Factors Which Influence Forage Quality and Effectiveness in Dairy Rations

Joe W. West

Animal and Dairy Science Department, University of Georgia - Coastal Plain Experiment Station, Tifton, GA 31793-0748 U.S.A.

# Take Home Messages

 # Introduction

Forages have always provided the base upon which ruminant nutrition is built. Proper feeding of dairy cattle involves the use of high quality forage, and is a key to efficient production. With greater emphasis on milk yield, the dairy cow is increasingly challenged to consume sufficient nutrients to support milk yield while maintaining sufficient fiber intake to support good rumen health and digestion. Most ration formulation programs strive to satisfy net energy requirements of the cow. Often this is difficult without reducing forage (and fiber) to dangerously low levels in the diet.

Forage quality is a complex interrelationship of many factors which influence intake potential, nutrient content, digestion, gut fill, passage rate, and partitioning of metabolized products within the animal. Man has known the essence of forage production for well over 100 years. Yet the consistent production of forages of high quality often eludes us still. This paper will discuss factors which influence forage quality that dairy producers can use to improve animal performance.

Differences Due to Forage Species

It is often assumed that the correlation between digestion and intake of the forage, and the relationship of neutral detergent fiber (NDF) with intake is constant among forages, but those relationships are not regular. Large differences exist among classes of forages and if not considered in ration formulation, can lead to performance below expectations. Legumes such as alfalfa and cool season grasses differ not only in their fiber content, but also in the rate and extent of digestion (15). Relative to alfalfa, grasses have a greater lag to the start of digestion and a slower rate of digestion, but have a greater extent of digestion. This greater extent of digestion offers the potential for greater energy availability, but slower digestive rates and greater ruminal retention times can result in lower intake, potentially offsetting gains from high digestibility.

Alfalfa has a high cell wall density because of extensive lignification, has low fiber digestion relative to grasses, but high total digestion (due in part to the high content of cell solubles). Thus, the digestible portion of alfalfa is rapidly digested and the remainder passes rapidly through the digestive tract. In comparison, grasses have a higher digestible fiber content, lower content of cell solubles, and a less dense cell wall structure.

Digestibility (and its depression) is a competition between digestion and passage rates. The more digestible and/or slower-digesting the cell wall, the greater potential for digestibility depression due to intake level, physical form, passage, or concentrate addition. As intake increases over the maintenance level, digestion of the more digestible fiber fractions (such as hemicellulose) becomes increasingly sensitive to passage effects, and can pass from the rumen undigested. At higher intakes, the slower digesting hemicellulose of grasses tends to have greater digestibility depressions than legumes such as alfalfa.

A major factor which could enhance intake of forages by cattle is to simply lower the cell wall content. This is a major reason for the advantages of legumes over grasses, and the advantage of immature forages over those of greater maturity. Buxton et al. (3) presented a comparison of fiber concentrations and digestibilities, TDN content, and proportions of nutrients from NDF (Table 1). These data show that a predominance of the TDN for the cool season legumes comes from sources other than NDF, while a greater proportion of nutrients for cool and warm season grasses comes from NDF. This proportion increases as the plant matures and digestibility declines. As the proportion of nutrients which must be derived from the digestion of fiber increases, it becomes more likely that limitations to intake will occur.

Further clouding the issue is the fact that NDF apparently differs by source. Diets comparing corn silage, alfalfa, and Coastal bermudagrass balanced with concentrate for similar NDF contents showed that the greatest fat-corrected milk (FCM) yield for each diet occurred at a similar NDF content (36%), but the milk yield at the optimum dietary NDF content varied by forage source. Milk yield was least for the coastal bermudagrass hay diets, corn silage based diets were intermediate, and alfalfa diets yielded the most FCM (7). Changes in FCM corresponded to differences in dry matter intake (DMI) among diets. Despite similar NDF contents, DMI and milk yield differed among the forage sources, suggesting that factors beyond simple dietary fiber levels influence DMI and milk yield.

Mertens and Ely (9) reported that the primary differences between alfalfa and Coastal bermudagrass were in NDF and lignin contents, and the proportion of NDF that is slow-digesting. In addition to the greater NDF content, the NDF of coastal bermudagrass, proportionally, has a larger fast digesting fraction, a larger slow digesting fraction, and a smaller indigestible fraction than alfalfa (9). The greater lignification of alfalfa is actually an advantage because the high cell wall density leads to less bulkiness (and less gut fill), with a moderately rapid rate of digestion (16).

Table 1. Neutral detergent fiber (NDF), digestibility, and total digestible nutrients (TDN) in forage herbage with specified levels of NDF.

