Fiber digestibility is a critical factor in dairy nutrition. It is involved in many parameters used to balance rations and evaluate forages. It affects DMI and gut fill capacity, as well as total chewing time both eating and ruminating, not to mention milk and milk components production. A new acronym, uNDF is being used in the dairy nutrition lexicon and showing up on some forage analyses relative to NDF digestibility. Here is an explanation of what it is and what it is not.
As part of a small group of researchers working with the CNCPS model determining fiber pools and rates digestion, it became clear that the distinction between undigested fiber and indigestible fiber needed to be clearly defined. For mathematical modeling purposes, iNDF is a required input, however, in the rumen of a dairy cow, iNDF is never approached, but rather uNDF is the more functional predictor of the fiber component. Understanding these distinctions will help clarify differences for both modeling digestion kinetics and evaluating forage quality.
What is uNDF?
uNDF is the undigested NDF residue after fermentation at a given length of time. It is used to estimate NDF digestibility (NDFD) and is expressed as either a percentage of NDF or percentage of DM. Therefore, uNDF must be accompanied with an indicator of the length of fermentation time; such as 24, 30, 48, 90, 120, 240 h. To be truly correct and a much more accurate predictor, uNDF should also be expressed on organic matter (om) basis to account for residual ash of the forage and any soil contamination of the material. The wet chemistry method of uNDF analysis is a gravimetric procedure and small amounts of mineral/ash can greatly affect the analysis and calculations of digestibility. Therefore, as examples:
William H. Miner Agricultural Research Institute
Corn harvest is mostly completed although a significant number of farms in the Northeast and upper Midwest are facing later harvest of possibly immature or frost-damaged corn silage. Corn development has been delayed and so the risk of frost damage and the need to harvest immature corn silage has increased. Depending on the planting date and type of silo, projected harvest dates run well into October for some parts of the U.S. and Canada.
Nutritional Value of Immature Corn Silage
Immature corn silage will typically be higher in crude protein, NDF, and sugar content, but lower in starch than normal corn silage. Because dry matter content is often lower, the risk of poor fermentation is higher and the resulting silage may cause reduced dry matter intake. The energy value of immature corn silage ranges from 80 to 95% of normal corn silage. The reduction in energy content of slightly immature corn is not as great as you might expect because, although the NDF content of the whole plant is higher, the stalk fiber is less lignified and more digestible. Since many factors influence the nutrient composition of immature or frosted corn silage, an actual analysis of your specific corn silage is needed. Wet chemistry analysis may be better than NIR because calibrations for normal corn silage may not fit well with immature silages. You should discuss this issue with the forage testing lab that you use to determine whether their NIR analysis accurately predicts the nutrient profile of immature or frost damaged corn silage. In order to obtain the most accurate estimate of the silage’s true energy and feeding value, be sure to analyze the silage for CP, NDF and NDF digestibility, starch and starch digestibility in addition to the usual values reported on a typical forage analysis. Knowing the total fermentable carbohydrate content of your corn silage is critical for ration formulation.
Feeding New Crop Corn Silage
Regardless of whether the 2014 crop was harvested at ideal maturity or less than ideal, most dairy farms will have two sources of corn silage to feed: last year’s carryover silage which has fully fermented and steeped plus new crop corn silage. Feeding immature and/or frost damaged new-crop corn silage adds yet another silage feeding challenge. Minimizing variations in herd productivity with changing sources of corn silage requires a good understanding of what happens to the nutritive value of silage with time in the silo. Corn silage is a dynamic feed – amount and digestibility of nutrients vary with storage time and our ration formulation strategies must accommodate these changes: Continue reading
by David R. Mertens, Ph.D, Mertens Innovations & Research LLC
To read Part I Indigestible Residue – What it is? Why does it occur? click here.
After we have established the biological rationale for indigestible NDF (iNDF), we are in a position to discuss its implications and impact on digestion kinetics. In 1969, USDA-ARS scientist Dale Waldo of Beltsville, Md., was the first to correctly postulate at a conference that the key to describing and understanding digestion of fiber was related to indigestibility. He sited some long-term in vitro data and concluded that if the indigestible fiber was subtracted from total fiber, the remaining potentially digestible fiber might follow first-order reaction kinetics. This was the breakthrough that led to the mathematical description of the dynamics of fiber digestion.
