Forage Quality for Ruminants: Plant and Animal Considerations1

The Professional Animal Scientist 11 :121-1 31

REVIEWS Forage Quality for Ruminants: Plant and Animal Considerations1 DWAYNE R. BUXTON2, DAVID R. MERTENS3, and KENNETH

J. MOORE4

Agricultural Research Service, USDA, and Iowa State University, Ames, IA 50011

Abstract Forage quality is a function of nutrient concentration, intake, nutrient availability, and partitioning of metabolized products within animals. Of these factors, intake potential is the major determinant of production by animals fed forage-based dietsj however, it is one of the most difficult aspects of forage quality to determine or predict because variation among animals has a large influence on intake. Physical fill limits intake of forages with high cell wall concentrations when fed to animals with high energy demand. Hence, grasses, with their higher cell wall concentration typically have lower intake than le' gumes. Energy availability of forage is also limited by cell wall concentration because cell contents are almost completely digested, whereas forage cell

l Th.is paper includes data presented by the senior author to the 28th Pacific Northwest Animal Nutrition Conference, Boise, ID, October 26-28, 1993. 2Field Crops Research Unit, ARS, USDA, De pt. of Ag ronomy, Iowa State University, Ames IA 50011. ' , 3U.S. Dairy Forage Research Center ARS USDA, Mad ison, WI 5 3706 . " 4Dept. of Ag rono my, Iowa State University, Ames, IA 50011 . ' Reviewed by L.

J. Boyd and J.

E. Oldfield .

walls are slowly digested. Thus, the proportion of cell walls to cell contents is a major determinant of energy availability in feeds . Protein digestion by ruminants is complex. When crude protein concentration in herbage drops below 7% of dry matter, ruminal fermentation of forages may be limited and protein requirements of animals may not be met. Additionally, inefficient use of protein in high quality forages may limit performance of high producing animals. Usually, only about 25% of the forage protein escapes degradation (rom the rumen. Efficiency would be improved if a larger portion of forage protein passed (rom the rumen undegraded so that it can be degraded in the intestines where absorption is more efficient. Another important plant factor influencing forage quality is herbage maturity. Systems are now available for determining maturity of both legumes and grasses that will become more important as aids for predicting forage quality before forages ~re han:ested or grazed. Forage quality 15 also mfluenced by the environment in which forages are grown and by soil fertility and these cause year-to-year, seasonal, and geographical variation in forage quality even when herbage is harvested at the same stage of maturity.

Introduction

Forage quality is determined by animal performance when forages are fed free choice (ad libitum) to livestock. It is a function of nutrient concentration in the herbage (including energy), intake potential, nutrient aVailability, and partitioning of metabolized products within animals and is usually estimated by in vitro chemical analysis of the plant. Although plant characteristics are major determinants of forage quality, animal variation can also impact its assessment. In most situations, intake of energy (mainly from digested carbohydrates) and protein determine animal performance and forage quality. Intake of available energy is primarily a function of plant cell wall concentration because cell walls limit intake and digestibility. Much of the complex carbohydrate in forages is contained in cell walls, which cannot be degraded by mammalian enzymes. Thus, animals must depend on microbial fermentation in the rumen (cows, sheep, an d goats) or hind gut (horses) to obtain the energy in cell walls, and forages predominate in the diets of animals with these specialized gut architectures. This adapta(Key Words: Forage, Grasses, legumes, Ruminants, Digestibility, Cell tion of animal to t he diet is so intertwined that ruminants essenWall Contents.)

0;

122

tially require the fiber in plant cell walls, and forages provide the fiber to livestock, which is required for normal rumen function. Fiber stimuiates the cardia! region of the reticulum to induce regurgitation, rumination, and ruminal motility. Available energy and metabolizable protein in forages are often low relative to animal requirements. Herbage age and maturity are the most important factors determining the amount of available energy and protein in forages. Additionally, plant environment, including soil fertility, causes changes in forage quality, which can lead to year-toyear, seasonal, and geographical variation in forage quality even when harvested at the same stage of maturity. We discuss each of these relationships in this article. Although minerals and vitamins contribute to forage quality (48), we will not consider nutrients beyond energy and protein. Neither will we consider other plant factors, such as canopy display to grazing animals (8), which influences forage quality by affecting intake, or accumulation of chemicals or other anti-quality factors that lower forage quality, as discussed by Bacon (4) and Bush and Burton (9).

Forage Intake Potential The amount of forage consumed is the major determinant of production by animals fed forage-based diets, yet it is one of the most difficult aspects of forage quality to determine or predict. Intake often accounts for more than twice as much variation in animal performance as does herbage digestibility (31). Intake potential of a forage is difficult to determine because variation among animals has a large influence on intake of a particular forage. The stimulus for animals to consume forage is influenced by their physiological status. For example, when consuming the same forage ad libitum, lactating cows consume more than mature animals that are simply maintaining

Buxton et al.

their body condition. Hence, most tables describing feed quality characteristics do not indude ad libitum intake potential. Before the intake potential of a forage can be evaluated, variability in intake due to animals must be minimized or eliminated (26). The major regulators of forage intake when animals are fed unrestricted diets are associated with physical, physiological, and psychogenic factors (31). We emphasize only physical limitations in this article because of their relevance to forages. Physical limitations occur when animals eat until the rumen is full. Therefore, physical fill limits intake of forages with high NDF (an estimate of cell wall) concentration when fed to animals with high energy demands, such as high producing dairy cows. This is why NDF concentration is negatively related to intake potential of a forage. Within a plant species, intake is usually positively correlated with digestibility because digestibility is inversely related to NDF concentration. Digestibility is subject to less variation among animals than intake and is an important item in evaluation of forage quality.

