Protein Content in Modern Sheep Rations
Table of Contents
A number of developments have occurred in the Ontario sheep industry to warrant re-evaluating how to meet the protein requirements of the ewe flock as well as market lambs. The developments that are of importance are:
Proteins are complex molecules built from amino acids, which have nitrogen (N) as a critical cornerstone element. Ruminants possess the unique ability to convert nitrogen in various water soluble forms into protein. This allows ruminants to take advantage of feeds which could be toxic to other species of livestock. The conversion occurs by the rumen bacteria taking nitrogen (which may even come from true protein, or its building blocks, amino acids) and energy available in the rumen creating amino acids and then bacterial protein. This bacterial protein is then washed out of the rumen and becomes the staple protein source in the forage-fed ruminant. There are essentially 4 nitrogen source classes although they have differing names in various literature.
Rumen Available Crude Protein - Degraded Intake Protein (DIP)
Rumen Escape protein N - Undegraded Intake Protein (UIP)
It is becoming increasingly accepted that animals utilize amino acids, not "crude protein". The measure of CP is still relevant on account of the rumen's ability to manufacture and alter protein from other nitrogen and protein sources. However, some dietary proteins do reach the hindgut intact, so their amino acid profile is very relevant. As a result, this concept is becoming increasingly popular. It is protein that is not available in the rumen, but is available for enzymatic breakdown in the true stomach and intestine. It is popularly referred to as "bypass protein". This term is inaccurate, as the protein does actually pass through the rumen. These proteins are lightly heat-treated (e.g. roasted soybeans) or of animal origin (e.g. fishmeal) which renders them unavailable to microbes, but available to the animal. Many amino acids in such "ruminally undegradable" proteins are scarce in microbial protein. Another terminology is Undegraded Intake Protein (UIP). In high producing animals, it is recommended that 25%-35% of the total CP be represented by this fraction.
Indigestible Protein - Indigestible N
This fraction passes into the feces and is unavailable to the animal.
Ideally, the crude protein make-up of a diet should provide some of each of the three classes of protein. This allows the animal to have a balanced supply of amino acids. It is often advantageous to supplement the rumen escape fraction to ensure the full compliment of amino acids is available to high producing animals.
Products such as meat and blood meal have been safely and effectively used as "bypass" protein supplements in ewe and feedlot lamb protein supplements. With scrutiny beyond the ruminant meat meal ban placed on those proteins, feed and animal industry personnel have sought to replace these with vegetable proteins to assure consumers of safe foods.
The concept of limiting amino acids has been used effectively in the monogastric species. It is suggested that the limiting amino acids for ruminants fed typical diets (corn and forage) are methionine (first) followed by lysine. As a result, the vegetable protein source blend should be high in those amino acids. Table 1 details feeds containing high methionine and/or lysine levels. The table clearly shows that animal feeds such as fishmeal and bloodmeal (which are not included under the ruminant meat meal ban) have a place in ruminant diets. Unless clear triggers are given to remove these feeds from rations (e.g., branded product specifications), neither the law nor the biology of the animal require it. For vegetable feeds, a number of vegetable ingredients have amino acid profiles which allow substitution for animal proteins, and show there is a possibility to deliver these amino acids in "all vegetable matter" diets. However, vegetable proteins need to be evaluated carefully. For example, although a feed may have a high percentage of UIP as methionine, total UIP levels might be low. There may also be palatability issues, and anti-nutritional or metabolic issues associated with a given feed at high levels.
Other emerging options to meet limiting amino acid needs are rumen protected amino acids. To date these are cost prohibitive, but they may become more important as animal protein meal use is challenged.
As a ruminant, sheep are capable of recycling dietary and metabolic nitrogen. This recycling is accomplished, by drawing urea from the blood and secreting it during rumination in the saliva. This sparing trait appears to be slightly better developed in sheep than in cattle, so presumably sheep can tolerate low nitrogen rations better than cattle. In fact, one research study indicated lambs could be fed oscillating CP levels with no performance difference when compared to animals fed the mean CP level on a continual basis. It has long been accepted that severe dietary nitrogen excesses can have a toxic effect. High blood urea nitrogen (BUN) may be a factor in sub-optimal fertility as is now suspected in dairy cows. As many ewes in Ontario that are being bred out of season are flushed on high quality pastures, there is the possibility that high BUN may be implicated in cases of poor flock fertility. Certainly the feeding of any supplemental protein on pasture for flushing or lactating ewes can be demonstrated to be excessive by formulating the ration using NRC or Sheep Ration Formulation Program (Ontario Ministry of Agriculture, Food and Rural Affairs, 2001, Version 2.0.1) recommendations. Here it is recommended that only energy is supplemented, preferably a low CP source. Whole corn is ideal.
Excess nitrogen output has become an environmental concern. The most cost effective method in reducing N output in the flock is to ensure the source nutrient (protein) is not overfed. To achieve this, ensuring less protein of higher quality (better balanced) is preferable to feeding more lower quality protein, that is unbalanced or non-protein nitrogen.
Meeting the CP recommendations or requirements in modern sheep operations may be more that just ensuring an abundant supply of crude protein, or diet nitrogen. Sheep rations should be formulated using a number of principles:
National Research Council. 1985. Nutrient Requirements of Sheep: Sixth Revised Edition. National Academy Press.
National Research Council. 1996. Nutrient Requirements of Beef Cattle: Seventh Revised Edition. National Academy Press.
National Research Council. 2001. Nutrient Requirements of Dairy Cattle: Seventh Revised Edition. National Academy Press.
Table 1. A Partial List of High Methionine and/or Lysine Feedstuffs
Adapted from the Nutrient Requirements of Beef Cattle, 7th Revised Edition 1996 (NRC, 1996). Those above 1.75% of UIP as methionine and those above 4.0% lysine are listed. Animal proteins are included as reference feeds. Bolded items are those that have high levels in both amino acids.
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