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Using forage fiber analyses to get the most from your cows

Rick Grant Published on 30 June 2014
cows in open pen with tractor

Essential forage concepts

Increasingly, our goal is to successfully feed higher-forage diets to dairy cattle. The two components of forage fiber – digestibility and physical effectiveness – must be in balance to optimize feed intake, promote rumen health and boost milk component output.

Forage fiber analysis can be complex, but let’s focus on the essential analyses we should consider and what they mean to the cow.

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Three key measures of forage quality are:

  • Lignin-to-NDF ratio and NDF (neutral-detergent fiber) digestibility
  • Undegradable or indigestible NDF
  • Physically effective NDF (peNDF)

Measures of forage quality and what they mean

Measures of NDF digestibility can be used on-farm for benchmarking and relative ranking of forages, allocating forages to proper cow groups and ration formulation to optimize forage inclusion in the diet.

As a chemical measure, lignin-to-NDF is a good predictor of forage digestibility and target lignin-to-NDF ratios are well known, such as less than 15 percent for alfalfa and less than 6 percent for corn silage.

But lignin-to-NDF does not always accurately predict NDF digestibility for corn silages and haycrops within maturities or cuttings. Cornell University scientists have found that 30-hour NDF digestibility may vary by as much as 20 percentage units for hybrids with very similar lignin and NDF content.

Undegradable NDF measured after 24 hours of in vitro fermentation is useful for measuring rates of NDF digestion and predicting dry matter intake due to its relationship with rumen fill. The peNDF of a forage or TMR reflects its particle size and ability to stimulate chewing and healthy rumen function.

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Getting the greatest response from your cows to higher-forage fiber digestibility

The amount of forage fiber that can be consumed by a dairy cow is largely determined by its NDF digestibility.

For example, increasing the 48-hour NDF digestibility of an alfalfa-grass mix from 60 percent (mediocre) to 76 percent (excellent) allows you to formulate a diet with 71 percent rather than only 61 percent forage. Typically, NDF digestibility can be as high as 65 to 80 percent for grasses and 50 to 60 percent for legumes.

How much forage-NDF can a lactating dairy cow actually consume? Traditionally, we have assumed that a cow could only eat approximately 0.9 percent of her bodyweight as forage-NDF. That limited the amount of forage in the diet. However, recent evidence shows that much larger amounts of forage-NDF can be consumed by dairy cows and the target should be to achieve at least 1 percent of bodyweight.

In fact, Cornell researchers have summarized forage-NDF intake data and found that pastured cows consume as much as 1.8 percent of bodyweight. So we need to aim high in forage-NDF intake when we feed forages containing highly digestible NDF.

Response to forage digestibility varies with milk production

Based on data summarized by Michigan State scientists over a decade ago, for every one percentage unit increase in forage fiber digestibility, we can expect 0.4 pounds per day more dry matter intake and 0.55 pounds per day more fat-corrected milk.

Recently, Minnesota researchers reported that for diets containing more than 40 percent corn silage, you could expect 0.26 pounds per day more dry matter intake and 0.31 pounds per day more fat-corrected milk.

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However, cows do respond to higher-NDF digestibility forages differently based on their level of milk production or stage of lactation. As an example, Nebraska researchers found that higher-producing, earlier-lactation cows responded positively to increased corn silage NDF digestibility. In contrast, lower-producing, later-lactation cows did not respond in milk production to the greater NDF digestibility.

In fact, some lower-producing cows actually responded negatively in milk yield when fed higher-digestibility corn silage. We need to understand how cows respond to improved fiber digestibility at different production levels so that we don’t squander the value of higher-quality forages.

On the farm we need to carefully inventory forages of varying quality and then allocate the highest-digestibility forage to the high-production pens. Feeding high-NDF digestibility forage to lower-producing cows will result in no milk response at best and may actually reduce milk yield compared to feeding more appropriate-quality forages.

PeNDF

PeNDF is the fraction of NDF that stimulates chewing and contributes to rumen digesta mat formation. Forages and TMR of adequate particle size promote healthy rumen function and efficient production of milk and milk components. Physically effective NDF is the product of NDF and a “physical effectiveness factor.”

The gold standard method for measuring a physical effectiveness factor is to dry-sieve a forage or TMR sample and measure the percentage of particles retained on the 1.18-mm screen and larger. Of course, this method cannot be used on-farm to make real-time decisions concerning chopper settings or TMR particle size.

However, an effective on-farm method has recently become available for measuring the physical effectiveness factor on the farm.

A commonly used tool for assessing particle distributions, the Penn State Particle Separator, has been retrofitted with a 4-mm screen that provides similar measures of peNDF compared with the standard method. This 4-mm screen is available through the NASCO catalog.

Miner Institute scientists summarized 12 studies and found that a peNDF between 20 and 24 percent of ration dry matter resulted in the best efficiency of fat-corrected milk production. For peNDF less than 20 percent, efficiency declined presumably due to greater risk of rumen acidosis. Above 25 percent, peNDF efficiency also declined presumably due to the filling effect of excessive NDF consumption.

The old adage “size isn’t everything” also applies to forage particles. Forage-NDF particles from different forages of similar particle size can elicit substantially different chewing responses. The classic example is straw – we know that straw NDF stimulates 1.5 times the chewing response as alfalfa of similar particle size.

At Miner Institute, we have found that as NDF digestibility increases for a range of forage types, the fragility increases also. Fragility refers to how quickly a particle breaks down when chewed or processed. In the future, we will need to consider fragility along with peNDF to most effectively formulate high-forage rations.

Don’t forget the management environment

Don’t risk losing the advantage of high-quality forages with a poor feeding environment. Factors such as limited feed availability or accessibility (i.e., pushups and feeding for refusals) and overcrowding can result in abnormal feeding behavior, compromised rumen function and inefficient milk production.

The bottom line

When forage NDF digestibility and peNDF are in balance, we observe:

  • Higher rumen pH and fiber fermentation
  • Greater microbial protein production
  • Improved milk components
  • Greater intake and milk yield
  • Higher peak milk and better persistency
  • Less bodyweight loss in early lactation
  • Improved body condition and reproduction

The management target should be feeding forages with high NDF digestibility to higher-producing cows capable of responding, aiming for forage-NDF intake in excess of 1 percent of bodyweight, and ensuring sufficient peNDF to maintain healthy rumen function and efficient milk production. PD

PHOTO: Image by Mike Dixon.

Rick Grant
  • Rick Grant

  • President
  • William H. Miner Agricultural Research Institue
  • Email Rick Grant

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