Forage Quality¹

A. T. Adesogan, L. E. Sollenberger, and J. E. Moore²

Introduction

Forage quality is discussed by everyone involved with the production and utilization of forages. There are many different opinions about what forage quality is, and what causes one forage to have a higher quality than another. One of the best definitions of forage quality was stated by a hay producer who sold alfalfa to mid-western dairy producers: “milk in the bucket.” This is an excellent working definition, because it gets right to the heart of what quality in a forage has to be all about; that is, animal production. No matter what a forage looks, smells, or tastes like, or how much protein or fiber it has, if it does not provide nutrients adequate for growth or milk production, it is not a high-quality forage.

Forages consumed by Florida livestock vary in quality due to differences in genotype, maturity, season, and management. When quality is low, forages alone may not support desired rates of animal performance. In such cases, it is necessary to supplement with protein and energy. The many combinations of forages and supplements make it difficult to determine the most economical supplementation program. It is necessary to know 1) requirements of the specific animal, 2) quality and composition of the specific forage, and 3) effects of supplements on utilization of that forage. In order to make decisions about matching forages to animals and supplements, it is necessary to have some estimate of forage quality prior to feeding hay or grazing pasture. Furthermore, it is necessary to have some idea of the economic value of forage in order to compare forage-based programs to other alternatives.

The objectives of this chapter are to define forage quality, discuss factors affecting forage quality, and discuss the relationship between forage quality and economic value of forages.

What is Forage Quality?

Animal performance, whether growth or milk production, depends on the animal's potential for production, how much dry matter (DM) the animal eats, and the nutritive value of the DM consumed. Forage intake is affected by the amount of forage available; characteristics of the forage consumed; the gut capacity, performance level, health and genotype of the animal, environmental factors such as prevailing temperature and humidity, and management factors such as the level of supplementation and stocking rate. Intake is affected by many characteristics of forages such as particle size of stored forages, amounts of fiber, protein, and minerals in the DM, and how fast undigested DM passes through the animal. Other factors affecting intake of hay are molds and any other substance that make the forage unpalatable. Intake of pasture is also affected by the nature of the sward. Accumulations of dead forage or manure on pasture will decrease intake, and a dense, leafy canopy will increase intake. “Voluntary forage intake” is used to describe how much forage DM an animal will consume when adequate amounts of palatable forage are available, and when no supplements of protein and energy are fed; it is assumed that adequate minerals are available either in the forage or as supplements. Energy and protein supplements may either increase or decrease forage intake, depending on the composition of the forage and the composition and amount of supplement being fed.

Forage nutritive value is often described as crude protein (CP) concentration. In fact, when asked about the quality of a forage, many people will answer with the CP percentage. An even more important measure of nutritive value, however, is the concentration of “available” energy. For many years, total digestible nutrients (TDN) has been used as an overall measure of available energy. In the past 20 years, there has been increasing use of digestible, metabolizable, and net energy, but TDN is still an acceptable and easily understood measure of nutritive value.

Changes in voluntary forage intake do not always parallel changes in TDN concentration; therefore, an overall definition of forage quality must combine both intake and TDN (e.g., voluntary intake of TDN). Forage quality can also be defined as animal performance, because of the close relationship between animal performance and TDN intake. Whatever definition is used, it applies only under the restrictions that 1) forage availability is not limiting intake, and 2) no supplemental energy or protein is fed. In addition, if forages are being compared in terms of animal performance, animals must have the potential to either grow or produce milk, and this potential must be uniform across forages.

“Quality Index” (QI) was developed in Florida to combine voluntary forage intake and TDN concentration into one value. Thus, QI includes more information on overall forage quality than do CP and TDN. In effect, QI is a measure of the voluntary intake of TDN relative to the maintenance TDN requirement. It is related closely to animal performance, under the limitations described above. When QI=1.0 the intake of TDN just meets the maintenance requirement. Examples of the relationship between QI and animal performance are shown in Table 1.

The term “relative feed value” (RFV) is another overall measure of forage quality used by the National Forage Testing Association (NFTA). The RFV value represents voluntary intake of digestible dry matter relative to a standard forage, and it has been used to set hay prices by the NFTA. Recently, the NFTA adopted the use of another index called “relative forage quality” (RFQ) instead of RFV because RFQ is based on more accurate equations for estimating the energy value and voluntary intake of forages. However, the RFQ value represents voluntary intake of TDN relative to a standard forage, therefore RFQ and RFV can only be used to compare forages on a relative basis. In contrast, QI can be used as an absolute measure of forage quality or potential animal performance because it relates directly to the energy requirements of animals.

