REGULATION OF FEED CONSUMPTION IN DAIRY COWS : ________________________________________________ The mechanisms involved in feed consumption and appetite regulation are complex. The following summaries briefly describe the factors that are involved in the regulation of feed consumption in dairy cattle. The two most important factors for dairy cattle that are not under any type of stress are the physiological and the chemostatic regulatory mechanisms of feed consumption. Physiological regulation normally occurs when less digestible feeds, such as low-quality forages or bulky feeds (hulls, etc.) are being fed. Chemostatic regulation occurs when less bulky feeds that contain higher digestible nutrient contents are used. Physiological regulation: Physiological regulation is based on the volumetric capacity of the digestive tract, in dairy cattle this specifically relates to the capacity of the rumen. Sensors located in the rumen of the dairy cow sense when the rumen is distended (full), at which time a signal is sent that causes the animal to stop consuming feed. This type of regulation occurs with feeds that contain low digestible nutrient densities and are bulky (low weight per unit volume), such as straws and other low-quality forages. Dramatic reductions in consumption occur when these types of feeds are fed. These feeds normally have low digestibility because they are high in lignin, silica and acid detergent fibre, which depress the digestibility of the nutrients that they contain. As forage quality declines, the digestive passage rate becomes slower, resulting in a greater fill factor and causing a reduction in feed intake. This same type of reduction in feed intake also occurs when low-bulk density feed stuffs are mixed into complete rations. In most cases, physiological fill is a factor only when dairy cattle are being fed very low-quality feeds. Chemostatic regulation: This mechanism functions when blood levels of specific metabolites rise, sending a signal that causes the animals appetite to be depressed. In the case of dairy cattle, volatile fatty acids are the metabolites that cause the signal to be sent; a few hours after a cow has consumed a meal, the volatile fatty acid levels in the rumen start to rise as a result of rumen fermentation of the ingested substrates. The dietary digestible energy levels are directly related to the amounts of the metabolites that are produced. Peak volatile fatty acid production normally occurs in the rumen two to three hours after a high-concentrate (high in readily available carbohydrates) ration has been consumed and four to five hours after a high-forage (high in cellulose) ration has been fed. The volatile fatty acids that are produced in the rumen are then absorbed and the levels in the blood rise. Once a certain level of volatile fatty acids in the blood has been reached, the appetite of the animal will be depressed. The volatile fatty acids are continuously absorbed and metabolized by the cells, so when the blood volatile fatty acid level declines the animals appetite will increase again. Many other factors have been shown to have an influence on feed consumption, including health, parasite load, digestive disorders, forage quality, environmental stress and water quality
Posted on: Sun, 03 Aug 2014 16:28:17 +0000
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