The maximum cell wall concentration of diets that will not hinder intake and animal production can be as high as 70 to 75% NDF in dry matter for mature beef cows and as low as 15 to 200Al NDF for fattening ruminants. The optimal concentration of NDF in diets of high producing dairy cows at peak lactation can be as low as 27 to 29%. These levels of NDF allow for adequate energy and maintain adequate fiber in the diet (30). Typical NDF concentrations at the mid-flowering stage of maturity ranges from 25 to 70% in forage leaves and from 40 to 85% in stems (fable 1). In addition to cell wall concentration, the filling effect of a forage is determined by rate of disappearance of cell walls from the rumen by digestion and passage. Passage from the rumen requires both particle size reduction and escape through the reticulo-omasal orifice of the rumen (30). Plant cell wall material must be chewed and digested to reduce its size before it can pass out of the rumen through this small opening. Grasses, with their high NDF concentration (fable 1), typically have lower intake potential than legumes. Likewise, because leaves

TABLE 1. Typical neutral detergent fiber (NDF) concentrations and in vitro dry matter digestibility (IVDMD) of perennial grasses and legumes at mid flowering a • IVDMD

NDF

Species

Leaves

Stems

Leaves

Stems

(% of dry matter)

Alfalfa Red clove; White clover Berseem clover Birdsfoot trefoil Tall fescue Smooth bromegrass Orchardgrass Reed canaryg rass Bermudagrass Switchgrass

25 25 25

55 40

25 50 50 50 50 65 b 70

55 70 70 70 70

aReferences: 5, 10, 13, 14, 15, 27. bTotal herbage in vegetative stage (17).

85

75 73 76 68 75 59 59 59 59 65 b 51

55 70 60 56 51 51 51 51 38

Forage Quality for Ruminants

123

This also emphasizes the critical nature of the amount of cell wall material in forage on the overall availability of energy-yielding nutrients to ruminants. Most of the energy obtained from legumes comes from the large proportion of the dry matter in these plants that is cellsoluble. This characteristic difference between legumes and grasses or between immature and mature forages not only impacts availability but may also affect metabolic efficiency. Cell contents result in different end products of digestion and require less metabolic and digestive work than cell walls, resulting in both greater digestibility and greater efficiency of metabolism. Digestibility, expressed as dry matter, organic matter, energy, or total digestible nutrients (fDN), is the most commonly used index of energy availability. Total digestible nutrients is the sum of digestible protein, carbohydrates, and lipids (ether extract). Digestible ether extract is multiplied by 2.25 to account for the higher energy concentration of lipids compared with proteins and carbohydrates. The TON requirement of ruminants varies from a low of SOOAl of dry matter in diets of mature animals to 75 to 85% in diets of lactating females and rapidly growing young animals. The ,--_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _--, TON value of forages often ranges from one-half to two-thirds of the TABLE 2. Representative neutral detergent fiber (NOF), digestibility, and dry matter (fable 2) . total digestible nutrients (TON) in forage herbage with specified levels Epidermal layers of plants restrict of NOF. microbe and enzyme access to forage particles. Hence, ruminal microbes Proportion of normally gain access to cell walls NDF NDF nutrients Species Concentration Digestibility TON from NDF through cut or sheared surfaces of herbage. Plant tissue types are (%DM) (% NDF) (% DM) (% TDN) colonized and degraded by rumen Cool-season legumes microorganisms to different extents 30 48 70 20 40 45 64 28 (fable 3). Mesophyll and phloem 50 40 56 36 cells are degraded more completely and before the outer bundle sheath Cool-season grasses 50 70 71 49 and epidermal cells. Attachment of 60 66 66 60 bacteria to sclerenchyma and vascu70 59 58 72 lar bundle cells occurs less readily and these tissues are slowly and Warm-season grasses 60 64 65 60 incompletely degraded. Because 70 56 56 70 80 50 stems generally have more tissues 46 85 resistant to digestion than leaves,

have low NDF concentrations (especially legume leaves; Table 1), they are consumed more readily than stems. Grasses require more chewing than legumes, again, because of their high cell wall concentration and because grasses do not fracture into small particles during chewing as readily as do legumes (56). This probably contributes to the decreased rate of passage and increased filling effect of grasses compared with legumes.

organic matter in forage crops. Whereas plant cell contents are almost completely digestible, the availability of plant cell walls to ruminant livestock varies greatly depending on their composition and structure (33). The reason that legumes are more digestible than grasses is because they contain less cell wall material, not that their cell walls are more digestible (fable 2). In fact, the cell walls of legumes are more lignified and less digestible than those of grasses. Similarly, the major factor improving the nutrient availability of immature Energy from Forages. Constituforage compared to mature forage is the lower cell wall and higher cellents in plants that provide most of the energy for animals are carbohysoluble concentrations of immature drates, proteins, and lipids. Carbohy- forage. Thus, the most significant drates provide up to 80% of the way of improving forage quality is to energy for ruminants from herbage, reduce the cell wall concentration. whereas lipids contribute less than Once the lower limit of cell wall 5%. The gross energy, i.e., caloric concentration is attained that is content obtained by combUstion, of critical for plant growth and survival forages provides little nutritional or for animal health, digestibility of information. The portion that is cell wall material becomes the only digestible is more important in remaining limitation to nutrient determining energy availability to availability in forages. animals. Ruminants normally digest 40 to The most meaningful division of 70% of the dry matter in cell walls plant dry matter into energy-yielding (fable 2). The proportion of the total components for animals is between energy in forages that is extracted cell walls and cell contents. Cell from cell walls varies from 20 to 400Al walls account for 40 to 800/0 of the for legumes to 50 to 80% for grasses.

Nutrient Availability

i24

Buxton et ai.

TABLE 3. Summary of plant tissues and their relative degradability. Based on data from Akin and Chesson (1) and Wilson (56). Tissue

Function

Relative degradability

Epidermis Xylem Phloem Parenchyma Collenchyma Sclerenchyma

Dermal Vascu lar Vascu lar Metabolic Structural Structural

None to low None to low Low to moderate Moderate to high Moderate to high None to low