Table 1. Relationship between forage Quality Index and animal performance.

Quality Index	Heifer gain, lb/day	Milk production, lb/day*
1.0	0	0
1.4	0.6	10
1.8	1.1	20
2.2	1.6	30
* Assuming no change in body condition.

Factors Affecting Forage Quality

Forage quality is affected most by variations in genotype, maturity, season, and management. Other “anti-quality” factors may be encountered occasionally.

1. Genotype.

Legumes generally have higher quality than grasses. This is because legumes have a higher intake due to a higher percentage of rapidly digested leaves. Legumes also have higher CP concentrations, but TDN concentrations of legumes and cool-season grasses are quite similar. Generalizations about quality of grasses are risky, but temperate or cool-season grasses like rye and ryegrass often have higher quality than do tropical or warm-season grasses like bermudagrass and bahiagrass. There is, however, much variation within, as well as among, grass genera.

2. Maturity.

The stage of forage regrowth at time of cutting hay (i.e., days between harvests) or grazing pastures (i.e., rest period in rotational grazing) has a major influence on quality. Forage quality begins to decline as soon as forages start to regrow due to the accumulation of stems and deposition of lignin in both leaves and stems. Maturity of legumes and cool-season grasses can be assessed by determining the reproductive stage of growth. For warm-season grasses, however, weeks of regrowth is a better indicator of maturity because flowering may begin shortly after regrowth begins.

Table 2 provides examples of the effects of genotype and maturity on the quality of typical forage grasses in Florida. Each value represents several cuttings made from different cultivars in different years, so these values are just a general reference point. These data suggest that digitgrass and limpograss tend to have higher quality than do bahiagrass, bermudagrass, and stargrass, especially at later stages of maturity. These differences are seen in voluntary intake.

With respect to maturity effects on perennial grasses, the most dramatic difference is the decrease in voluntary intake between 6 and 8 weeks. These data, and others, show that after 8 weeks regrowth, forage quality will generally be less than needed for maintenance. Exceptions are digitgrass and limpograss that maintain a somewhat higher quality when mature than do the other grasses. Limpograss and digitgrass are excellent forages for fall stockpiling because they maintain higher TDN than do other grasses. Mature limpograss and digitgrass often are low in CP, however, and require protein supplementation for maximum utilization.

3. Season.

Seasonal effects on forage quality have been noted in grazing trials in Florida where forage regrowth intervals have been kept constant. A “summer slump” was observed in that gains of grazing cattle were less during the summer than in spring and fall. That this is an effect of environment on forage and not animals was suggested by a direct comparison of bahiagrass with dwarf elephantgrass. The summer slump was dramatic with bahiagrass but not apparent with elephantgrass even though similar cattle grazed adjacent paddocks of the two grasses. Summer slumps in quality of warm-season grasses have been observed with hay harvested after similar regrowth intervals on different dates throughout the growing season (Table 3). Summer regrowth may have lower quality because high temperature increases lignin deposition, and high rainfall increases growth rates and maturation.

In the case of hay made in Florida, the negative effects of season and maturity on forage quality may be additive. Spring harvests are made generally after short regrowth periods while summer harvests are made after long regrowth periods because of heavy summer rainfall. Therefore, the quality of bermudagrass hay is highest when harvested in the spring or early summer.

4. Management.

Pre-harvest management for maximum quality of hay or silage involves weed control and frequent cutting (see maturity, above). Some producers harvest every 4 or 5 weeks throughout the season, making either hay or silage depending on rainfall. The quality of hay or silage will never increase during harvesting and storage, but post-harvest decreases in quality can be minimized by careful management. Post-harvest management requires avoiding rain damage, proper curing of hay or wilting for silage, and minimizing losses during storage. Leaching of nutrients decreases forage nutritive value, and growth of mold may decrease palatability and intake.

For maximum quality, pastures should be managed to maintain a leafy canopy that is free of weeds and dead herbage, and is grazed uniformly without many ungrazed patches. There is much controversy about how to achieve that objective. Some grazing experts contend that frequent rotation is desirable. Others feel that if stocking rate is matched carefully to forage availability, then there is little advantage to frequent rotation. The requirements of a particular forage and the objectives of the livestock operation often are the most important factors influencing choice of rotation frequency. In addition, it is necessary to avoid over-grazing because lack of available forage will have a major negative impact on animal performance regardless of forage nutritive value and potential quality.

