10 January 2010

6 minute read

Forage quality

Forage quality can be defined simply as the ability of the dairy cow to digest and utilize the nutrient components provided by the forage source . The higher the content and digestibility of the nutrients, the higher the quality of the forage. The highest-quality and most digestible forage is young herbage, because it contains the lowest amount of structural carbohydrates (cellulose, hemicellulose) and lignin.

As a forage matures, its digestibility, rate of digestion and CP content decline, causing the cow to derive fewer nutrients from the forage, lowering the quality. A decline in the quality of forage has an impact on the amount of other feedstuffs that the animal is able to consume. The slower passage time of the forage results in a reduction in intake of not only the forage but also other feeds that the animal is consuming.

Table 1 shows that, as alfalfa matures, its digestibility and CP content decline, reducing the amounts of nutrients that the cow can obtain from the alfalfa and, thus, also reducing intake. Forage quality has also been shown to have an effect on dry matter consumption, especially when low-quality forages are being fed.

The quality of the forage being fed to dairy cattle has a dramatic impact on not only dry matter consumption but also the proportion of nutrients that are being provided by other feedstuffs. Table 2 gives an example of the effect that changing forage quality has on nutrient consumption.

¹Based on providing enough DM intake and nutrients for a 600kg cow to produce 20kg of milk (3.5 per cent butter and 3.2 per cent milk protein). Nutrient requirements for producing 20kg/milk per day include 10.26kg of total digestible nutrients (TDN), 2.086kg of crude protein (CP) and 16.2kg dry matter intake.

In the Table 2 example, the amount of concentrate (in this case wheat bran) required to maintain the same energy intake increases from 5.35 kg when good-quality alfalfa is fed, to 12.59 kg when straw is fed. Whenever the quality of the forage declines, the amount of concentrate required to be fed increases, if the same dietary energy level is to be maintained. In Table 6 the amount of dry matter consumption remains constant, but as forage quality declines, dry matter consumption also declines, so even greater quantities of concentrate will have to be fed.

In Table 2, the CP intake ranges from 2.87 kg down to 2.28 kg, which is still above the 2.09 kg minimum required to produce 20 kg of milk. As forage quality declines, the quantity declines and the amount of CP it provides decreases. Lower CP intakes also have a tendency to reduce feed intake because CP stimulates rumen fermentation, which increases dry matter intake.

Normally, 40 per cent of roughage is considered the minimum level required when formulating ratios for lactating dairy cattle. The length of the dietary roughage component must also be considered. An inadequate amount of roughage or reducing the length of the roughage, so that there is not enough effective fibre, will cause butterfat depression and, often, digestive problems.

Visual appraisal of forage quality

Visual appraisal of forage can be useful in assessing its quality. The maturity of a forage can be estimated quite accurately by the number of buds, blossoms or seed heads that are present. Proper curing during the haymaking process can be assessed by the colour of the hay.

Colour can also be used to assess the extent of nutrient losses associated with leaching resulting from exposure to rain and weather. Bleached forages will have lower vitamin and CP contents. With legume-type forages, the leaf-stem ratio can provide a fairly accurate estimate of the nutrient content of the forage. When there are many leaves, the CP content is high; when there is more stem, the structural carbohydrates content will be higher and the digestible nutrients content lower. The CP content of a forage is closely correlated to its digestibility - the higher the CP content of a forage, the higher its digestibility will be.

Palatability of forage

The palatability of a forage is affected by its taste (sweet, salty, bitter, acidic), olfactory and textural characteristics. Taste is normally the major factor affecting palatability. Dairy cattle are non-selective consumers and readily consume a wide range of feeds. Almost all livestock show a preference for sweet, so feed consumption can often be increased by adding molasses to a ration. Salt can also be used to increase the palatability of a feed but, once it reaches a certain level, increasing the salt content will depress feed consumption. Palatability can play a role in feed consumption when the animals have a choice, but dairy cattle do not usually have a choice, so palatability is not a major factor in feeding dairy cattle. Palatability normally becomes a factor only when attempts are made to feed spoiled feeds to dairy cattle.

Processing of forage

The decrease in intake that occurs as a forage matures can be counteracted to some extent by reducing the physical size of the forage (through chopping or grinding), which will allow it to pass through the rumen at a faster rate. The passage rate out of the rumen is based on particle size and density. Small, dense particles are passed out of the rumen more quickly than larger forage particles (most of which are less dense and float), which are retained in the rumen. As the ruminal passage rate increases, exposure to the digestive processes decreases and the overall digestibility of the forage declines but, because more can pass through the digestive tract, the animal will increase its dry matter consumption and the net result is usually that the cow's digestible nutrient intake increases slightly. This is one of the reasons for chopping forages prior to feeding. Chopping is most beneficial when low-quality forages are fed, but forages should not be chopped into pieces that are too small, as this can result in a depression of milk butterfat.