stems are low in dil!f'stihilitv (Table rumen can be up to five times faster I), and stem digestibility d~clines . for legumes than for grasses (21). more rapidly with increasing plant Digestible NDF intake for cattle maturity than does that of leaves (14, consuming grasses is generally greater 15). Differences between leaf and than for cattle consuming legumes stem digestibility, however, are because grasses contain more digestnormally less in grasses than in ible NDF than legumes (Table 2). legumes (Table 1). Bottom stems are Animals may adapt to increased cellmore mature than top stems and of wall concentration in grass diets by lower digestibility, especially in increasing the maximum gut fill or legumes. Digestibility declines along rate of ruminal clearance (31). Weiss the stems of alfalfa (Medicago sativa and Shockey (52) found that lactatL.) and birdsfoot trefoil (Lotus ing dairy cows consuming ad libitum corniculatus L.) at about 2 percentage diets of alfalfa or orchardgrass units per node (14). Nodes and (Dactylis glomerata L.) silage coninternodes of alfalfa stems have sumed 43% more digestible NDF, similar digestibilities, whereas red 14% more NDF, and 8% less dry clover (Trifolium pratense L.) internmatter when fed orchardgrass rather odes are about 4 percentage units than alfalfa. Digestible dry matter more digestible that nodes (D. R. intake was similar for alfalfa or Buxton, unpublished data). Generorchardgrass silages and there was no ally leaves of cool-season grasses are difference in milk production. The more digestible than those of warme.xpected smaller intake of season grasses (Table 1) because they orchardgrass dry matter compared to have a greater proportion of mesoalfalfa at similar digestibilities has phyll cells (56). Leaf blades of grasses also been confirmed by other studies are about 10 percentage units more (49). digestible than leaf sheaths (23). Forage Protein. Nitrogen in forage Legumes generally have a more can be divided into true protein and rapid digestion rate of potentially nonprotein nitrogen (NPN). Most of digestible NDF than grasses (30). the NPN is in nucleic acids, free Additionally, immature forages have amino acids, amides, and nitrate. a faster rate of NDF digestion than When sufficient carbohydrate or more mature forages. Leaves are other sources of energy are available retained in the rumen for a shorter for microbial growth, NPN is contime than stems because of both verted to ammonia in the rumen and faster rates of NDF digestion and used for microbial protein synthesis. higher rates of passage. The number True protein normally comprises 60 of times that ingested hay is chewed to 800A> of the herbage nitrogen. and the time spent masticating is More than 900A> of nitrogen in most greater for late-cut than for early-cut forages is in cell solubles and is hays and greater for grasses than for readily digestible. legumes. As a result, rate of disapProtein digestion by ruminants is pearance of late-cut hay from the complex and involves degradation

and loss of protein from the rumen, transformation of forage protein to microbial protein, and ultimately digestion and absorption of amino acids from microorganisms and herbage that move out of the rumen (6,29). Under some circumstances, animal production is limited because excessive amounts of forage nitrogen are lost from the rumen and from the animal. The extent of protein degradation by rumen microbes is governed by the proteolytic rate and length of time plant residues are retained within the rumen. After proteins are degraded to amino acids and peptides, they can be assimilated by microorganisms and used to synthesize microbial protein or they can be deaminated and metabolized for energy. When amino acids are deaminated, ammonia is released into the rumen. lf this ammonia is absorbed through the ruminal wall into the bloodstream, it is detoxified in the liver by conversion to urea. Some of the urea is recycled to the rumen via saliva and through the ruminal wall, but substantial amounts are excreted in the urine. Transformation of forage protein to microbial protein in the rumen is determined predominantly by availability of energy for microbial growth. Hence, feeding of an energy source, such as grain, along with forage improves the effidency of utilization of protein Nand NPN. Protein synthesized by microbes often accounts for 50 to 800/0 of the nitrogen reaching the small intestine. When available energy is adequate, microbial protein can furnish animals 'With enough protein fOi maintenance and some growth. When limited by energy availability in forages, microbial protein may not be produced in enough quantity for optimum performance during rapid growth, late pregnancy, or early lactation. Protein reqUirements for livestock usually are expressed as crude protein (nitrogen x 6.25). Crude protein requirements range from 7% of dry matter for mature beef cows to 19% for high producing, lactating dairy

125

Forage Quality for Ruminants

COWS (39, 40). On a world-wide basis, average crude protein concentration is about 10% of dry matter for warm-season grasses such as bermudagrass [Cynodon dactylon (L.) Pers.] and switchgrass (Panicum virgatum L.), 13% for cool-season grasses such as tall fescue (Festuca arundinacea Schreb.) and orchardgrass, and 17% for legumes such as alfalfa and red clover (32). Many high-quality, improved forages have higher values. That in corn (Zea mays L.) silage, however, is usually less than 10%. Leaf blades generally have twice as much crude protein as stems. Addition of nitrogen fertilizer increases crude protein concentration of grasses as well as yield, but the protein concentration usually remains less than that for legumes. To more efficiently use protein in high-quality herbage, either the fraction of protein that is degraded must be decreased or the availability of energy must be increased to recapture a greater proportion of the protein that is degraded to ammonia. Usually animal production is improved if more forage protein passes from the rumen undegraded so that it can be degraded in the intestines and absorbed as amino acids rather than being absorbed as ammonia from the rumen. Protein passing from the rumen to the intestine in an undegraded form is called escape or bypass protein (6). On average, only about 25% of the forage protein escapes degradation in the rumen (29,32). The proportion of escape protein is often greater in mature herbage (22, 29). The effect may be influenced by plant species because Mullahey et al. (37) found that escape protein remained relatively constant with advance in maturity of smooth bromegrass (Bromus inermis Leyss.), but increased with maturity of switchgrass and reached 70% at the ripe seed stage (Figure 1). We emphasize that a higher proportion of escape protein in mature herbage does not mean that more protein is available to animals that consume

mature herbage because mature herbage has a low crude protein concentration (Figure I), which limits the influence of a high proportion of escape protein. Plant species with moderate levels of tannin, such as birdsfoot trefoil and sanfoin (Onobrychis viciifolia Scop.), have a higher proportion of escape protein than those without tannin such as alfalfa, red clover, and white clover (Trifolium repens L.) (2, 6). The amount of escape protein is higher for red clover than for alfalfa or white clover. Broderick (6) reported that average escape protein was 21% for alfalfa and 29% for red clover. Yellow-flowered alfalfa (Medicago facata) has a somewhat greater escape protein percentage than blue-flowered M. sativa (7).

Metabolic Efficiency Variation in metabolism and partitioning of absorbed nutrients among forages accounts for much less variation in animal performance than do intake potential and availability of nutrients. Digestible energy obtained from forage is generally used less efficiently than equal digestible energy from grain. Additionally, when grasses and legumes are fed at similar levels of intake, energy from legumes may be used more efficiently for tissue gain than grasses (51). Although the difference between grasses and legumes in efficiency of energy use is not universally accepted, many studies suggest a greater efficiency in converting metabolizable energy of legumes to net energy for production. This difference between grasses and legumes may be related to the additional energy used by the digestive tract to process the greater cell wall concentration of grasses or to the differences in end products absorbed from the digestion of cell walls vs cell contents. Information in Table 2 shows that the proportion of total TDN coming from digested cell walls is much less for legumes than for grasses. Some evidence indicates that the potential difference in efficiency

relates to differences in protein absorption. Because legumes have higher protein concentrations, a higher proportion of the digestible energy absorbed is derived from amino acids. This difference in protein concentration may have its biggest effect on the performance of young, rapidly growing animals. If the carbon skeletons from deaminated amino acids are used as glycolytic precursors to convert acetate to fat tissue, however, higher protein concentration may also improve energetic effiCiency of acetate utilization.

