Fiber is a critical component of the plant cell wall and provides both structural support and protection against disease and weather stress. Fiber is resistant to degradation by the digestive enzymes of most monogastric animals. Ruminants, on the other hand, have the ability to release the energy from part of the fiber structure thanks to the activity of the rumen and hindgut microbiota populations. The ruminant and its diverse microbiota population play a unique role in sustainable agriculture through this ability to release energy from fiber without competing with humans for their own nutrition.
But all forages are different in terms of fiber quality and digestibility, which at the end of the day determines its nutritional value for the animal. From the field to the farm, from the seed to the trough, which are the factors that can influence fiber quality and digestibility?
What is fiber quality?
Plant material is composed of carbohydrates, protein, oils and minerals. The basic carbohydrate structure (Figure 1) is a combination of cell content and cell wall. The cell wall forms the majority of the plant structure.
Figure 1: The different carbohydrate components of plant material.
When looking at nutritional quality of a forage fiber for ruminant nutrition, the most significant criteria is fiber digestibility, or NDFd (see boxed text). pdNDF and NDFd represent important analysis for ruminant nutrition as it has been associated with production response and allowed producers to calculate the amount of energy release and microbial protein in an advanced (dynamic) model (Fox, 2003).
Oba and Allen have determined that, for every 1% unit increase in NDF digestibility, milk production increases by 0.24 kg/d and Fat Corrected Milk (FCM) increases by 0.25 kg/d (Oba and Allen, 1999).
Figure 2: Fiber digestibility depending on rumen conditions and fiber type (MB Hall, 2014).
The life cycle of fiber
This is a concept that examines both the development and degradation of fiber from the soil to the plant to the silo to the cow. The development of fiber is influenced by the environment in which the plant is grown and the inherent morphological differences between plant species. The degradation of fiber is influenced by farming practices and rumen dynamics.
Figure 3: The life cycle of fiber: the different factors affecting fiber development and degradation.
Environmental factors can have a significant impact on fiber formation and degradability. These include:
The soil conditions and fertilizers:
The effect of soil fertility is more specifically linked to plant growth and development, which, in turn, could affect fiber digestibility. For example, lignification increases in plants growing under conditions of low soil fertility.
Even though maturity has great influence on fiber digestibility, warm temperatures can alter the digestibility of the forage at the same maturity stage. For instance, Ohlsson (1991) found that a temperature increase from 10 to 20°C lowered digestibility at equivalent maturity by 7 percentage units in red clover and 5 percentage units in timothy. On the other hand, when temperatures are suboptimal for growth, there is an increase in soluble sugars as the sensitivity of photosynthesis is favored over growth (Putnam and Orloff, 2016).
Excessive amounts of water are associated with a depletion of soil oxygen that will retard plant growth and yield. Plants die after two to three days due to lack of oxygen at the roots. Drought conditions reduce plant growth, which delays maturation and results in an increase of the leaf:stem ratio, an increase of soluble carbohydrate concentration in plant tissue (Mueller and Orloff, 1994) and can lead to improved digestibility.
Photoperiod and light intensity:
Photoperiod (amount of daylight) and the intensity of the light provides the total amount of light available for photosynthesis. In general, increasing light dilutes the amount of cell wall production in favor of non-structural carbohydrates (cell content). Van Soest (1996) calculated that every one-hour increase in day length can increase digestibility by approximately 0.2 percentage units. The shortening photoperiod has a negative effect on digestibility but is somewhat offset by cooler temperatures. Cloud cover and shade can lead to a decrease in fiber digestibility.
The crop factors
There are factors inherent to the plant variety, genetics, seasonality and maturity. For instance:
- Legumes, although they contain lower fiber levels than grasses, tend to have lower fiber digestibility due to the physical structure of the plant.
- Grasses contain more potentially digestible fiber than legumes but advancing maturity will lower digestibility.
- When comparing corn silage to alfalfa, the corn silage NDFd is often higher than alfalfa, but the rate (speed) of digestion is much slower.
Finally, fiber digestibility decline with advancing maturity. This is largely related to:
- The accumulation of stem mass that eventually exceeds leaf mass;
- A shift towards higher cellulose and lignin concentrations and a lower pectin concentration.
The peoples’ factors: the impact of growing and harvesting practices
Any decision that an individual makes regarding planting, harvesting and storing forages could have a potential impact on fiber digestibility. Understanding the implications of these decisions is important to understand the potential fiber available for energy release by the ruminant.
Important factors include:
- Plant density: for corn silage for instance, increasing planting density increased corn silage yield with some sacrifice in starch content and fiber digestibility. The NDF usually increases as plant populations increase because there is less grain in the crop at the higher plant densities.