Generally, there is little effect of fertilizer on forage quality except that CP will be increased for a period of time following N fertilization. If forage CP is low in unfertilized grass, then N fertilizer will often improve forage intake and animal performance.

5. Other factors.

Examples of anti-quality factors are noxious weeds, nitrates, and prussic acid. These factors can be avoided generally by proper management, but may be very serious when they occur. In some cases, insects can defoliate forages thus decreasing forage quality. Cattle grazing improved forages grown under very wet conditions (i.e., standing water) are observed to have low rates of performance, but the reasons for this effect are not well defined.

Assessing Forage Quality

The discussions above have pointed out that forage quality can vary widely due to many factors. Tables of forage composition and quality have very little value in practice (including Tables 2 and 3 in this article). Tables can give a general idea of major factors affecting quality, but it is unlikely that tabular values will be accurate for a particular cutting of hay or silage, or for a pasture on a particular day. Therefore, in order to formulate rations or plan supplementation programs, it is important to sample forages regularly and frequently, and have them tested in a qualified laboratory.

Sampling is the most important factor affecting the accuracy of a forage test. Samples must represent forage being consumed by animals. Unless extra effort is taken to insure that samples are representative, it is not worth sending the samples to a laboratory. A probe is recommended for sampling hay bales. For pasture sampling, hand-plucking forage from the grazed canopy is recommended rather than clipping to ground level.

Many laboratories are capable of analyzing forage samples accurately for constituents such as CP, Acid-Detergent Fiber, Neutral-Detergent Fiber (NDF), and minerals. No laboratory can “measure” TDN or intake directly, however, because they are animal responses. Laboratories may “estimate” TDN and intake, however, using conversion formulas. These formulas are based on research trials with animals where TDN and voluntary intake have been compared statistically to characteristics of the forage measured in the laboratory. These conversion formulas are valid only for the type of forages used in the research. In many cases, laboratory managers may not know the origin and limitations of the formulas that they use. Furthermore, the formulas recommended by the National Forage Testing Association (NFTA) are not valid for use with the warm-season grasses commonly used as forages in Florida. Relative Feed Value as estimated by NFTA formulas underestimates the quality of warm-season forage grasses.

Economic Value of Forage Quality

The economic value of forages may be estimated in relation to the cost of a complete high-concentrate ration for a specific livestock operation, such as a heifer development program. Initially, the cost of a high-concentrate ration for a particular rate of gain is calculated. Next, the cost of supplement needed to achieve the same rate of gain with a particular forage is calculated. Then, a Concentrate Equivalent Value (CEV), $ per pound of dry matter, is calculated as follows:

CEV = (TC - SC) / FDMI

where:

TC = total cost of gain, $/head

= cost of high-concentrate ration, $/head

= high-concentrate intake, lb/head × $/lb

SC = cost of supplement, $/head

= supplement intake, lb/head × $/lb

FDMI = forage DM intake, lb/head

The difference between TC and SC is the cost of the forage portion of the ration when the total feed cost is the same for the forage-plus-supplement ration as for the high-concentrate ration. Therefore, if the price of forage equaled its CEV, feed costs would be the same for both the forage-plus-supplement ration and the high-concentrate ration.

Intakes of concentrate, supplement, and forage may be expressed either on a daily basis, or as total intakes, as long as they are all expressed the same. Prices per lb of concentrates and supplements should be calculated according to their specific formulations. The CEV values can be converted from $/lb of DM to $/ton of air-dry forage.

Three heifer development programs giving continuous ADG of 1.1 lb, were used to calculate CEV of three forages (Table 4). A computer model was used to generate the data in this example. Commodity prices of January 1998, were used to calculate the prices of the high-concentrate ration and supplements. These calculations show a definite difference among the three forages in their economic value, in terms of CEV; the higher the QI, the higher the value. If the actual price of a forage is less than the CEV, then it would be economically advantageous to use the forage. On the other hand, if the actual price was equal to or higher than the CEV, feeding a limited amount of high-concentrate ration would be the most economical way to achieve the desired performance.

Implications

1. Forage quality can be defined as animal performance when animals have a capacity for production, forage is available free choice, and no supplemental energy or protein is provided.