Herbage Age There is usually a strong correlation between herbage age (time following initiation of spring growth or since previous harvest) and digestibility for most spring grown, coolseason forages (20). Digestibility typically decreases .3 to .5 percentage units per day during May and June in the northern U.S. with variation associated with geographical location and plant species. As plants advance in maturity, the leaf:stem ratio usually decreases. For example, Albrecht et al. (3) reported that the leaf:stem ratio of alfalfa decreased from 1.5 in the early vegetative stage to about .5 in the late flowering stage. Additionally, cell wall concen-

:sc

eil:' ~~

Swttchg . . . . .

~~

40

~- 30 20

.n C

S ei' D..Q

-8~

2u

...... 54

70 10 D..u 10

~-

" sm:lioth Bromeg:.!----f, ~------~.~J----~E~.-------4

14 12 10 •

• 4

1L4O--~11-0--1~1O--~2~--~~~0--~24O---2-80--2~80 Day of Year

Figure 1. Concentration of CP and escape protein percentage of switchgrass and smooth bromegrass. Data from Mullahey et al. (37). Maturity stage using the Nebraska System (Table 5) is shown for each harvest.

i26

tration within stems and most leaves increases, and digestibility decreases with age. Sanderson and Wedin (45) reported that dry matter digestibility of leaf blades of timothy (Phleum pratense L.) and smooth bromegrass declined with increasing maturity, whereas those of alfalfa and red clover retained relatively high digestibility with advancing maturity. As a result, digestibility of grass herbage usually declines faster with maturity than that of legume herbage and warm-season grasses decline faster than cool-season grasses. Although digestibility of most legume leaves remains high in old leaves, the concentration of crude protein decreases as rapidly in leaves as in stems (14). Fastest rates of decline in forage quality occur during primary (first) spring growth of grasses when reproductive development is occurring. Flowering alters the morphological development of most forage grasses because the terminal stops producing new leaves, causing a rapid shift toward a lower leaf:stem ratio. Buxton and Marten (15) reported that in Iowa, herbage digestibility of four cool-season grass species with a moderate number of reproductive tillers had an average rate of decline in digestibility of .39 percentage units per day, whereas the rate averaged .61 percentage units per day in plants with numerous reproductive tillers. By comparison, alfalfa herbage digestibility decreased .28 percentage units per day during spring growth (14), which is similar to data reported by Fick and Onstad (19). Brink and Fairbrother (5) reported that digestibility of berseem (Trifolium alexandrinum L.) and red clovers declined .2 percentage units per day in Mississippi. As forages advance in maturity, voluntary intake also declines. Minson (32) reported an average decline of .39 g/kg BW·75 per d during spring growth. The decline is probably a function of fill limitations in the animal from an increase in cell wall concentration with maturity.

Buxton et al.

forage test before decisions have to be made, such as deciding the optimal date for harvesting or grazing of forages to obtain a desired forage quality. Nutrient testing of forages in . laboratories for forage quality will still be necessary for ration balancing. Fick and Mueller (18) described the maturity staging system commonly used for alfalfa (Table 4). A modification of this system has been developed for red clover (42) . No one staging system is widely adapted for perennial forage grasses. One of the first shown to have utility was that developed by Simon and Park (47) . This system relies on the most mature stems to determine maturity. Because perennial grass swards are highly diverse in forage maturity, this system has limitations for predicting forage quality (15). Three new systems have been developed that overcome this limitation. The first is that developed by Moore et al. (35; Table 5; the Nebraska system), which Forage Maturity is a generic system for application to all perennial grasses. The second was Although we have considered forage age, under most circumstances developed for perennial warm-season grasses by Sanderson (44) and the it is forage maturity that is more closely related to forage quality. The third was developed for bermudadistinction between age and maturity grass by West (53). With the exception of the Simon is sometimes overlooked because and Park system (47), these staging they are usually confounded in natural environments. Forages systems utilize representation of all tiller or stem maturities in a forage mature more rapidly in warm summer temperatures than in cool spring sward. Two methods of calculating the mean stage are used; one uses and fall temperatures, but slower during water stress. average stage weighted for number of shoots at each stage (Mean Stage by Emphasis is now being placed on Count or MSC) and the other uses developing relationships between average stage weighted for dry weight maturity and forage quality. In the future, we will likely see more impor- of shoots at each stage (Mean Stage by Weight or MSW). Mean Stage by tance placed on precisely describing the maturity of forages consumed by Count is faster to determine than MSW and equations have been livestock and in developing better relationships between forage matudeveloped to convert MSC to MSW rity and forage quality so that matu- in the alfalfa system (36). Because all stems or tillers in a rity can be used as a predictor of forage quality at the time of harvest stand are considered in these new systems, as opposed to visual descripor grazing (20, 34). This will not tors such as "booting","heading", replace the need for laboratory "first flower", or "full bloom" where testing of forages. Forage maturity, however, will be used for making only the most mature stems are taken decisions in those situations in which into account, these new systems there is not time enough to obtain a describe the average shoot in the

Protein concentration is also higher in immature herbage. The average decrease in crude protein concentration with advancing age for several species and experiments summarized by Minson (32) was .22 percentage units per day. For most species, the fastest rate of decline occurs in very immature herbage with a rate that slows as herbage becomes more mature (19). When moderate to high rates of nitrogen fertilizer are applied, the protein concentration of immature grasses can be high with a faster rate of decline (15). Although concentrations of crude protein decrease in both leaves and stems, the decline is accelerated because stems make up a larger portion of total herbage as reproductive tillers mature. A slower rate of decline normally occurs in aftermath growth of grasses because less stem material is produced.