It has been stated that long-term in vitro measurements are irrelevant because feeds do not stay in the rumen of dairy cows longer than about 48 hours. First, it can be demonstrated with the correct passage model that significant fiber does stay in the rumen for longer than 48 hours. In fact, the rumen is designed to selectively retain fiber so that it has adequate time for digestion. Second, this comment misses the point that the reason for measuring digestion for more than 48 hours is to determine the potential extent of digestion and assure that kinetic parameters are conceptually correct and accurately determined.
by David R. Mertens, PhD Mertens Innovations & Research LLC
In Digestion Takes Time, I pointed out that digestion kinetics involves lag, rate, and potential extent of digestion, and indicated that potential extent was the most important of the three. Most, if not all, indigestible residue is indigestible NDF. There are minerals in feeds that are not digested, but our focus is on the organic matter in feeds because it provides the energy, protein and major nutrients needed by animals. There is no evidence that neutral detergent soluble organic material is indigestible. Note that I refer to indigestible NDF (iNDF) when I refer to potential extent of digestion, not undigested NDF (uNDF). From both a kinetic and biological perspective, iNDF defines material that can never be digested by anaerobic fermentation in the rumen even at infinite fermentation times. The plateau or asymptote of the digestion curve (see Figure 1, Digestion Takes Time) certainly indicates that digestion is not complete (does not go to 100%).
We typically use measurements of uNDF (what has not been digested after a specific time of in vitro fermentation) to estimate iNDF. Originally, in vitro fermentations of 72 or 96 hours were used to estimate iNDF, and more recently, 200 to 240 hours in vitro or 10 to14 days in situ are being suggested for estimating iNDF. But we need to remember that these uNDF are estimates of iNDF that would have to be measured at infinite times of digestion. The longer the fermentation, the closer uNDF comes to iNDF. But what fermentation time is practical for measuring uNDF that is also sufficient for estimating iNDF for use in current nutritional models and feeding systems?
Howard Jensen, DVM, MS, ACAN
Heat stress is just around the corner and despite the advances in environmental cooling, it continues to cost the dairy industry millions of dollars every year.
The negative impacts of heat stress include:
- decreased milk production
- increased metabolic disorders
- compromised milk components
- reduced reproductive performance
- slowed growth
- rumen acidosis
- cow death
The cow responds to heat stress with:
- drooling and sweating
- circulatory adjustments
- endocrine changes
- altered eating patterns, including decreased feed intake
- panting and an increased respiratory rate
What are the effects of the cow’s response to heat stress on rumen function?
Panting increases respiration rate, which sets off a negative chain of events. An increased respiration rate causes more CO2 to be exhaled which decreases CO2 in the blood stream. Because the buffering mechanism of blood requires a 20:1 HCO3 (bicarbonate) to CO2 ratio, when more CO2 is lost through an increased respiration rate, the kidneys secrete HCO3 in order to maintain the proper ratio.
William H. Miner Agricultural Research Institute
Summer’s heat and humidity are just around the corner. We need to be prepared with effective heat abatement systems that will cool our cows and allow them to respond to the diets fed. Unless a cow is properly cooled, adjusting dietary ingredients and feeding highly digestible forages will not result in the expected responses in rumen fermentation, feed intake, or milk production. According to recent research from Iowa State, the temperature-humidity index (THI) should be 68 (or even as low as 65) for high-producing cows. Our modern dairy cows have been selected based on heat-producing processes such as milk synthesis and consequently heat abatement has become ever more critical.
There is considerable information available on cow-cooling systems, so this article focuses on important aspects of forage quality and cow behavior that mitigate the negative consequences of heat stress.
Heat stress predisposes the cow to rumen acidosis
Classic data from Missouri show that heat stress conditions may reduce rumen pH to levels well below 6.0 (Figure 1). During heat stress, there is less bicarbonate in the saliva to buffer and maintain a healthy rumen pH (Baumgard et al., 2014). Heat-stressed cows ruminate 20 to 25% less, drool (so the saliva and its buffers don’t enter the rumen), pant, and are more likely to sort their diet or slug-feed. Consequently, heat stress is not a good time to feed to a clean bunk which may also encourage cows to eat faster. Overcrowding at the bunk or stall will also reduce rumination by as much as 1 to 2 hours daily.
All of these negative behavioral changes during hot, humid weather contribute to lower rumen pH and predictable negative consequences for rumen VFA profiles, rumen biohydrogenation, and milk production. Figure 1 shows that the extent of reduction in rumen pH associated with hot, humid conditions is virtually identical to reducing dietary forage content from 65 to 35% of dry matter! Under these low-pH conditions, we can expect inefficient rumen fiber fermentation and lower microbial protein output – both of which result in lower milk and milk component production. A traditional dietary recommendation during heat stress has been to feed less forage and more concentrates in an attempt to lessen the heat load of digestion. However, if we feed a diet that is borderline in fiber and high in starch, that will only exacerbate an already compromised rumen pH and microbial fermentation.
Figure 1. Influence of temperature and humidity or dietary roughage content on rumen pH (Mishra et al., 1970).