- Cutting height: generally, there will be an improvement in fiber digestibility as cutting height increases because the bottom of the plant is more highly lignified and less spoilage from the soil occurred (chemically and microbiologically speaking). For grass and alfalfa, it is recommended to cut between 7 to 10 cm.
- Dry matter content at harvesting: for corn silage it is recommended to harvest at 32% to 38% DM
- Length of cut: it will impact rumen passage rate and feeding behavior (sorting). For corn silage, it is recommended between 13 and 19 mm (0.5 to 0.75 inches).
The silo factor: impact of storage and fermentation
Although the amount of available fiber in forages is determined by the maturity of a crop when harvested and how well chopped it is, the fermentation of the forage also plays a critical role in how much of the fiber remains viable for feedout.
Fermenting or ensiling forage is a method used globally to preserve chopped forages for feed. The forage is stored in a silo/bunker and as much air as possible is removed by rolling. This allows lactic acid bacteria (LAB) to grow, which they do so by uptilting a small amount of sugar found in the forage to produce lactic acid, which lowers the pH to an optimal value. This reduces the growth of undesirable microorganisms, which would otherwise degrade the nutrient and energy content of the silage and can result in significant physical dry matter (DM) losses which includes fiber content.
Using a silage inoculant introduces large numbers of LAB organisms to the forage helping drive the fermentation more efficiently and quickly, resulting in a quicker pH drop, reducing DM losses more retained nutrients, energy and fiber levels. However, when the silage is opened for feeding it is once again prone to spoilage. Air can now leak into the silage and this can cause the silage to heat and spoil through yeast and mold growth, causing significant DM and fiber losses, and even worse losses through unpalatable spoiled silage.
Utilizing silage inoculant containing combinations of specialized lactic acid bacteria including homofermentative Lactobacillus Plantarum CNCM MA18/5U, Pediococcus pentosaceus NCIMB 12455, Pediococcus acidilactici CNCM I-3237 and the unique Lactobacillus hilgardii CNCM I-4785 and Lactobacillus buchneri NCIMB 40788 both of which work in combination to not only improve the fermentation, but ensure silage stability when opened. They preserve the nutritional and energy values, decreasing the DM losses through the fermentation period (figure 4) and prevent the silage from heating and spoiling when opened. This results in better quality silage as more of the original nutritional, energy and fiber fractions are maintained until fed. In addition, some silage inoculants can contain specific enzyme formulations that release small amounts of soluble sugar from the forage and increase the fiber digestibility of certain forages.
Figure 4: Dry matter losses after 30 and 150 days of fermentation (grass silage, 34.5% DM University of Turin, Italy unpublished
MAGNIVA Platinum 3 treated grass silage reduced DM losses after 30 and 150 days of fermentation. Meaning more retained nutrient and fiber levels for feedout.
Plant fiber is an essential component of ruminant diet and fiber quality, in terms of digestibility is an important lever of farm profitability, especially in the actual context of increasing raw material price. This quality is a multifactorial element linked to the whole fiber life cycle. The environment in which the plant is grown, its intrinsic parameters, but also farm management practices from planting to harvesting and silage preservation have the potential to alter fiber degradation and fermentation in the rumen.
- Fox D. G., T. P. Tylutki, L. O. Tedeschi, M. E. Van Amburgh, L. E. Chase, A. N. Pell, T. R. Overton, and J. B. Russell. 2003. The Net Carbohydrate and Protein System for evaluating herd nutrition and nutrient excretion. CNCPS version 5.0. July 29, 2003.
- Mueller, S. C. and S. B. Orloff. 1994. Environmental Factors Affecting Forage Quality. 24th California Alfalfa Symposium (Proceedings), Redding, CA, University of California Cooperative Extension.
- Oba M. and M. S. Allen. 1999. Evaluation of the importance of the digestibility of neutral detergent fiber from forage: effects on dry matter intake and milk yield of dairy cows. J Dairy Sci. 82(3):589-96.
- Ohlsson, C. 1991. Growth, development, and composition of temperate forage legumes and grasses in varying environments. Retrospective Theses and Dissertations. 9563. Accessed Jan. 4, 2021, Available at: https://lib.dr.iastate.edu/rtd/9563.
- Putnam, D. H. and S. Orloff. 2016. Agronomic Factors Affecting Forage Quality in Alfalfa. Proceedings of California Alfalfa and Forage Symposium, Reno, Nevada.
- Van Soest P. J. 1967. Development of a Comprehensive System of Feed Analyses and its Application to Forages. J. Animal Science. 26:119-128.