2. Forage quality varies widely due to variations in genotype, maturity, season, management, and anti-quality components. Because of all these factors and their interactions, tables of forage quality and nutritive value are unlikely to provide useful information about a particular forage. Therefore, it is important to test forages frequently. Samples must be taken carefully to insure that they are representative of forage being consumed.

3. Forage testing can provide accurate data on CP and mineral concentrations, but errors may occur in estimating TDN and intake. Formulas for estimating forage TDN and intake from laboratory analyses must be appropriate for the specific type of forage being tested.

4. Forage quality has a major impact on the economic value of forages for livestock production. Generally, high-quality forage costs more to produce than low-quality forage, and a higher price is required in order to make a profit. High-quality forage may have a high economic value for livestock production, but no forage is worth more than its Concentrate Equivalent Value.

Tables

Table 2. Effects of grass and maturity (age in weeks) on forage quality^a

Grass	TDNb			Voluntary Intakec			Quality Indexd
	4	6	8	4	6	8	4	6	8
Bahia	56	55	54	2.3	2.1	1.7	1.2	1.1	.9
Bermuda	57	52	44	2.3	2.2	1.8	1.3	1.1	.8
Star	60	53	49	2.4	2.5	2.1	1.4	1.3	1.0
Digit	60	58	57	2.5	2.7	2.2	1.5	1.5	1.2
Limpo	63	63	56	2.5	2.3	2.2	1.5	1.4	1.2
a Adapted from Brown and Kalmbacher, p. 79-87, in 47th Annual Florida Beef Cattle Short Course Proceedings, May 1998 (summary of research with sheep by J.E. Moore and W.R. Ocumpaugh). b Total Digestible Nutrients, percentage of dry matter. c Intake of dry matter expressed as percentage of body weight. d Voluntary TDN intake relative to maintenance requirement; 1.0 = maintenance.

Table 3. Quality of Coastal Bermudagrass hay harvested at different maturities and seasons^a.

Item	Weeks of Regrowth	Harvest Date
		6/14	7/12	8/9	9/6	10/4
TDN, %b	4 6 8	55 52 52	57 51 51	52 47 46	53 49 47	46 48 44
QIc	4 6 8	1.4 1.3 1.3	1.4 1.4 1.1	1.3 1.0 0.9	1.3 1.2 1.1	1.1 1.2 0.8
ADG, lbd	4 6 8	0.57 0.34 0.16	0.78 0.48 0.07	0.72 -0.04 -0.39	0.63 0.42 0.07	0.28 0.22 -0.39
a Adapted from Nelson et al., Louisiana Agr. Exp. Sta. Bull. 730, October, 1980. b Total Digestible Nutrients, percentage of dry matter. c Quality Index. d Average daily gain, in pounds/day; feeding trial conducted with steers from December through February for all hays.

Table 4. Concentrate Equivalent Value (CEV) of three forages in comparison to a high-concentrate ration for growing heifers from 450 lb to 650 lb in 180 days^a

Program		Total Feed, lb/hdc		Concentrate Cost, $/hdd	Forage Cost, $/hde	CEV, $/ton as fedf
Forage (QIb)	Concentrate	Forage	Conc.
A (1.80)	None	2390	0	0	129	97
B (1.45)	9% CP supplement	2170	450	35	94	79
C (1.10)	14% CP supplement	1370	1240	102	27	36
None	High-concentrate ration	0	1710	129
a Moore and Kunkle, p. 119-124, in 47th Annual Florida Beef Cattle Short Course Proceedings, May 1998. b QI = Quality Index. c Total intake per head, on dry matter basis; assume all feeds are 90% dry matter as fed. d Cost of supplements or high-concentrate ration per head; prices of concentrates per ton as fed: 9% CP = $137, 14% CP = $148, high-concentrate ration = $136. e Forage cost per head = total feed cost ($129, equivalent to cost of high-concentrate ration) - cost of supplemental concentrate. f Forage cost divided by forage intake, corrected to 90% dry matter as fed.

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Footnotes

1. This document is SS-AGR-93, one of a series of the Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First Published June 2002. Revised April 2006. This publication is also a part of the Florida Forage Handbook, an electronic publication of the Agronomy Department. For more information you may contact M. B. Adjei (mba@ufl.edu). Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. A. T. Adesogan, associate professor, Department of Animal Sciences; L. E. Sollenberger, professor, Agronomy Department; J. E. Moore, professor emeritus, Department of Animal Sciences; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.