 

Species

NDF

Concentration, % DM

NDF

Digestibility, % NDF

TDN, % DM

Proportion of nutrients from NDF, % TDN

Cool-season legumes

30

48

70

20

40

45

64

28

50

40

56

36

Cool-season grasses

50

70

71

49

60

66

66

60

70

59

58

72

Warm-season grasses

60

64

65

60

70

56

56

70

80

50

46

85

From Buxton et al. (3).

 Effect of Forage Species on Intake

One of the ultimate indicators of forage quality is intake by the cow. A negative correlation exists between the content of digestible DM or digestible cell wall in the forage and DM intake (15). The relationship is less for legumes and greater for grasses, indicating that intake changes are less sensitive to changes in digestibility for the legumes and increasing maturity can have a greater impact on intake for grasses.

However, perhaps one can take advantage of the inherent digestibility advantages for some grass species by producing the highest quality (very highly digestible) forage possible. Dairy cows fed diets based on orchardgrass (a cool season grass) or alfalfa consumed more of the alfalfa containing diets (5, 17). Despite lower DMI for orchardgrass based diets, intake of digestible DM was equal for the two forages. This was because of the greater DM digestibility of the orchardgrass diets, due largely to the far greater NDF digestibility of the grass compared with alfalfa (75.1 vs. 49%) [17]. Exploiting the greater NDF digestion of grasses may be the means by which grasses work best in dairy rations.

The choice of forage species greatly affects the intake potential for dairy diets. Daily intake of forages by cattle was greatest for cool season legumes, and was similar among cool season and warm season grasses [Table 2] (11). The ADF and NDF content was least for legumes, intermediate for cool season grasses, and greatest for warm season grasses. The correlation of digestible DM, DMI and digestible DMI with ADF and NDF content of forages is negative (Table 3), meaning that these variables decline as the amount of ADF and NDF in the forages increase. Thus the grasses, especially the warm season grasses that are highly productive in a hot environment, are at a disadvantage when compared with legumes and cool season grasses.

Table 2. Digestibility and intake values for forage classes fed to cattle.

 Item

Cool season

legumes

Cool season

grasses

Warm season

grasses

Intake, g/kg BW.75

94.8

89.0

90.0

DM digestion, %

62.8

66.9

59.8

Forage ADF, %

35.9

38.3

42.7

Forage NDF, %

49.5

65.3

74.5

Adapted from Reid et al. (11).

Table 3.Correlations between cattle responses and fiber concentrations.

 Item

DMD1

DMI2

DDMI3

NDFI

ADF, %

-.39

-.52

-.55

.30

NDF. %

-.32

-.41

-.43

.5

Adapted from Reid et al. (11).

1Dry matter digestibility.

2Dry matter intake.

3Digestible dry matter intake.

4NDF intake.

Relationship of NDF and DMI

Diets containing large quantities of forages can have an effect on feed DMI, because of the amount of fiber present, the digestibility of the fiber, and the passage rate of undigested residues from the digestive tract. There are differences in the digestibility and rate of digestion for different forage species. However intake is considered more important than digestibility for influencing digestible DMI from forages. When both grasses and legumes were considered, relative contributions to intake of digestible DM were 70% for DMI and 30% for digestibility (4).

Mertens (8) suggests that the maximum NDF intake (NDFI) that will not reduce milk yield below the cow's potential is 1.2% of body weight (BW). However, the potential for distension of the rumen may allow for greater content of bulky feeds. Ruminal pool size of NDF for Coastal bermudagrass was projected to be 1.38% of BW compared with 1.1% of BW for alfalfa (9). If this is true, then one would expect that cows may be able to consume a greater percentage of NDF with this grass and maintain intake. Work in Georgia supports this, for Holsteins fed diets containing 15 and 30% Tifton 85 bermudagrass (NDF percentage of TMR of 39.5 and 46.6%) consumed as much feed DM as those fed diets containing similar amounts of alfalfa but with lower TMR NDF content (NDF percentage of TMR of 33 to 35%) (18).

Florida workers (12) compared four ensiled forages (dwarf elephant grass, forage sorghum, bermudagrass, and corn silage) in diets formulated for 31, 35, and 39% NDF. Not surprisingly, DMI declined with increasing dietary NDF content, while NDFI increased from 1.15 to 1.32% of BW with increasing NDF. Intake of dwarf elephantgrass and corn silage was greater than intake for bermudagrass and forage sorghum, which was consistent with the greater extent of in situ digestion of DM, NDF, and cellulose for the elephantgrass and corn silage. Milk yield was least for the forage sorghum, which had the lowest fiber quality by analysis. The authors concluded that a measure of fiber digestibility would help to explain differences in fiber quality among dietary sources, that a measure of undigested NDF intake could describe the indigestible and slowly digestible portion of the diet that occupies space in the digestive tract, and a measure of digestibility by analysis could help to describe the potential energy content of the forage, and ultimately, the diet. Differences among species in the rate of digestion and passage of fiber suggest that analyses beyond the traditional chemical analyses are necessary to describe the forage and its energy content and intake potential.

