turned out on pasture in spring.
However, such forage could effectively replace byproduct feeds or lower non-fiber carbohydrates levels in the diet. Research in 2012 showed using high-quality ryegrass forage as a portion of a corn silage and alfalfa diet could result in a healthier diet.
We continue to learn more about forage feeding and forage digestibility, and cows continue to milk more and more as we do. Forage nutrient density and digestibility are limiting factors in dairy cows achieving their maximum genetic potential.
Improvements in forage genetics are coming on the market. We will continue to research which combinations of forage will maximize profitability, sustainability and productivity of both our animal and forage production systems. PD
References omitted due to space but are available upon request. Click here to email an editor.
-
James Paulson
- Dairy Extension Educator
- University of Minnesota
- Email James Paulson
