Forage evaluation: Wading through a sea of numbers

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Forage analysis reports from a commercial feed testing laboratory often can contain more than 70 potential lab analyses. All those numbers have value in specific situations, but rarely, if ever, would one want or need all 70 results for a single sample. Some of the numbers are needed mainly for routine ration formulation (e.g., mineral concentrations), and some of the measurements might be needed to help solve specific problems (e.g., mycotoxin concentrations).

A core set of analyses should be conducted routinely on all forage samples. This core set of information is needed for ration formulation and can be used to evaluate the overall quality of the forage, estimate an actual or relative economic value of the forage, and for inventory management (assigning different quality forages to different types of animals). Analyses beyond this core may be needed to fine-tune diets and adjust evaluation decisions under specific situations. As discussed in a previous article (Proper Use of Forage Composition Data for Formulating Diets for Dairy Cows), using an average of data from two or more samples is much better than using data from a single sample.

The core set of analyses (Table 1) are dry matter (DM), neutral detergent fiber (NDF), in vitro NDF digestibility (IVNDFD), net energy for lactation (NEL), crude protein (CP), and ash. Each analysis will be discussed below.

Dry Matter (DM)

The concentration of DM (or conversely moisture) in forage is the single most important number when making forage harvesting decisions because it determines whether hay or silage will store properly. Forages harvested too wet for hay or too wet or dry for silage or baleage are at a high risk for spoilage or overall low quality after storage. The DM concentration of stored forage is needed to adjust the as-fed amounts added to the diet. Hay and silage with DM concentrations outside the acceptable range should be discounted heavily when purchasing because of the high risk of spoilage. Forage with the improper DM concentration may not be an acceptable feed for any class of livestock (e.g., it may be moldy), or you may need to limit its use to lower production animals (e.g., bred heifers).

Neutral Detergent Fiber (NDF)

The NDF concentration is related to the economic value of forages because forage NDF is considered an essential nutrient (needed to maintain rumen and cow health). Therefore, higher NDF concentrations could enhance the value of a forage; however, increased concentrations of forage NDF can lower intake and milk yield, which would reduce the value of a forage. Almost always, the economic costs of high NDF concentrations outweigh the costs of low NDF concentrations. The NDF concentrations in haycrop forages (grasses, legumes, and mixtures) have a much stronger correlation to intake and milk yield than does the NDF concentration of corn silage. This is because the NDF concentration in haycrop forages is positively correlated with plant maturity (i.e., as plant maturity increases, NDF concentrations usually increase). As maturity advances, digestibility of DM and NDF tends to decrease. As a corn plant matures, the grain component increases and this dilutes NDF so that NDF concentration in corn plants often has a negative correlation with plant maturity and with fiber digestibility. The NDF concentration has less value in estimating quality of corn silage than for other types of forage.

In vitro NDF Digestibility (IVNDFD)

This basic assay was developed more than 50 years ago, but it has not gained widespread use until very recently. The assay requires access to a rumen-cannulated animal, making it an expensive and difficult assay to conduct routinely. However, with improvements in equipment and advancement in near infrared (NIR) technology, this test has almost become routine. A wide range of incubation times are available (from <12 hr to more >200 hr). The data from various incubation times may have value in different situations, but for routine analyses, a 30-hr incubation is probably best. It is not so long as to reduce important differences among forages, but it is long enough to get more repeatable results. Because digestive activity of rumen fluid cannot be standardized across labs or even within a lab on different days, the actual numeric value has limited utility, but IVNDFD can be used to compare different forages (best done when forages are assayed at the same time by the same lab) or to compare a forage to a lab mean. Higher values are associated with greater intake and milk yield. Theoretically, higher IVNDFD should also mean greater energy concentrations (the digestible fiber will contribute to digestible energy); however, because of the negative relationship between intake and digestibility, forages with greater IVNDFD often do not actually (as measured in a cow) have greater concentration of energy, but they often result in increased energy intake. Because NDF concentration of corn silage is poorly correlated with nutrient value, IVNDFD is especially useful for corn silage. It also has value for haycrop forages, but knowing NDF concentration is often adequate to make quality decisions for those forages.

Net Energy for Lactation (NEL)

Because this article is written for a dairy audience, NEL is used; however, other energy terms, such as total digestible nutrients, net energy for gain, etc., could be used. All labs estimate NEL (it is not measured), and several different equations are available. When comparing forages, make sure the equations used to estimate NEL are the same; otherwise, you may not actually be comparing energy values for the forage but comparing equations. On average, about 65% (good quality alfalfa) to 75% (good corn silage) of the economic value of forages is derived from its value as an energy source. As the estimated energy concentration increases, the economic value of the forage usually increases because less supplemental energy will need to be purchased. However, greater NEL concentration in a forage does not necessarily mean that feeding the forage will increase NEL intake. Based on the physiologic state of the cow, increasing the NEL concentration of the diet (e.g., by feeding a forage with greater NEL) may reduce DM intake so that energy intake is constant. The NEL concentration of forages should be evaluated in concert with IVNDFD (remember higher IVNDFD usually does not mean greater NEL). Because equations differ, it is not possible to give specific goals, but higher is usually better.

Crude Protein (CP)

The CP concentration is needed because it is used in essentially all ration formulation software and because CP impacts the economic value of forages. Higher concentrations are usually better because that should reduce the costs of supplemental protein. However, because CP from forages is usually highly degradable (i.e., high rumen degradable protein) with only moderate digestibility, the economic value of forage CP is less than CP from many protein supplements. Based on expected degradability and digestibility, 1 lb of CP from good quality grass or legume forage is worth about 0.65 times the value of 1 lb of CP from soybean meal. For example, if soybean meal CP is valued at $0.25/lb, then alfalfa CP would be worth about 0.25 x 0.65 = $0.16/lb. For forages that are heat damaged (hay baled too wet or silage made too dry), the CP concentration can grossly overestimate the value of a forage. When heat damage is suspected, have the lab estimate available protein using the concentration of acid detergent insoluble protein.

Ash

Determining ash is simple and inexpensive, but it accounts for a significant amount of variation in the energy concentrations of forages (ash has no energy, so high ash forages usually have less energy than forages with less ash). High concentrations of ash in forages can indicate soil contamination which can reduce the availability of important nutritional minerals, especially copper (high concentrations of iron in forage also are a good indication of soil contamination).

Take-Home Message

Although not adequate for complete diet formulation, evaluating analytical results for six critical measures (DM, NDF, IVNDFD, NEL, CP, and ash) allows you to quickly determine overall quality and economic value of forages.

 

Table 1. Suggested Forage Quality Goals (lactating cows)

Measurement

Goal

Dry Matter, %

Corn silage

30% to 38% (wetter for bunkers; drier for upright silos)

Haycrop silage

30% to 55% (wetter for bunkers; intermediate for bags and conventional vertical silos; drier for bottom-unloading, sealed silos)

Baleage

40% to 60% (wetter is better)

Hay

80% to 86% (wetter for small rectangular bales; drier for large rectangular bales)

Hay with chemical preservative

75% to 80%

NDF, % of DM

Legumes

35% to 42%

Cool-season grasses

48% to 55%

Legume-grass mixes

Weighted average depending on proportion of legume to grass

Corn silage

See comments in article

IVNDFD, % of NDF

> Lab average for specific forage type

CP, % of DM

Higher is usually better

NEL, Mcal/lb

Higher is usually better

Ash, % of DM

Legume silage

< 12

Legume hay

< 11

Grass silage

< 10

Grass hay

< 9

Corn silage

< 6


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