Maturity Effects

Voluntary intake of cell-wall (fiber) components and cellulose has a highly negative relationship for several grasses, while there was no relationship for these components in alfalfa (14). So despite rations balanced for similar NDF content, those based on grasses usually have lower DMI than those based on legumes.

As plants mature, crude protein decreases, fiber increases, and digestibility and energy content decline. This is true for alfalfa hay (Table 4) and sorghum (Table 5). Sorghum does not demonstrate fiber changes as large as those of alfalfa, but the digestibility of sorghum declines dramatically (Table 5).

Table 4. Effect of stage of maturity on nutrient content of alfalfa hay.

 Stage of maturity

TDN,

% of DM

Crude protein

Crude

fiber

Neutral detergent fiber

Vegetative

65

22

24

41

Bud

62

20

27

44

Early bloom

58

17

31

48

Mid bloom

56

16

33

50

Full bloom

54

15

35

52

Mature

52

13

37

55

 As forages mature, there is generally a point at which the accumulation of digestible DM declines despite increasing forage DM yield. At this point the general effect is both a decline in energy content of the forage as well as reduced intake. Intake of DM declines with increasing ADF and NDF content of the forage, and digestibility declines with increasing lignin content of the forage (15). These responses are relatively well known, and the obvious means to minimize the effects of maturity is to harvest at optimum maturity. However another common approach is to reduce the content of low quality forages and substitute another feedstuff (such as grain) of greater energy density; the rationale being that high energy feeds offset the lower energy content of the forage. However when alfalfa was harvested at pre, early, mid, and full bloom and fed at 80, 63, 46, and 29% of diet DM, with the remainder from concentrates, the most milk was produced for diets containing the pre-bloom alfalfa (Table 6, 6). Cows fed diets containing 80% of pre-bloom alfalfa actually produced more milk than cows fed diets of only 29% of full bloom alfalfa and 71% concentrates. High energy concentrates could not overcome the intake depressing effects of mature alfalfa coupled with the low energy content.

Table 5. Chemical constituents and digestibility coefficient of sorghum silage harvested at six stages of maturity.

 

 Stage of maturity

 Component

 Early bloom

 Bloom

 Milk

Late milk to late dough

 Dough

 Hard dough

DM%

23.2

24.6

25.3

28.6

29.6

30.8

Percentage of dry matter

Crude fiber

Crude protein

Nitrogen free extract

Ash

Acid detergent fiber

Neutral detergent fiber

Neutral detergent soluble

27.0

8.4

58.2

4.6

35.7

68.4

31.6

27.8

8.0

58.1

4.3

37.5

69.4

30.6

26.2

7.3

60.1

4.5

36.1

65.3

34.7

24.1

7.0

63.0

4.1

33.3

64.3

35.7

26.1

5.8

62.4

3.6

34.0

64.1

35.9

27.8

5.9

58.5

4.3

34.1

63.9

36.1

Digestibility coefficients %

DM

Crude fiber

Crude protein

Nitrogen free extract

Acid detergent fiber

Neutral detergent fiber

Neutral detergent soluble

65.21

68.8a

52.8a

65.3a

57.6a

65.9a

62.7ab

57.8ab

64.1a

42.6ab

59.6a

51.4ab

57.6ab

58.6b

56.9ab

59.3a

37.8b

58.8a

49.1abc

56.6ab

57.6b

57.7ab

48.8b

34.6b

64.6a

42.8bcd

52.1bc

68.1a

50.3b

44.5b

15.1c

58.3a

39.7cd

44.9c

60.9b

52.1b

45.2b

14.8c

60.6a

38.1d

43.2c

68.2a

abcdMeans in row with same letter not different at P < .05.

Black, Ely, McCullough and Sudweeks (1980). J. Anim. Sci. 50:617-624.

Table 6. Effect of alfalfa maturity and concentrate percentage on milk yield (kg/d) of dairy cows.

 

Concentrate % of DM

Alfalfa maturity, bloom

Differences

pre to full bloom

Pre

Early

Mid

Full

20

38.1

34.1

28.0

25.1

13.0

37

38.7

36.6

31.9

26.9

11.8

54

43.0

40.7

34.5

32.9

10.1

71

44.6

42.0

34.8

34.8

9.7

Difference, 20 to 71% concentrate

 

6.5

 

7.9

 

6.8

 

9.7

 

Adapted from Kawas et al. (6).