127

Forage Quality for Ruminants

TABLE 4. The 10 stages of alfalfa development described by Fick and Mueller (18). Stage name

Stage number

Stage description

Early vegetative Mid vegetative Late vegetative Early bud Late bud Early flower Late flower Early seed pod Late seed pod Ripe seed pod

o

No reproduction, shoots <15 cm " shoots 15 to 30 cm long " shoots> 30 cm long 1 to 2 nodes with visible buds ~ 3 nodes with visible buds One node with an open flower ~ 2 nodes with open flowers 1 to 3 nodes with green seed pods ~ 4 nodes with green seed pods Nodes with mostly brown seed pods

1 2

3 4 5 6

7 8 9

TABLE 5. Primary and secondary growth stages and their numerical indices and descriptions for staging growth and development of perennial grasses (35). Stage

Index

Vegetative-Leaf development VO 1.0 Vl (1/N) + .9 a V2 (2/N) +.9 Vn (n/N) + .9

Description

Emergence of first leaf First leaf collared Second leaf collared Nth leaf collared

Elongation-Stem elongation

EO

2.0

El E2 En

(1/N) + 1.9 (2/N) + 1.9 (n/N) + 1.9

Onset of stem elongation First node palpable/visible Second node palpable/visible Nth node palpable/visible

Reproductive-Floral development

RO Rl R2 R3

R4 R5

3.0 3.1 3.3 3.5 3.7 3.9

Seed development and ripening SO 4.0 51 4.1 S2 4.3 S3 4.5 S4 4.7 S5 4.9

Boot stage Inflorescence emergence/first spikelet visible Spikelets fully emerged/peduncle not emerged Inflorescence emerged/peduncle fully elongated Anther emergence/anthesis Postanthesis/fertilization

stand. For example, an alfalfa stand at "first flower" based on visual observation is not the same as a stand at Stage 5 (early flower, Table 4) of the Fick and Mueller system. In fact, "first flower" is approximately equal to Stage 3 based on MSW and "full bloom" is approximately equal to Stage 5 (20). In addition to these staging systems, Hintz and Albrecht (25) described a fast prediction method for alfalfa using plant height and maturity stage of the most mature stem in a sample, which they found provided better prediction equations in Wisconsin than those based on MSW or MSC with the Fick and Mueller system. West et a1. (54) described the use of staging sytems in wheat (Triticum aestivum L.) to predict forage quality. Much information has been developed relating the alfalfa staging system to traits associated with forage quality (20), but very little is available relating the recently developed grass systems to forage quality. Most of the research with the newer grass staging systems has been in describing developmental morphology of perennial grasses with respect to time and environmental variables (35) although a number of studies have been initiated to determine the relationship between forage quality and developmental morphology of tiller populations using the Nebraska system. 100 1 - -1

r========:::::;-,

80

::E C

80

1: Caryopsis visible Milk Soft dough Hard dough Endosperm hard/physiological maturity Endosperm dry/seed ripe

aWhere n equals the event number (number of leaves or nodes) and N equals the number of events within the primary stage (total number of leaves or nodes developed). General formula is P + (n/N) -.1; where P equals primary stage number (lor 2 for vegetative and elongation, respectively) and n equals the event number. When N > 9, the formula P + .9(n/N) should be used. ~--------------------------------------------------~

~ 40r-----~----~-----4----~

CI)

a..

20 °1~--~~--~--~~-~ 1.5 2 2.5 3

Mean Stage Count

Figure 2. Concentration of crude protein and in vitro dry matter digestibility (IVDMD) as a function of Mean Stage Count using the Nebraska System (Table 5) for smooth bromegrass and intermediate wheatgrass grown near Mead, NE during the 1990 growing season.

Buxton et al.

An example of using the Nebraska system to determine the relationship between forage quality parameters and stage of development is presented in Figure 2. Smooth bromegrass and intermediate wheatgrass (Thinopyrum intermedium (Host) Barkw. & D. R. Dewey) were grown in plots at the University of Nebraska Agricultural Research and Development Center near Mead, NE 68041. Tiller samples were collected from six replicated plots of each species at 11 sampling dates during the 1990 growing season. Excellent predictive equations of forage quality parameters were developed as linear (crude protein) or quadratic (digestibility) functions of MSC and MSW. Coefficients of determination for predicting digestibility and crude protein concentration as a function of MSC were .95 and .99 for smooth bromegrass and .90 and .97 for intermediate wheatgrass, respectively. Crude protein concentrations were similar between species with respect to MSC (Figure 2). However, at any given maturity, intermediate wheatgrass had a higher digestibiliity than smooth bromegrass.

composition of plant parts. Environmental factors that have the greatest effect on nutritive value of forages are temperature, water deficit, solar radiation regimen, and soH nutrient availability. Temperature usually has the greatest influence. Temperature. Optimal growth temperatures are near 20°C for coolseason species compared with 30 to 35°C for warm-season species. At temperatures below the optimum for growth, soluble sugars accumulate because of the lower temperature sensitivity of photosynthesis compared with that of growth (38). A rise in temperature normally increases rate of plant development and reduces leaf:stem ratios and digestibility. Increasing temperature lowers forage quality even when compared at the same morphological stage. Each 1 °C increase in temperature will generally decrease digestibility of forages .3 to .7 percentage units with only minor effects on crude protein concentration (41, 55, 58). This is one reason that forages produced at northern latitudes or high elevations in the U.S., with their low temperatures, tend to be of high quality.

The depressed digestibility associated with elevated temperatures is usually attributed to higher NDF concentrations (Table 6). Additionally, the NDF of forages grown under warm temperatures is usually less digestible than that of forages grown under cool temperatures. During spring growth, the effect of warming temperatures interacts with advancing maturity to cause a more rapid decline in forage quality with time than occurs during summer growth (22, 50). Spring-grown forage can be of very high quality if harvested early, but because of the rapid rate of decrease in quality, poor timing of harvest or grazing will have a large negative impact on quality. Because the majority of forage from coolseason, perennial species is produced in the spring, harvesting and grazing per unit of land proceed at a slower rate than during the summer when yields are lower. As a result of the high spring yields, mismanagement has a greater negative effect then on total forage quality than occurs during the summer. Furthermore, temperatures are not increasing with the advancing season during latesummer and the effect of advancing

Plant Environment Plant environment usually has a smaller effect on forage quality than on forage yield, and environmental effects on forage quality are generally less than those of forage maturity (II, 12). Environmental effects are integrated through plant physiological processes and reflected in forage growth rate, developmental rate, yield, and herbage quality. Year-toyear, seasonal, and geographical variation in environment alter forage growth, development, and herbage quality even when forages are harvested at similar morphological stages. This results in inconsistent performance of animals that consume the forage. Environment often exerts its greatest influence on forage quality by altering leaf:stem ratios, but it also causes other morphological modifications and changes in chemical

TABLE 6. Effect of growth temperature on chemical and digstibility characteristics of leaf blades and stem plus sheath of perennial forages (57). lignin Species

Plant part

8ermudagrass

Leaf Stem Leaf Stem Leaf Stem Leaf Sterna Leaf Stem

22°(

32°(

NDF

22°(

32°(

- - (%of DM)

Switchgrass

Panicum laxum Perennial ryegrass Alfalfa

aLeaf sheath only.

