PASTURE QUANTITY AND QUALITY Lawrence D. Muller
Pennsylvania State Universitya
Pasture quantity and quality are the two most important factors in maximizing the amount
of nutrients obtained from pasture. However, questions arise as to what is high quality pasture
and how we can develop and maintain a high quality sward in abundant quantity.
Just like harvested forages, the nutritional quality of pastures is related to the maturity of
the forage when harvested. Dairy producers strive to achieve legume forage with 20-23% crude
protein (CP), 26-30% acid detergent fiber (ADF), 38-42% neutral detergent fiber (NDF), and a
net energy for lactation (NEL) of 0.60 – 0.68 Mcal/NE/lb when harvested as stored forage.
With GOOD grazing management, grasses harvested by cows should be in a vegetative
state and an approximate height 8 to 10 inches, depending on the type of grass. Grasses
harvested as hay or silage are typically more mature than when harvested as pasture, thus pasture
will be of higher quality than stored forages. Likewise, legumes such as clover or alfalfa are
usually grazed at an earlier stage of growth than when harvested as stored forage. Therefore,
with excellent grazing management we should expect to have higher quality forage when
harvested as pasture than when harvested as stored forage. However, pasture quality will depend
on many factors, including geographic location, environmental conditions (temperature,
humidity, precipitation), types of grass and/or legume, and grazing management.
Seasonal Forage Growth and Quality
Table 1 shows the average nutrient composition for cool season grass pastures. These are
average values and should be used as a guide. Forage testing and monitoring of nutrient
composition are needed to more closely monitor the supplemental feeding program. In general,
pastures containing some legumes are higher in nutrient value and will likely be consumed in
higher amounts than only grass pastures. The type of grass and the inclusion of legumes will
influence pasture composition. The seasonal effects on nutrient composition are illustrated with
columns titled spring and summer. For most grasses and grass/legume mixtures, the protein and
energy content are higher in spring and fall and lower during the summer. These fluctuations in
nutrient content are closely correlated with the annual growth cycle of the forage. In appendix
Table 1 is a more complete nutrient composition for several different grasses and legumes.
a
Department of Dairy and Animal Science ([email protected])
1
Table 1. Average nutrient composition for cool season grass pasture and legumes
a
over a grazing season.
Predominately grass (cool season) Grass with legumes
Nutrient
Spring
Summer
Spring
Summer
Crude protein (CP), % DM
21-25
18-22
22-26
20-24
RUPb, % of CP
20-25
25-30
20-25
25-30
RDPc, % of CP
75-80
70-75
75-80
70-75
ADF, % DM
24-28
28-34
21-25
25-30
NDF, % DM
40-45
48-55
32-37
38-43
NE, Mcal/
0.7 1-0.76
Non-fiber carbohydrate (NFC)d, % DM 15-20
0.66-0.72
0.74-0.80
0.70-0.74
12-15
18-24
15-20
Fat, % DM
3-4
3-4
3-4
3-4
Ash, % DM
7-9
7-9
8-9
8-9
1.1-1.3
1.1-1.3
Ca, % DM
.50 - .75
.50 - .75
P, % DM
.30 - .35
.30 - .35
.30-.35
.30-.35
Mg, % DM
.15 - .20
.15 - .20
.18-.24
.18-.24
K, % DM
2.0 - 3.0
2.0 - 3.0
2.5 – 3.5
2.5-3.5
a
Summarized from Fales et al., 1995 and five other studies in the Northeast USA.
b
Rumen undegradable protein
c
Rumen degradable protein grazing and feeding management decisions.
NFC = 100 – (CP + NDF + Fat + Ash)
d
In Table 2 is shown the change in nutrient composition of a mixed mostly grass pasture in
the Northeast USA throughout the grazing season. During periods of peak growth (spring and
fall), CP, RDP, NFC, and NEL are higher, while NDF is lower. The reverse happens during the
summer months, when growth rates are lower, fiber increases, and nutritional value declines.
Table 2. Variation in nutrient composition of a mixed mostly grass pasture throughout the
grazing season (adapted from Rayburn, 1991).
May
June
July
Aug
Sept
Oct
CP, % of DM
24
22
20
20
22
24
RDP, % of CP
74
73
72
71
71
74
NDF, % of DM
47
49
51
52
48
45
NFC, % of DM
16
15
14
13
15
16
NEL, Mcal/lb
0.73
0.70
0.66
0.66
0.72
0.75
2
The nutrient composition of an orchardgrass pasture during a grazing season from a Penn
State study is shown in Figure 1. These results are from well-managed pastures that were grazed
at a stocking rate of 1.3 cows per acre and had nine grazing rotations during a six month grazing
season. Quality remained high with this well-managed pasture where grass was grazed in the
vegetative state and should be representative of most intensively-managed pastures. Crude
protein remained greater than 20% for the entire grazing season. Fiber increased and in vitro dry
matter digestibility (IVDMD) decreased during the summer. These results suggest the analysis
of pasture perhaps three times per year to coincide with changing seasons may be adequate when
pasture availability is not limiting.
The values presented in Tables 1 and 2, and Figure 1 can be important when formulating
feeding programs with pasture. The variability in nutritional composition throughout the grazing
season or BETWEEN grazing years emphasizes the need for some re-formulation of the dairy
ration throughout the grazing season. In addition, regular forage testing is important as the
nutritional quality changes. When formulating rations with stored forages, forage tests are
usually taken when a change in bunkers occurs, when the field harvested changes within a silo,
or when a new source of forage is fed. The same applies for pastures.
