Author: Charles C. Stallings, Extension Dairy Scientist and Coordinator of Forage Testing, Virginia Tech
Publication Number: 404-124, posted February 2005
Forage quality has typically been determined by measuring the dry matter, crude protein, fiber, and estimated energy content. Forage testing labs are now able to estimate the actual digestibility of feeds by using newly available tests.
The bottom line is forages should be analyzed regularly (every 4 to 6 weeks) for dry matter, protein, fiber, and energy. If regular forage testing cannot be done, the lower end of the range for protein and energy can be used for ration formulation. This would result in less under formulation and result in adequate supplementation under most conditions. Mineral analysis may be needed in some situations.
| ADF % | NDF % | NDF: ADF | RFV | |
|---|---|---|---|---|
| Alfalfa A (mature) | 40 | 51 | 1.28 | 105 |
| Alfalfa B (immature) | 29 | 36 | 1.24 | 170 |
| Grass | 32 | 50 | 1.56 | 119 |
| Dry matter | Crude protein | A.D.1 fiber | TDN | Net energy | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| %DM | SD | %DM | SD | %DM | SD | %DM | SD | MCAL | SD | |
| Hays | ||||||||||
| Alfalfa | 87.0 | 8.8 | 18.9 | 2.8 | 31.8 | 6.0 | 65.4 | 5.4 | .64/lb. | .06 |
| (range) | (78-96) | (16-22) | (26-37) | (60-71) | (.58-.70) | |||||
| Mixed | 86.7 | 9.0 | 12.9 | 4.4 | 39.1 | 7.6 | 59.1 | 3.2 | .58/lb. | .03 |
| (range) | (78-96) | (9-17) | (32-47) | (56-62) | (.55-.61) | |||||
| Grass | 81.3 | 18.2 | 10.5 | 2.8 | 40.4 | 5.6 | 55.5 | 6.8 | .54/lb. | .07 |
| (range) | (63-99) | (8-13) | (35-46) | (49-62) | (.47-.61) | |||||
| Small grain | 87.2 | 4.9 | 8.9 | 3.4 | 40.6 | 5.8 | 59.2 | 4.6 | .58/lb. | .05 |
| (range) | (83-92) | (6-12) | (35-46) | (55-64) | (.53-.63) | |||||
| Silages | ||||||||||
| Corn | 38.3 | 6.1 | 7.2 | 2.9 | 28.0 | 4.1 | 66.9 | 1.9 | .66/lb. | .02 |
| (range) | (32-44) | (4-10) | (24-32) | (65-69) | (.64-.68) | |||||
| Alfalfa | 49.6 | 13.3 | 23.6 | 3.0 | 35.8 | 5.4 | 62.4 | 4.5 | .61/lb. | .05 |
| (range) | (36-63) | (21-27) | (30-41) | (58-67) | (.56-.66) | |||||
| Mixed | 34.1 | 15.6 | 16.3 | 4.5 | 31.8 | 6.6 | 66.1 | 5.0 | .65/lb. | .05 |
| (range) | (19-50) | (12-21) | (25-38) | (61-71) | (.60-.70) | |||||
| Grass | 32.9 | 11.0 | 13.6 | 4.7 | 35.4 | 6.2 | 62.4 | 7.2 | .61/lb. | .08 |
| (range) | (22-44) | (9-18) | (29-42) | (55-70) | (.53-.69) | |||||
| Rye | 34.4 | 14.0 | 14.3 | 4.0 | 34.2 | 5.5 | 60.9 | 8.3 | .59/lb. | .09 |
| (range) | (20-48) | (10-18) | (29-40) | (53-69) | (.50-.68) | |||||
| Small grain | 37.8 | 16.4 | 10.4 | 3.7 | 34.5 | 5.5 | 61.5 | 3.3 | .60/lb. | .04 |
| (range) | (21-54) | (7-14) | (29-40) | (58-65) | (.56-.64) | |||||
| Sorghum | 51.5 | 20.1 | 10.6 | 3.4 | 39.0 | 8.2 | 57.5 | 2.5 | .56/lb. | .02 |
| (range) | (31-72) | (7-14) | (31-47) | (55-60) | (.54-.58) | |||||
Fiber digestibility in corn silage and other forages is becoming more of a consideration. Since most fiber digestion occurs in the rumen, the main concern is with its digestibility in rumen contents. Typically a 48-hour in vitro NDF digestibility is conducted. Results from variety testing in Virginia indicate there are differences in fiber digestibility in corn varieties. Also year to year variation most likely occurs in fiber digestibility due to more lignification during certain growing seasons. Michigan State University research (Oba and Allen, Journal of Dairy Science 82:589, 1999) indicates that a 1% increase in ration-neutral detergent fiber digestibility will result in a 0.37 lb./cow/day increase in dry matter intake (digestibility and intake are related) and 0.64 lb. more 4% fat corrected milk.
If silage is harvested at a mature stage (black layer), the kernels will be hard and difficult for the cow to digest. Corn silages above 45% dry matter many times will have kernels that are hard. Kernel digestibility will be reduced but an analysis on the silage might indicate high energy content because reduced kernel digestibility is not considered. The reason for this is the lab must grind the feed and in doing so the kernel is broken and appears digestible. Therefore, a general lab analysis will not detect reduced digestibility of kernels. It is possible to do an in vitro starch digestibility but the process of sample preparation is still a problem. Corn silage harvested with kernel processing equipment will have greater starch digestion and energy content than unprocessed silages. Some forage testing labs will consider this when calculating energy content.
When there is heating in hay-crop silages and hays during ensiling and storage, the result is protein that is bound in the fiber. This has been termed acid detergent fiber protein or acid detergent insoluble nitrogen. There is a direct relationship between "bound" protein and reduced protein digestibility and this should be accounted for in balancing rations. There also appears to be a reduced amount of energy in heat-damaged forages. Some laboratories estimate protein digestibility based on amount of heat damage measured as acid detergent insoluble nitrogen. Typically, if "bound" nitrogen is less than 12% of total nitrogen, the forage is not considered heat damaged.
Reduced digestibility of forage fiber, silage kernels, or forage protein can result in failure of the ration to meet expectations. Year to year variation in corn silage quality many times can be related to fiber and/or kernel digestibility. Laboratory tests do not always detect these changes but the more refined estimates of dry matter, fiber, or protein digestibility can bring more knowledge of how cows digest feed.
The type and amount of acids produced during storage can give an idea of the adequacy of the fermentation, and ammonia nitrogen content can indicate if excessive protein is broken down during storage. Yeast and molds cause adverse biological processes that can lead to poor-quality silage. Below is a fermentation profile of well-fermented corn silage from Corn Silage Production Management, Virginia Cooperative Extension publication 424-015.
| pH | 3.6-4.0 |
|---|---|
| Lactic acid | 4%-6% of dry matter |
| Acetic acid | less than 2% |
| Butyric acid | less than .1% |
| Propionic acid | less than .5% |
| Ethanol | less than .5% |
| Ammonia nitrogen | less than 5% of total nitrogen |
| Yeast and molds | less than 100,000 colony forming units/gram |
Note that the pH needs to be below 4.0 for stable corn silage. The dominant acid should be lactic. If other acids such as acetic, butyric, or propionic become predominate, this indicates a shift in the profile that results in poorer quality and may, as a result, not be as palatable to the cows. Ethanol fermentation should be avoided. Also high levels of ammonia are indicative of excessive proteolysis of proteins. These measures can aid in problem-solving situations but are probably not warranted on a routine basis because most silages will have a normal fermentation profile if ensiled under anaerobic conditions with dry matter contents of 30% to 42%.
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