Introduction
In Coopworth Technical Bulletin 7 we discussed the different components of feeds and focused mainly on metabolisable energy (ME). I am confident that you will have been around your farm thinking how much ME is in the grass now compared with 3 months ago. We also spoke briefly about protein. The focus of the bulletin is to discuss protein in a bit more depth.
Protein
Protein is important for the formation of body tissue, milk, enzymes and hormones. Understanding the protein needs of ruminants is more difficult than for energy because the activity of the micro-organisms in the rumen can enhance or depreciate the protein supply from a feed. However it is more common for energy rather than protein to be a limiting factor in the diet. The protein content of feeds varies from around 8 – 35% of the dry matter.
The protein requirements of ruminants have until very recently been expressed in terms of "crude protein" requirements. This misleading term has now been replaced with the term "metabolisable protein" (MP). As with energy the MP is the protein, which is absorbed by the animal, after which it can be used for body maintenance and production.
There is also more to the protein story and the need to appreciate that protein comes from two sources – microbial protein and undegraded dietary protein (UPD; also called "bypass" protein). Microbial protein is produced by rumen microbes, from so-called "rumen degradable protein"(RDP) in the feed. The RDP is essential for high rates of production of rumen microbes, which not only adds to the metabolisable protein supply of the animal, but also supports high rates of feed digestion and feed intake. Undegraded dietary protein is the part of the dietary protein, which is not degraded (broken down) in the rumen. The UDP (by-pass protein) escapes degradation in the rumen and subsequently undergoes digestion and absorption in the small intestine and utilisation at tissue level.
The proportion of RDP and UDP in the protein of any feed is expressed as its "protein degradability". The degradability of crude protein in high quality ryegrass can be as high as 75-80% (i.e. 75-80% of the protein is RDP and only 20-25% is UDP). As can be seen, ryegrass has no shortage of RDP, allowing for high rates of production of rumen microbes. In fact the RDP content is often in excess of requirements and is excreted by the animal as urea in urine. There is much discussion on the topic of "by-pass" protein, and plants that contain higher levels of by-pass protein.
Protein requirement of the growing lamb.
In a very young animal much of the gain in weight is in the form of protein (i.e. muscle as opposed to fat). These animals have a small maintenance requirement for ME and a high requirement for MP. Herbages with a low rate of degradation in the rumen barely meet the requirement for UDP in the small lamb. In other words the small lamb requires a high level of MP per unit of ME consumed. This is often not met in pasture. It seems that lambs weaned at less than 15 kg bodyweight may suffer from protein deficiency and may be incapable of reaching their full growth rate potential until they reach 20 kg bodyweight.
Legume diets do however support higher rates of gain. This may occur because less protein is degraded in the rumen which may be attributed to legumes tending to have a faster rate of passage, i.e. they do not stay in the rumen as long.
If you need to wean early, ensure lambs have access to legume based swards.
Alternative pasture species.
There is presently much discussion about ‘super-legumes’. These legumes contain condensed tannins (CT). Some of the species referred to are:
birdsfoot trefoil (Lotus Corniculatus) lotus major (Lotus pedunculatus)
sulla (Hedysarum coronarium) flowers of white clover (Trifolium repens)
What do condensed tannins do?
Plants differ in the type and concentration of CT they contain, therefore the effects of CT-containing legumes may vary. But the overall function of CT is the same.
CT convert plant proteins into ‘rumen protected proteins’ or ‘protected proteins’. That is these proteins are protected from breakdown by microbes in the rumen. Thus more protein is available to the animal. There has been some very good lamb growth rates achieved with lambs grazing Lotus.
CT containing legumes have potential to increase growth rates in lambs. However there are still challenges in successfully incorporating these legumes on farm. Maybe you could consider growing them as a perennial greenfeed crop or as a summer-autumn greenfeed crop when renovating pastures.
Protein supplementation of the ewe.
There are two issues here :
It appears there is scope to improve the productivity of both the ewe and the lamb by supplementing with by-pass protein (UDP) or by feeding legumes containing CT. The challenges remain in sourcing a suitably cheap feed high in UDP, or successfully incorporating CT plants in our farming system. Have you considered planting perhaps 15-20% of your farm in another species?
The protein content of feeds is given in the Feed Value chart you received with Technical Bulletin 7. However this lists only the Crude Protein content.
Lynda Clark Publicity Officer, Coopworth Sheep Society
If you require further information, please contact the Technical Secretary, Chris Logan, PO Box 169, Lincoln University.