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Factors Influencing Forage Nutritional Value
The following section will discuss a number of the factors influencing the nutrient content, digestibility, and subsequent nutritional value of a forage. In addition to these factors, additional factors have been discussed in other portions of the module. Data are presented to illustrate trends; actual values vary with specific conditions.
The primary factor influencing the nutritional value of forages is the
forage maturity at the time of harvest.
Table 8.1 describes the developmental stages of legumes and grasses.
Stages
of maturity |
Definition* |
|---|---|
Legumes |
|
Late vegetative |
No visible buds, flowers of seed pods. Stem at least 12 inches tall |
Early bud |
visible flower buds on at least one stem (1%) |
Mid bud |
50% of the stems have at least one bud |
Late bud |
75% of the stems have at least one bud, no visible flowers |
First bloom |
Flowers on at least one stem (1%) |
1/10 bloom |
10% of the stems have at least one flower |
Mid bloom |
50% of the stems have at least one flower |
Full bloom |
75% of the stems have at least one flower, no visible seed pods |
Grasses |
|
Vegetative |
Leaves only, stems not elongated (specify height) |
Stem elongation |
Stems elongating. Specify early or late jointing. |
Boot |
Flower head is enclosed in flag leaf sheath and not showing |
Heading |
Flower head emerging or emerged from flag sheath but not shedding pollen |
Anthesis |
Flowering stage, anthers shedding pollen |
Milk stage |
Seed immature, endosperm milky |
Dough stage |
Well-developed seed, endosperm doughy |
Ripe seed |
Seed ripe, leaves green to yellow brown |
| *Randomly select 100 stems from the fields and determine the percentage of stems at the most mature stage of development. | |
As a forage matures, maturation is associated with a decrease in the nutrient
content, digestibility, and subsequent nutritional value of the forage.
As a plant matures the contents of water, protein, nonstructural carbohydrates
(i.e. energy), minerals, and vitamins decrease. In addition, the contents
of the structural carbohydrates and lignin increase. Lignin is an indigestible
compound. The rate and magnitude of change in nutritional value is dependent
on a number of factors such as plant species and environmental conditions.
Lignin is, in part, responsible for the decrease in digestibility as a
plant matures. Lignin forms indigestible complexes with cellulose, hemicellulose,
and proteins. In general, the amounts of lignin in the various plant species
are: legumes > warm-season grasses > cool-season grasses. For legumes,
a range for lignin is from less than 6% to more than 10%. For grasses,
a range for lignin is from less than 3% to more than 7%.
Table 8.2 describes the stage of maturity and corresponding range for
crude protein and neutral detergent fiber contents for legumes and grasses.
Composition,
% DM |
||
|---|---|---|
Stage
of Maturity |
CP* |
NDF** |
Grasses |
||
Pre head |
>18 |
<55 |
Earlyhead |
13-18 |
55-60 |
Head |
8-12 |
61-65 |
Post head |
<8 |
>65 |
Legumes |
||
Pre bloom |
>19 |
<40 |
Early bloom |
17-19 |
40-46 |
Mid bloom |
13-16 |
47-51 |
Full bloom |
<13 |
>51 |
*CP
= Crude protein **NDF = Neutral detergent fiber |
||
Table 7-6 on page 125 of the text describes seasonal changes in carbohydrates,
lignin, and cell solubles for four species of grasses. Table 7-9 on page
128 of the text describes the effect of maturity on crude protein content
of grasses (averaged over eight grasses). For alfalfa, the dairy NRC publications
state the decrease in crude protein and TDN for early vegetative to full
bloom alfalfa as 23% to 15% and 63% to 55%, respectively. Table 7-3 on
page 121 of the text describes effect of maturity on digestibility of
alfalfa.
Figure 8.10 illustrates the stage of maturity at harvest to maximize digestible
dry-matter yield for legumes and grasses.
In addition to these data, additional data is available in the literature
for other species and parameters.
One significant consequence of the decrease in digestibility as the plant
matures is the association between digestibility and dry-matter intake;
as digestibility decreases, dry-matter intake also decreases. Dry-matter
intake is essential for nutrient intake to support maintenance and productive
functions. As the digestibility of a forage decreases, the digestion and
subsequent passage rates are also decreased. Therefore, dry-matter intake
of the forage is limited by the physical volume required to contain the
feedstuff prior to passage through the GI tract. Note, crude protein and
mineral deficiencies also have the ability to limit intake. For crude
protein, the typical crude protein content to maintain intake for a pasture
forage is 7.5-8.0%. At this time, there is not an effective method to
accurately relate forage digestibility and dry-matter intake.
Additional factors influencing the nutritional value of a forage include
plant species and environmental and soil conditions. Plant species, especially
between grasses and legumes, influences the nutritional value of the forage.
Also within the classifications of grasses and legumes, the nutritional
value varies. Environmental conditions such as the progression rate of
the season and temperature and moisture levels and patterns of the season
will influence the nutritional value of the forage. Soil conditions will
also influence the nutritional value of a forage. For grass forages, nitrogen
fertilization can significantly increase the crude protein content and
dry-matter yield. Further, for a grass-legume forage system, excess nitrogen
fertilization may result in the loss of the legumes in the system. As
described in the text, a number of other associations between fertilization
and nutritional value have been suggested.
In addition to the change in nutrient content of the forage plant, the
change in leaf to stem ratio also impacts the nutritional value of the
forage. In general, the nutritional value of the leaves is greater than
that of the stems. As legumes and grasses mature, the ratio of leaves
to stems decreases, thus decreasing the nutritional value of the forage.
In comparison to grasses, the extent of leaf loss is greater in legumes.
Table 8.3 describes the stage of maturity and corresponding percentage
of leaves for legumes and grasses.
Stage
of Maturity |
Percent Leaves
|
|---|---|
Grasses |
|
Pre head |
>50 |
Earlyhead |
40-50 |
Head |
30-40 |
Post head |
20-30 |
Legumes |
|
Pre bloom |
40-50 |
Early bloom |
35-45 |
Mid bloom |
25-40 |
Full bloom |
<30 |
A number of the factors influencing the nutritional value of forages discussed
in this module are in the control of the forage manager. Effective management
of these factors such as appropriate selection of forage species, fertilization
practices, and selection of stage of maturity at harvest will increase
the overall nutritional value of the forage system. In addition, maintenance
of the forage at a vegetative state, especially for grasses, can improve
the nutritional value of the forage system.
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