During periods of summer heat and drought, additional forages may need to be provided
to lactating dairy cows. However, at other times of year, perhaps only concentrate, mineral
and/or a rumen undegradable protein source is all that is necessary to maintain animal health and
productivity. One of the most important concepts with grazing is to frequently monitor forage
quality, especially the first few years, and to be flexible and be prepared for change, depending
on weather patterns, forage growth, and changing nutritional composition.
80
% of DM
70
60
50
CP
NDF
IVD MD
40
30
20
10
0
Grazing Cycle
Figure 1. Nutrient composition of orchardgrass-based pasture during six month grazing with 9
rotations per paddock.
3
If cows are moving to pastures that have a different forage composition, forage testing is
recommended. Keep in mind that forage tests are always subject to errors and good judgment is
necessary when interpreting results and developing feeding strategies.
Summary:
Well-managed pastures, regardless of the plant species, can be high in nutrient quality
and often exceed the nutrient composition of high quality stored forages. Good grazing
management is essential to maintaining quality pasture. Changes in season trigger changes in
rate of plant growth and subsequent changes in nutrient composition. The continual changing of
pasture quantity and quality during the grazing season provides challenges to dairy producers
using a grazing system. Use of available information about forage quality and nutrition can lead
to sound grazing and feeding management decisions.
Keys to High Quality Forage Production
•
•
•
•
Know when the forage canopy is ready to be grazed
Know how much residual to leave before moving to another paddock
Know how long it takes to use the canopy to the desired residual
Use a stocking density which allows quick removal of forage and uniform distribution of
excreta
4
Reference Table 1. Average nutrient composition of typical high quality pastures (dm basis).
DM, %
CP, %
RUP, % of CP
Sol.P,% of CP
ADF, %
NDF, %
NEL, Mcal/lb
NSC/NFC, %
Ca, %
P, %
Mg, %
K, %
S, %
Mn, ppm
Cu, ppm
Zn, ppm
Fe, ppm
Se, ppm
Vit. A, IU/lb
Vit. D, IU/lb
Vit. E, IU/lb
Fat, %
Ash, %
1
Cool Season
Grass,
Spring
20
20-22
20
40
26-30
43-47
.72-.75
15-20
.50
.30
.16
3
.22
70
8
28
146
0.04
40,000
-250
3
10
Cool Season
Grass,
Summer
20
18-20
30
35
30-34
48-54
.68-.72
14-18
.50
.30
.18
3
.22
70
8
28
146
0.04
40,000
-250
3
10
Cool Season
Grass,
Fall
20
20-22
25
40
24-28
42-46
.71-.74
17-21
.50
.30
0.20
3
.22
70
8
28
146
0.04
40,000
-250
4
10
MMG1
Pasture,
Spring
20
21-23
20
40
25-29
41-45
.73-.77
16-21
.75
.28
.16
3
.22
70
9
28
205
0.04
40,000
-250
4
11
MMG = Mixed Mostly Grass.
5
MMG1
Pasture,
Summer
20
20-22
30
35
29-33
45-50
.68-.72
13-18
.75
.28
.18
3
.22
70
9
28
205
0.04
40,000
-250
4
11
MMG1
Pasture,
Fall
20
22-24
25
40
24-28
40-44
.73-.77
16-20
.75
.30
.21
3
.22
70
9
28
205
0.04
40,000
-250
4
11
Legume
Pasture,
Spring
20
21-25
15
40
26-30
37-41
.73-.76
16-20
1.3
.30
.22
3.3
.22
56
9
30
194
0.04
40,000
-250
4.30
11
Legume
Pasture,
Summer
20
21-25
20
35
28-32
39-43
.68-.72
15-19
1.3
.30
.22
3.3
.22
56
9
30
194
0.04
40,000
-250
4.30
11
Reference Table 1 continued. Average nutrient composition of typical high quality pastures (dm basis).
DM, %
CP, %
RUP, % of CP
Sol.P, % of CP
ADF, %
NDF, %
NEL, Mcal/lb
NSC/NFC, %
Ca, %
P, %
Mg, %
K, %
S, %
Mn, ppm
Cu, ppm
Zn, ppm
Fe, ppm
Se, ppm
Vit. A, IU/lb
Vit. D, IU/lb
Vit. E, IU/lb
Fat, %
Ash, %
1
2
Legume
Pasture,
Fall
20
27
20
40-45
25-29
33-38
.72-.75
16-20
1.3
.30
.26
3.31
0.24
56
9
30
194
0.04
40,000
-250
4.30
13
MML1 Pasture,
Spring
20
22-24
20
40
26-30
35-39
.73-.77
15-20
1.0
.30
.16
3.3
.20
60
9
28
186
0.04
40,000
-250.00
4
11
MML1 Pasture,
Summer
MML1 Pasture,
Fall
20
22-24
25
35
28-32
40-45
.68-.72
15-20
1.0
.30
.22
3
.20
60
9
28
186
0.04
40,000
-250
4
11
20
23-25
20
40
24-28
36-40
.72-.76
15-20
1.0
0.30
.23
3
.20
60
9
28
186
0.04
40,000
-250
4
13
MML = Mixed Mostly Legume.
Sor-Sud = Sorghum-Sudangrass.
6
Sor-Sud2
Pasture,
30 inches
20
15
36
35
34-38
56-60
.63-.67
13-16
.55
0.27
.28
2.1
0.1
68
9
34
267
0.03
36,000
-216
3
9.5
Sor-Sud2
Pasture,
40 inches
22
13
38
32
38-42
60-65
.60-.64
11-14
0.50
0.30
0.30
2.60
0.12
68
9
34
268
0.03
34,000
-204
3.10
9.5
Kale Tops,
Brassicas
12
16-18
37
35
28-32
38-42
.74-.78
26-30
1.61
0.34
0.30
2.78
0.29
52
17
36
93
0.03
---4
10