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Beef Breeder MANUAL
Figure 3.1. The ruminant digestive tract.
3.2. Concepts of Nutrition and Feeding
3.2.1. Feed Intake
The generally accepted principle is that the amount of feed eaten by ruminants is limited by;
• Rate of feed intake • Capacity of the rumen
• Rate of passage of digested material from the rumen • Water intake.
If the rate of digestion can be increased, then the rate of passage will also increase, and which in turn allows
the animal to consume more feed. With a higher rate of digestion in the rumen, greater quantities of nutrients
are available for absorption in the small intestine, with a consequently higher level of animal production. If the
rate of digestion is slow, feed intake is limited by the full rumen.
If the rumen is full of low quality feed, rumen contractions may cease. This is called ruminal stasis, and impaction
problems can arise with this condition, sometimes leading to death from starvation or metabolic diseases if cattle are
not supplemented with other more nutritious feeds.
Capacity of well-developed adult bovine rumen:
≈ volume of 200L 44 gal. drum.
Other factors that can have an impact to reduce the intake of a balanced diet, thus leading to production
losses are; • Limiting access to feed or the amount of feed on
ofer • Environmental stress, e.g. heat which can lead to a
reduction in intake • Plant species selection by the animal
• Moisture content of feed • The mineral content of feed.
While it may not be important for livestock managers to know the precise nutritional composition of the
various feeds, it is important that they understand the concept of diferent nutrients, the need for general
nutrient balance, the classes of feed, and their nutritional strengths and shortcomings. They must also understand
how the ruminant digestive system functions in order to understand the impact of poor feeding practice, and
how it might be remedied.
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Cold Winter Climates
3.2.2. The First Limiting Nutrient
In 1843, a German chemist, Justus von Liebig, published the law that bears his name, stating that;
“Growth is controlled, not by the total of resources available, but by the scarcest resource limiting factor.”
Liebig used the image of a barrel Fig. 3.2, now called Liebig’s barrel, to explain his law. In simple terms, Liebig’s
barrel is used to illustrate that the growth of a living organism is limited by the nutrient in shortest supply
i.e. the irst limiting nutrient in the same way that the capacity of a barrel with staves of unequal length is
limited by the length of the shortest stave. While this was originally proposed for plants, it also
applies to animal growth and activity. Therefore, if any one of the nutrients in Fig. 3.2 is in lower supply than
the animal requires for a particular level of performance e.g. live weight growth, milk production etc., the
animal cannot reach that level of performance until the deiciency is overcome. Increasing the supply of other
nutrients does not improve performance, and if the deiciency is extreme, the animal may die.
Figure 3.2. Liebig’s Barrel - The volume of liquid held by the barrel can only be increased once the “Energy” stave is repaired. The barrel can only be illed to the top when both “Energy” and “Protein” staves have been repaired.
Note: The Liebig Barrel concept is equally applicable to all nutrients. A similar result would be obtained if, for example, the “Phosphorus” stave was incomplete.
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Beef Breeder MANUAL
3.2.3. Digestibility
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