1.3 3.4 1.8 4.2 1.6 3.0 1.4

2.2 6.7 2.6 6.8 2.3 5.0 1.8

NO

NO

.8 8.6

1.4 9.9

37 57 44 64 38 51 29 38 10 42

NDF digestibility

22°(

32°(

(% of NDF)

51 64 39 65 45 55 33 50 11

42

75 60 74 65 76 55 78 78 42 36

62 41 67 42 65 42 66 69 22 30

Forage Quality for Ruminants

maturity often results in a slower decline in forage quality than occurs during the spring (22). Water Stress. Drought usually inhibits tillering and branching of forages and hastens death of established tillers. Leaf area is reduced because of accelerated rate of senescence of older leaves. Both nitrogen and soluble carbohydrates are mobilized and moved out of leaves as they die. Under severe prolonged water stress, leaves are lost and some perennial species may go into dormancy, which causes most nutrients to be translocated from leaves to roots and results in poor forage quality. Water stress typically slows development of forages (fable 7). If the leaf loss associated with drought is not severe and a temperature rise does not occur with drought, dry growing conditions may actually increase the leaf/stem ratio and improve forage digestibility. Water stress of alfalfa, sufficient to cause a 50% yield reduction, improved digestibility but not crude protein content because crude protein concentration increased in stems and decreased in leaves (fable 7). Similar results have been reported for other forage species (43, 46). Halim et al. (24) concluded that slowing of plant maturation and growth during water stress accounted for much, but not all, of the changes in forage quality.

The effect of water stress on crude protein concentrations has been inconsistent in many other studies, which probably reflects whether the stress was great enough to cause leaf senescence and on the distribution of nitrogen within the soil profile in relation to the limited available soil water. If both nitrogen and water are present in the same soil horizon, they may be taken up together and herbage crude protein concentration may be unaffected or may be increased if nitrogen is more available than soil water. If subsoil water is ample and most of the soil nitrogen is near the surface, however, growth may continue with reduced nitrogen uptake so that herbage crude protein concentration declines. Solar Radiation. Changes in photoperiod can also influence forage quality. Work summarized by Deinum et al. (16) indicates that each increase of 1 h in day length can increase digestibility by about .2 percentage units. Thus, a lengthening photoperiod during spring and a long photoperiod during early summer have a positive effect on forage quality, whereas shortening photoperiod during late summer and fall have a negative effect. However, cool temperatures in the spring and in the fall contribute to high forage quality and these contradictory effects partially cancel the effect of lengthening photoperiod. The long

TABLE 7. Response of alfalfa to various irrigation levels when harvested on a common date (24). Irrigation level

Maturityb

Leaf/stem

IVDMDC

2.5 3.2

.72 .72 .63 .61 .60

65 65 64 64

(% FC)a 65 77 88 100 112

CP

(% of DM)

3.4 3.5 3.8

aField capacity. bS ee Table 4. cln vitro DM digestibility.

63

21 21 22 22 21

129

summer days in the northern U.S. with associated cool temperatures should generally contribute to high forage quality, however. Additionally, periods of cloudy weather may lower forage quality. Bright sunshine just before harvest or grazing normally increases nonstructural carbohydrates in herbage and increases forage quality (12, 20). Diurnal variation also occurs in the concentration of nonstructural carbohydrates of forages with lowest values before sunrise and highest values in the late afternoon. Lechtenberg et a1. (28) found that digestibility of alfalfa was about 1.6 percentage units greater in the late afternoon than in the early morning. Leaf starch concentration also increased by 10% of dry matter during the daylight. This change caused a shift in leaf:stem ratio from 1.1 in the early morning to 1.5 in the late afternoon. Additionally, protein concentration shows diurnal fluctuation with highest concentrations in late afternoon (12). Some of the advantage of higher quality forage in the late afternoon may be lost when harvested for hay because of high respiration rates associated with these soluble plant fractions. Additionally, waiting until afternoon to cut forage for hay will increase the drying time and may increase the potential of rain during drying. It may be possible to take advantage of diurnal variation under some grazing systems and when forages are ensiled, however. Nutrient Availability. Normal variation in soil nutrients has only small effects on forage quality. Nitrogen fertilization has the greatest impact and usually raises crude protein concentration of grasses as already discussed. Forage species with typically low crude protein concentration, such as warm-season grasses, can have improved digestibility following nitrogen fertilization because the increased nitrogen may provide a better balance of available nitrogen and energy, causing rumen microbe activity to be stimulated (12).

130

Buxton et ai.

Conclusions Of the variation in energy intake among forages, approximately 6S to 75% may be related to dry matter intake, 20 to 30% to differences in digestibility, and S to 15% to differences in metabolic efficiency. The central role of intake in determining forage quality is illustrated by the observation that intake of legumes is 20 to 3()% greater than that of grasses when digestibilities of these forages are similar. Cell wall concentration is the most important plant characteristic affecting intake because it is related to the filling effect and digestibility of forages. Measurement of digestibility is an important step in evaluation of forage quality. Digestibility is subject to less variation among animals than intake. Digestibility is also a function of cell wall concentration and variability in cell wall digestibility. Digestibility is correlated positively with intake within a species. Herbage age and maturity generally have a larger influence on forage quality than environmental factors. Environmental factors, however, cause deviations in forage quality even when forages are harvested at the same maturity. Producers control herbage maturity by selecting the harvest or grazing date. As greater refinements in forage quality are required to meet future needs of high producing animals, the need to accurately predict forage quality before harvest or grazing will become more important. This will place increased emphasis on a better understanding of the effect of environmental factors on forage quality.

France, 4-11 October 1989. p. 1753. Association Francaise pour Ia Production Fourrager, Versailles, France. 2. Albrecht, K. A., and G. A. Broderick. 1993. Ruminal in vitro degradation of protein from different forage legume species. In 1992 Research Summaries. p 92. U.S. Dairy Forage Research Center, Madison, WI.