David R. Mertens, Ph.D, Mertens Innovations & Research LLC
Understanding digestion kinetics is relevant because time and potential extent of digestion are becoming crucial limitations for high-producing animals. When animals produce more, they eat more, and this pushes digesta through the stomach and intestines more rapidly. Faster rates of passage through the digestive tract result in shorter retention times, which means that less time is available for digestion.
Composition of feeds tells us what it is, digestibility tells us how much is available to the animal in a specific fixed, or static situation, but digestion kinetics tells us what is potentially available and how quickly it can be made available to animals in all situations. This explains why digestion kinetics is the future direction for describing feeds and why efficient feeding programs will be based on digestion kinetics. Our discussion of digestion kinetics will be connected to our understanding about how the cow’s digestive system works and how we analyze feeds in the laboratory to assess nutritional value.
The real key to understanding digestion kinetics is that it is all about TIME, TIME, TIME.
All the biological processes of the cow take time and high producing cows run out of time. They produce milk per unit of time (milk per day). It takes time for the udder to synthesize the components in milk. It takes time for the liver and other tissues to metabolize absorbed nutrients. It takes time to digest feeds, especially fiber in feeds. And it takes time to eat and ruminate feeds and excrete wastes. Of all the biological processes needed for maintenance of life, growth, reproduction and lactation, the ingestion, chewing, digestion and passage of feeds take the most time and become the ultimate limit for animal survival and production.
William H. Miner Agricultural Research Institute
Chazy, NY 12921
Recently, I was asked the question of whether you should worry about day-to-day variation in forage NDF digestibility. Like any nutrient, NDF and its digestibility may vary across days as the forage inventory is fed out. The important question is whether or not it pays to measure this variation on-farm. Does day-to-day variability in forage NDF and NDF digestibility influence the cow’s feed intake or milk component output, and if so, by how much?
We are concerned about variation in forage NDF digestibility because of the established relationship between NDF digestibility and cow performance. The figure below summarizes the well-known information from Oba and Allen (1999) that predicts a 0.40 lb/day increase in dry matter intake and a 0.55 lb/day improvement in milk yield for each one percentage-unit increase in forage NDF digestibility. It also shows the lesser known relationship from Minnesota researchers specific to higher corn silage diets (>40% corn silage in ration DM). In this case we expect a 0.26 lb/day increase in intake and a 0.31 lb/day boost in fat-corrected milk. So, variability in NDF digestibility can have a substantial impact on cow response – but is it necessary to be concerned about day-to-day variation?
Conventional wisdom tells us that cows will perform better when fed a consistent ration in a consistent environment – cows don’t particularly like change! Consistent feed, feeding environment, and feeding behavior ought to promote more efficient rumen fermentation throughout the day and more uniform delivery of nutrients to the cow. Interestingly though, at this summer’s ADSA/ASAS meetings in Kansas City an invited paper from Utah State University presented information showing that dietary variability actually stimulated feed intake in some ruminants such as sheep. Perhaps controlled ingredient variability, while maintaining overall ration nutrient profiles, might stimulate feed intake in dairy cows as well? Food for thought – no research to-date – and a topic for another time.
William H. Miner Agricultural Research Institute
Fiber Group Continues its Work
An informal “Fiber Group” has been working on improving our ability to model rumen fiber turnover. The group consists of researchers from Cornell University, Miner Institute, University of Bologna, Fencrest, Mertens Innovation & Research, and various scientists from the industry. In particular, we have conducted feeding studies at Miner and Bologna to evaluate dietary uNDF and its effect on rumen fill, turnover, and dry matter intake. At Miner Institute the focus has been on corn and haycrop silage-based diets, while Bologna has focused on dry alfalfa hay-based diets due to the feed restrictions inherent in Parmigiano Reggiano cheese production. Cornell is planning studies to evaluate how uNDF from non-forage sources of fiber compares with forage uNDF.
What Does Our Research Tell Us So Far?
All of the details on diet ingredients, nutrient composition and cow responses can be found in the 2014 and 2012 Cornell Nutrition Conference proceedings (Cotanch et al., 2014; Grant and Cotanch, 2012). At Miner Institute, we evaluated diets:
- With a wide range in corn silage source and amount
- Ranging between 36 and 55% corn silage (DM basis)
- Containing conventional versus brown midrib corn silage that varied by 10%-units in NDF digestibility, and some diets have contained up to 10% added chopped straw to maintain chewing activity as forage percentage was reduced from 52 to 39% (DM basis).
- Overall, diets contained between 39 and 68% total forage.
In all studies, cows responded predictably to dietary NDF and NDF digestibility and were uniformly high-performing, averaging 61 lb/d dry matter intake and 99 lb/d solids-corrected milk production.