 Environmental Effects on Forage Quality

The environmental conditions in which the plant is grown affect the quality of the forage, though the effects are not as great as those of increasing maturity. Factors which have the greatest effect include moisture, temperature, solar radiation, and soil nutrients. In a review, Buxton et al. (3) reported that the optimal growth temperatures are near 20C for cool-season species and 30-35C for warm-season species. A rise in temperature reduces the leaf:stem ratio, which generally reduces forage digestion because of the lower digestibility of stems. High ambient temperatures also promote the production of lignin. Bermudagrass grown at 22C or 32C had 1.3 and 2.2% lignin in leaves and 3.4 and 6.7% lignin in stems for the two respective temperatures (13, 19). In addition to higher NDF contents for the forages grown at higher temperatures, NDF digestion was 75 and 62% for leaves and 60 and 41% for stems at the cool and hot temperatures, respectively. Lower quality for the forage grown at higher temperatures was consistent with the fiber and lignin contents and reflects the challenges associated with producing forages under hot conditions. Buxton et al. (3) reported that each 1C increase in temperature decreases digestibility of forages by .3 to .7 %. This is why forages grown in cooler regions are of higher quality than those grown in warm climates. There is little one can do to alter the effects of climate on forage digestibility, however, it is important to recognize that these changes do occur and account for them in ration formulation.

Physical Form of Forages

Physical form of forages affects digestion because of effects of particle size on outflow from the rumen. A rapid outflow of solids from the rumen has beneficial and negative effects. Rapid removal from the rumen means more space is available for additional feed intake, but beneficial rumination and chewing (which buffers the rumen) is reduced. Simulations revealed that pelleting alfalfa or bermudagrass reduced ruminal NDF digestion and total DM digestion in steers (9). However ruminal turnover time was more rapid so that intakes of DM and digestible DM were sharply increased over long hay.

Despite results which show greater DMI with chopping or pelleting of forages, one must also consider that these studies were often conducted with the forages in question as the sole feed source. Recent studies with dairy cows showed little increase in DMI when forages were chopped, perhaps because of their use in TMR and because of the large amount of concentrates fed. Chopped alfalfa hay ranging from .26 to .9 cm did not affect DMI but milk fat depression was prevented when mean particle length exceeded .25 inches in length (20). In a Canadian study, cows were offered diets with low (35%) or adequate (65%) forage content with two chop lengths of alfalfa silage, .5 and 1.0 cm (2). In the low forage diets the increase in chop length did not affect DMI but did increase FCM, while in the adequate forage diets longer chop length tended to decrease DMI. Interestingly, addition of hay to these diets increased DMI, but the addition of hay to the low forage diets was less effective than increasing particle length of the silage. This suggests that greater particle length of all forage sources is more effective toward maintaining DMI, but the addition of some hay helps to offset the negative effects of low fiber diets. It has been recommended that 15 to 20% of forage particles should be greater than 3.3 cm long to maintain normal rumen function (13). For dairy rations, attention to adequate particle length is of greater concern than fine particle size to stimulate intake. In addition, much of the particle length work has been done with alfalfa. Less is known about the effects of particle length from grasses and the effectiveness in dairy rations.

 # Conclusions

The use of NDF within forages is an indicator of forage quality and quality is closely linked to cow performance. However great variation exists between forage types and must be considered. Generally grasses are more fibrous than legumes and result in lower DMI.

The minimum NRC (10) recommendations for ADF and NDF are probably too conservative for many diets. However, this is very dependent on fiber source and physical form, which must be considered.

Forage particle size greatly influences the effectiveness of fiber. Allen (1) recommends no adjustment from 30% dietary NDF for silage with 5-10% of particles more than 3.8 cm; a decrease in NDF of 2 units when more than 15% of particles exceed 3.8 cm, and an increase in NDF of 2 units when few long particles exist.

There is a wide variation in the chemical composition, physical form, digestion rate, and rate of passage of NDF among feeds, making it very unlikely that any single value for NDF will apply in all feeding situations. In addition the interrelationships of fiber with other carbohydrate components (especially nonstructural carbohydrates and starch) are critical to the performance of the animal, and influence effectiveness of the fiber present in the diet. The overall balance of the diet is the determining factor for animal performance. Forage continues to provide the base for dairy rations, and forage quality, based on fiber and energy content, maturity, digestibility, and species differences which affect utilization, must be considered for optimal use of the forage.

 # References

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