16. Deinum, B.,]. Beyer, P. H. Nordfeldt, A. Kornher, O. Ostgard, and G. Bogaert. 1981. Quality of herbage at different latitudes. Neth. ]. Agric. Sci. 29:141.

3. Albrecht, K. A., W. F. Wedin, and D. R. Buxton. 1987. Cell-wall composition and digestibility of alfalfa sterns and leaves. Crop Sci. 27:735.

17. de Ruiter,). M., and). C. Bums. 1987. Digestible and indigestible cell wall carbohydrates of flaccidgrass, tall fescue, and Coastal bermudagrass. Crop Sci. 27:132.

4. Bacon, D. W. 1994. Fungal endophytes, other fungi, and their metabolites as extrinsic factors of grass quality. In Forage Quality, Evaluation, and Utilization. p 318. G. C. Fahey, Jr., M. Collins, D. R. Mertens, and I.. E. Moser (Eds.). American Society of Agronomy, Madison, WI.

18. Fick, G. w., and S. C. Mueller. 1989. Alfalfa: Quality, maturit-y; and mean stage of development. Information Bull. 217, College of Agric. and Life Sci., Cornell Univ., Ithaca,

5. Brink, G. E., and T. E. Fairbrother. 1992. Forage quality and morphological components of diverse clovers during primary spring growth. Crop Sci. 32:1043. 6. Broderick, G. A. 1994. Quantifying forage protein quality. In Forage Quality, Evaluation, and Utilization. p 200. G. C. Fahey, ]r., M. Collins, D. R. Mertens, and 1. E. Moser (Eds.) . American Society of Agronomy, Madison, WI. 7. Broderick, G. A., and D. R. Buxton. 1991. Genetic variation in alfalfa for ruminal protein degradability. Can.]. Plant Sci. 71:755. 8. Bums,]. c., K. R Pond, and D. S. Fisher. 1994. Measurement of forage intake. In Forage Quality, Evaluation, and Utilization. p 492. G. C. Fahey, ]r., M. Collins, D. R. Mertens, and 1. E. Moser (Eds.). American Society of Agronomy, Madison, WI. 9. Bush, 1., and H. Burton. 1994. Intrinsic chemical factors in forage quality. In Forage Quality, Evaluation, and Utilization. p 397. G. C. Fahey, ]r., M. Collins, D. R Mertens, and 1. E. Moser (Eds.). American Society of Agronomy, Madison, WI. 10. Buxton, D. R. 1990. Cell-wall components in divergent germplasm of four perennial forage grass species. Crop Sci. 29:213. 11. Buxton, D. R., and M. D. Casler. 1993. Environmental and genetic effects on cell wall composition and digestibility. In Forage Cell Wall Structure and Digestibility. p 685. H. G. lung, D. R Buxton, R. D. Hatfield, and]. Ralph (Eds.). American Society of Agronomy, Madison, WI. 12. Buxton, D. R, and S. 1. Fales. 1994. Plant environment and quality. In Forage Quality, Evaluation, and Utilization. pISS. G. C. Fahey, ]r., M. Collins, D. R. Mertens, and 1. E. Moser (Eds.). American Society of Agronomy, Madison, WI.

literature Cited 1. Akin, D. E., and A. Chesson. 1990. Lignification as the major factor limiting forage value especially in warm conditions. In Proc. XVI Int. Grassland Congr., Vol. III. Nice,

15. Buxton, D. R, and G. C. Marten, 1989. Forage quality of plant parts of perennial grasses and relationship to phenology. Crop Sci. 29:429.

13. Buxton, D. R., and J. S. Hornstein. 1986. Cell-wall concentration and components in stratified canopies of alfalfa, birdsfoot trefoil, and red clover. Crop Sci. 26:180. 14. Buxton, D. R.,]. S. Hornstein, W. F. Wedin, and G. C. Marten. 1985. Forage quality in stratified canopies of alfalfa, birdsfoot trefoil, and red clover. Crop Sci. 25 :273 .

NY.

19. Fick, G. w., and D. W. Onstad. 1988. Statistical models for predicting alfalfa herbage quality from morphological or weather data.]. Prod. Agric. 1:160. 20. Fick, G. w., P. W. Wilkens, and]. H. Cherney. 1994. Modeling forage quality changes in the growing crop. In Forage Quality, Evaluation, and Utilization. p 757. G. C. Fahey, ]r., M. Collins, D. R. Mertens, and 1. E. Moser (Eds.) . American Society of Agronomy, Madison, WI. 21. Grenet, E. 1989. A comparison of the digestion and reduction in ryegrass hay (Lolium italicum) in the ovine digestive tract. Br.]. Nutr. 62:493. 22. Griffin, T. S., K. A. Cassida, O. B. Hesterman, and S. R Rust. 1994. Alfalfa maturity and cultivar effects on chemical and in situ estimates of protein degradability. Crop. Sci. 34:1654. 23. Hacker,]. B., and D.]. Minson. 1981. The digestibility of plant parts. Herb. Abstr. 51:459. 24. Halim, R A., D. R. Buxton, M.]. Hattendorf, and R. E. Carlson. 1989. Water stress effects on alfalfa forage quality after adjustment for maturity differences. Agron. ]. 81:189. 25. Hintz, R w., and K. A. Albrecht. 1991. Prediction of alfalfa chemical composition from maturity and plant morphology. Crop Sci. 31:1561. 26. ]arrige, R, C. Demarquilly, ]. P. Dulphy, A. Hoden, ]. Robelin, C. Beranger, Y. Geay, M. ]ournet, C. Malterre, D. Micol, and M Petil. 1986. The INRA "fill unit" system for predicting the voluntary intake of foragebased diets in ruminants: A review.]. Anim. Sci. 63:1737. 27. Kephart, K. D., and D. R Buxton. 1993. Forage quality responses of C3 and C perennial grasses to shade. Crop Sci. 33:831. 28. Lechtenberg, V. 1., D. A. Holt, and H. W. Youngberg. 1971. Diurnal variation in non structural carbohydrates, in vitro digestibility, and leaf to stem ratio of alfalfa. Agron.]. 63:719 . 29. Merchen, N. R, and 1. D. Bourquin. 1994. Processes of digestion of forage-based diets by

