Rethinking Fiber Dynamics and Grass Forages

Rick Grant
William H. Miner Agricultural Research Institute

Grass Fiber Digestion and Rumen Retention Time

Recent research conducted at Miner Institute indicates that the mean rumen retention time for marked haycrop silage and corn silage of medium length (1.18-4.75 mm) is approximately 35 to 45 hours for cows consuming about 60 lb/d of dry matter and producing about 100 lb/d of milk. Mean retention time for small forage particles (<1.18 mm) is about 30 to 35 hours.

In separate studies with similarly productive and rumen-fistulated cows, we have observed a consistent relationship between undigested NDF (uNDF; measured at 240 hours of in vitro fermentation) in the TMR versus uNDF in the rumen of approximately 0.625. On a 24-hour basis, this equates to a passage rate of about 2.6%/h or a mean retention time of about 38 hours. So, we see consistency among several studies that all indicate that high-producing cows can do quite well on grass forages based on their inherent rumen dynamics.

The inference of this research is that highly productive dairy cows can effectively use grass forage as a source of fermentable NDF. The retention time in the rumen is sufficiently lengthy so that the greater extent of NDF digestion of grasses can be effectively exploited. Of course, the data in Table 1 makes it clear that maturity at harvest has a far larger impact on NDF digestion than type of forage (grass or legume) and the grass must be harvested early to support high feed intake and milk production.

An important forage management goal is to shorten the fermentation time required for the two NDF digestion profiles to cross. This can be accomplished by harvesting grass forage at earlier maturities with less lignified NDF or other approaches that enhance NDF digestion rate.

A recently published data set from Dairy One Forage Lab (Chase, 2012) shows that the normal range in 30-hour NDF digestibility for grass silage in the US is about 55 to 70% (normal range defined as the average ± one standard deviation). We need to target the upper end of this digestibility range to maximize response to grass forages when fed to highly productive dairy cows.

Rumen Fiber Dynamics

To optimize milk component production from forages we need to understand rumen fiber dynamics. Digestion characteristics of forage fiber influence eating and rumination behavior, rate of particle breakdown, rumen turnover and fill, dry matter intake, and rumen efficiency. Grasses, legumes, and grain-containing forages such as corn silage all behave differently in the rumen.

For example, legumes typically have more fragile NDF than grasses and their particle size decreases more rapidly with chewing. Across a wide range of forages we have observed a positive relationship between NDF digestibility and fragility measured as rate of particle size reduction. Highly lignified, low digestibility straw is often the least fragile forage-fiber and stimulates 1.5+ times the chewing per kilogram than higher quality legumes or grasses. In contrast, low-lignin, highly digestible forages such as brown midrib corn silage or early maturity haycrop silages are highly fragile and require relatively less chewing.

Grasses tend to increase the rumen pool size of long particles versus legumes (Kammes and Allen, 2012). Grasses naturally break into long and slender pieces when chewed compared with most legumes. The net effect of the longer forage particles with grass-based diets is slower passage rate of smaller particles from the rumen (i.e. selective retention), greater rumen fill, and mass of physically effective NDF. In essence, longer forage particles in the rumen act as a filter and modulate passage of particles that are otherwise sufficiently small and dense to escape.

Grass versus Legume Digestion Kinetics

High producing cows that have greater appetites and higher dry matter intake will be more quickly limited by rumen fill when consuming average or low quality grasses compared with legumes. Figure 1 illustrates typical rumen NDF digestion profiles for both legume and grass forages. The figure shows that legumes ordinarily have a 15-20% greater initial rate of NDF digestion versus grasses, but the extent of NDF digestion is 30-40% greater for grasses reflecting 30-40% less lignin.

Table 1 summarizes the average measured rate of NDF digestion, digestible NDF fraction, and lignin content (extracted from a detailed review of grass dynamics written by Mertens and Huhtanen, 2007). Averaged across maturity, grasses contain less lignin than legumes and so have a greater extent of NDF digestion with a slower rate of NDF digestion. Averaged across forage type, immature forages contain much less lignin and have greater rate and extent of NDF digestion. The fermentation curves in Figure 1 reflect the digestion data in Table 1.

Table 1. Effect of maturity and legumes/grasses on rate, digestibility (dNDF), and lignin content.

Jan 1 Table 1

For average grasses and legumes the rumen fermentation curves cross at approximately 24 to 30 hours. Beyond this time frame, the inherently greater extent of NDF digestion in grasses should be a nutritional advantage. An important consideration is the average time that a forage particle spends in the rumen in comparison to the point when grass NDF digestion exceeds that of legumes. If rumen residence time is too short, then the greater extent of NDF digestion for grasses will be of relatively little use to the cow.

Figure 1. Rumen retention time and typical NDF digestion profiles for grasses and legumes. The shaded area illustrates measured retention time of fiber particles in highly productive dairy cows.

Jan 1 Table 2


Selected Bibliography

Chase, L. E. 2012. Using grass forages in dairy cattle rations. Pages 75-85 in Tri-State Dairy Nutr. Conf., Fort Wayne, IN.

Kammes, K. L., and M. S. Allen. 2012. Rates of particle size reduction and passage are faster for legume compared with cool-season grass, resulting in lower rumen fill and less effective fiber. J. Dairy Sci. 95:3288-3297.

Mertens, D. R., and P. Huhtanen. 2007. Grass forages: dynamics of digestion in the rumen. Pages 1-20 in Proc. Ruminant Health and Nutr. Conf. Syracuse, NY.

Amaferm Application

Dr. Grant points out that, “The inference of this research is that highly productive dairy cows can effectively use grass forage as a source of fermentable NDF.” Digestibility converts to performance. Research by Oba and Allen found that a 1 unit change in NDFd can increase dry matter intake (DMI) by .37 lbs (.17 kgs) and milk production by .55 lbs (.25 kgs).

Amaferm is research proven to increase NDF digestibility across forage qualities and forage types.

Amaferm Application

Based on the Oba and Allen research, looking at even the lowest increase on Brome Hay, Amaferm improves DMI by 1.63 lbs (.74 kgs) and milk production by 2.42 lbs (1.1 kgs). If you apply those numbers to the TMR data, Amaferm improves DMI by 2.33 lbs (1.06 kgs) and milk production by 3.47 lbs (1.58 kgs).

Amaferm not only works on improving forage fiber NDFd, but total tract research shows a 9% improvement in digestion. Remember, digestibility converts to performance, Amaferm’s average milk increase in early lactation is 7%. For a high producing cow giving 100 lbs, with milk at $.17/lb ($.37/kg), feeding 5 grams of Amaferm for the first 150 days of lactation, the calculated return of increased milk production is $1.09/day or $163.50 per cow over the fed period.

For the easiest way to add Amaferm to your rations, visit Go to the CONNECT tab and click on Get More Info.

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