Use Forage Test To Diagnose Management Problems

Edward Rayburn
Agronomy Specialist
WVU Extension Service
4/2002
 

Forage tests are handy for more than balancing an animal's ration. They also can help you detect management problems in your livestock operation.

You can use the information reported on a forage test to improve forage quality. The first question to address is: Do I need to change my management?

Harvest management and legume content are the main determinants of forage quality. If the quality of pasture and hay is equal to or greater than that needed by the livestock being fed, you don't need to change management. However, if the forage quality is too low, you often can improve it through earlier harvest or by including legumes in the stand.

The required level of forage quality depends on the animal's nutritional requirement, the forage value, and the cost of energy and protein supplements. Generally, it is cheaper to grow good-quality forage than to buy supplements for poor-quality forage.

Percentage of moisture vs. dry matter indicates how well the forage was dried before storage. Hay crops should be baled when the moisture is less than 20 percent 9dry matter greater than 80 percent),. Haylage should be made when the forage moisture is 40 to 50 percent (dry matter 50 to 60 percent). Most hays will dry to 10 to 15 percent moisture (85 to 90 percent dry matter) during storage; however, some round bales can have higher moisture when wrapped for haylage or when stored outside. The lesson: Be sure to allow hay to dry adequately before baling.

Table 1 lists the total digestible nutrient (TDN) energy and protein requirements for different classes of cattle. It also shows the range of forage acid detergent fiber (ADF) associated with each energy level. This ADF is the best measure of fiber for forage. If ADF is high, digestibility will be low. Dairy cattle need a total ration ADF level about 21 percent to maintain butterfat porduction.

Keep in mind that forage crops increase in ADF as they grow and mature. Reduce the ADF in hay by harvesting at an earlier growth stage. When you are managing for ADF levels lower than needed by the livestock, forage yield may be reduced due to harvesting too early. Your challenge is to achieve a good balance between ADF content and yield.

Neutral detergent fiber (NDF) is the best measure of forage dry matter intake by livestock. The lower the NDF content, the higher the intake. Legumes are lower in NDF than grasses at the same growth stage but both increase in NDF than grasses at the same growth stage but both increase in NDF as they grow and mature. If you need for the animals to eat more forage, manage the stand for a lower NDF content. You can do this by harvesting at an earlier growth stage or by inter-seeding legumes.

Crude protein (CP) is a measure of the nitrogen in the forage. The CP is used by rumen bacteria in digesting forage and concentrates in the diet. The bacteria digest fiber and nonstructural carbohydrates in the ration, releasing volatile fatty acids that ruminants use for energy. Ruminants also digest the bacteria for energy and high-quality protein as they pass out of the rumen. The CP content of forage can be increased by adding legumes to the stand, by harvesting the forage at an earlier growth stage, or (to a small extent) by using higher rates of nitrogen fertilizer.

Digestible energy (DE) of forage is calculated from the measured fiber content. Several different energy systems are used in formulating livestock rations. The systems used in the United States are summarized in Table 2 using equivalent values data from the National Research Council. You can use this table to convert the feeding value of forage or grain supplement from one enrgy system to another. Equivalent values are listed for total digestible nutrients (TDN)m dugestubke energy (DE), metabolizable energy (ME), net energy maintenance (NEM), net energy gain (NEG), and net energy lactation (NEL).

Horses digest forages less efficiently than ruminants do. To estimate an energy value for horses, multiply the energy values in Table 1 by 0.80.

To increase the digestible energy in a forage, manage the stand to decrease forage adf content. Also, to increase digestible energy intake, decrease the forage NDF by adding legumes to the stand. Livestock will eat more grass-legume forage than straight grass, be it hay or pasture. Forages containing 25 to 30 percent legumes will allow yearling cattle to grow 0.25 to0.33 pounds per day faster than nitrogen-fertilized grass. Dairy cattle will produce 6 to 10 pound of milk per day more at the same level of grain feeding. High-producing dairy cattle often will need grain for supplemental energy since cool-season grasses and legumes are often short of energy compared to protein.

Table 1

Total digestible nutrient (TDN) and crude protein (CP) requirements for different cattle classes and the approximate forage acid detergent fiber (ADF, average range) content that will supply the indicated TDN.

Production Status

TDN

CP

ADF

-----------------%----------------

Growing steer 60-68 10-11 34 5
Dry cow 50 7 42 5
Last 3rd gestation 52-54 8 40 5
Lactating beef cow
Average 55-56 9 39 5
Above Average 63-64 11-12 345
Heifer 62-64 10-11 345
Lactating dairy cow
50-70 lb milk/day* 68-70 15-16 275

* Grain supplementation is needed at milk production levels over 50 lbs. per day.

Table 2

Equivalent values for total digestible nutrients (TDN), digestible energy (DE), metabolixable energy (ME), net energy maintenance (NEM), net energy gain (NEG), and net energy lacation (NEL).

TDN

DE

ME

NEM

NEG

NEL

%

--------------Mcal/lb ----------------

45 0.90 0.74 0.36 0.11 0.45
50 1.00 0.82 0.44 0.19 0.50
55 1.10 0.90 0.52 0.26 0.56
60 1.20 0.99 0.60 0.33 0.61
65 1.30 1.07 0.67 0.40 0.67
70 1.40 1.15 0.74 0.47 0.73
75 1.50 1.23 0.81 0.53 0.78
80 1.60 1.31 0.88 0.59 0.84
85 1.70 1.40 0.95 0.65 0.89
90 1.80 1.48 1.02 0.70 0.95