Forage Quality for Ruminants

ruminants. In Forage Quality, Evaluation, and Utilization. p 564. G. C. Fahey, Jr., M. Collins, D. R. Mertens, and L. E. Moser (Eds.). American Society of Agronomy, Madison, WI. 30. Mertens, D. R. 1993. Kinetics of cell wall digestion and passage in ruminants. In Forage Cell Wall Structure and Digestibility. p 535. H. G.Jung, D. R. Buxton, R D. Hatfield, and J. Ralph (Eds.). American Society of Agronomy, Madison, WI. 31. Mertens, D. R. 1994. Regulation of forage intake. In Forage Quality, Evaluation, and Utilization. p 450. G. C. Fahey, Jr., M. Collins, D. R. Mertens, and L. E. Moser (Eds.). American Society of Agronomy, Madison, WI. 32. Minson, D.J. 1990. Forage in Ruminant Nutrition. Academic Press, Inc., New York. 33. Moore, KJ., and R. D. Hatfield. 1994. Carbohydrates and forage quality. In Forage Quality, Evaluation, and Utilization. p 229. G. C. Fahey, Jr., M. Collins, D. R Mertens, and L. E. Moser (Eds.). American Society of Agronomy, Madison, WI. 34. Moore, K J., and L. E. Moser. 1995. Quantifying developmental morphology of perennial grasses. Crop Sci. 35:37. 35. Moore, K J., L. E. Moser, K P. Vogel, S. S. Waller, B. E. Johnson, and J. F. Pedersen. 1991. Describing and quantifying growth stages of perennial forage grasses. Agron. J. 83:1073. 36. Mueller, S. C., and G. W. Fick. 1989. Converting alfalfa development measurements from mean stage by count to mean stage by weight. Crop Sci. 29:821. 37. Mullahey,J.J., S. S. Waller, K.J. Moore, L. E. Moser, and T. J. Klopfenstein. 1992. In situ ruminal protein degradation of switchgrass and smooth bromegrass. Agron. J. 84:183. 38. Nelson, C. J., and L. E. Moser. 1994. Plant factors affecting forage quality. In Forage Quality, Evaluation, and Utilization. p 115. G. C. Fahey, Jr., M. Collins, D. R Mertens, and L. E. Moser (Eds.). American Society of Agronomy, Madison, WI.

39. NRC. 1984. Nutrient Requirements of Beef Cattle. (6th Rev. Ed.). National Academy Press, Washington, DC. 40. NRC. 1989. Nutrient Requirements of Dairy Cattle. (6th Rev. Ed.) . National Academy Press, Washington, DC. 41. Ohlsson, C. 1991. Growth, development, and composition of temperate forage legumes and grasses in varying environments. Ph.D. dissertation. Iowa State Univ., Ames (Diss. Abstr.91-26231). 42. Ohlsson, c., and W. F. Wedin. 1989. Phenological staging schemes for predicting red clover quality. Crop Sci. 29:416. 43. Peterson, P. R, C. C. Sheaffer, and M. H. Hall. 1992. Drought effects on perennial forage legume yield and quality. Agron. J. 84:774. 44. Sanderson, M. A. 1992. Morphological development of switchgrass and kleingrass. Agron.J.84:415. 45 . Sanderson, M. A., and W. F. Wedin. 1989. Phenological stage and herbage quality relationships in temperate grasses and legumes. Agron.J. 81:864.

131

acid-treated alfalfa and orchardgrass silages. J. Anim. Sci. 69:4644. 50. Van Soest, P. J. 1994. Nutritional Ecology of the Ruminant .. (2nd Ed.). Cornell University Press, Ithaca, NY. 51. Waldo, D. R, G. A. Varga, G. B. Huntington, B. P. Glenn, and H. F. Tyrrell. 1990. Energy components of growth in Holstein steers fed formaldehyde- and formic acidtreated alfalfa or orchardgrass silages at equalized intakes of dry matter. J. Anim. Sci. 68:3792. 52. Weiss, W. P., and W. L. Shockey. 1991. Value of orchardgrass and alfalfa silages fed with varying amounts of concentrates to dairy cows.J. Dairy Sci. 74:1933. 53. West, C. P. 1990. A proposed growth stage system for bermudagrass. In Proc. Forage and Grassland Conf., Blacksburg, VA 69 June 1990. p 38. American Forage and Grassland Council, Georgetown, TX. 54. West, C. P., D. W. Walker, R. K Bacon, D. E. Longer, and K E. Turner. 1991. Phenological analysis of forage yield and quality in winter wheat. Agron. J. 83:217.

46. Sheaffer, C. c., P. R Peterson, M. H. Hall, andJ. B. Stordahl. 1992. Drought effects on yield and quality of perennial grasses in the North Central States. J. Prod. Agric. 5:556.

55. Wilson, J. R 1982. Environmental and nutritional factors affecting herbage quality. In Nutritional Limits to Animal Production from Pastures. p 111. J. B. Hacker (Ed.). CAB, Farnham Royal, U.K

47. Simon, U., and P. H. Park. 1983. A descriptive scheme for stages of development in perennial forage grasses. In Proc. Int. Grassland Congr. 14th, Lexington, KY. 15-24 June 1981. p 416.J. A. Smith and V. W. Hays (Eds.). Westview Press, Boulder, CO.

56. Wilson, J. R 1993 . Organization of forage plant tissues. In Forage Cell Wall Structure and Digestibility. p 1. H. G. Jung, D. R. Buxton, R. D. Hatfield, andJ. Ralph (Eds.). American Society of Agronomy, Madison, WI.

48. Spears, J. W. 1994. Minerals in forages. In Forage Quality, Evaluation and Utilization. p 281. G. C. Fahey, Jr., M. Collins, D. R Mertens, and L. E. Moser (Eds.). American Society of Agronomy, Madison, WI. 49. Thomson, D.J., D. R Waldo, H. K Goering, and H. F. Tyrrell. 1991. Voluntary intake, growth rate, and tissue retention by Holstein steers fed formaldehyde- and formic

57. Wilson, J. R, B. Deinum, and F. M. Engels. 1991. Temperature effects on anatomy and digestibility of leaf and stem of tropical and temperate forage species, Neth. J. Agric. Sci. 39:3l. 58. Wilson, J. R , and D. J. Minson. 1983. Influence of temperature on digestibility of the tropical legume Macroptilium atropurpureum. Grass Forage Sci. 38:39.