Protein

Proteins are needed throughout life for growth and repair of the body and for synthesis of products such as enzymes, hor­mones, mucin, milk, and hair. As is true of monogastric ani­mals, the ruminants’ protein requirements consist mainly of amino acids that are absorbed from the small intestine.

Because the rumen microbial population synthesizes all of the amino acids essential to a goat, sheep, or cow from dietary protein and NPN and sulfur, the amino acid compo­sition of the feedstuffs is relatively unimportant. However, for maximum milk production it is advantageous to sup - ply some additional amino acids in the form of rumen undegradable protein or rumen-protected amino acids. For dairy cattle, methionine and lysine are considered to be the most limiting amino acids relative to high milk production. Fishmeal as a bypass protein is a good source of methio­nine, but heat-treated soybean meal is not. Zinc-methio- nine and zinc-lysine chelates have been used to supplement these amino acids in dairy cattle (Kung and Rode 1996). There is very limited evidence that supplementation with protected methionine or lysine improves milk production in goats that are not deficient in zinc (Salama et al.

Protein Systems and Requirements

Protein requirements for the goat were originally expressed in terms of total crude protein (CP), which is simply deter­mined by multiplying the nitrogen content of the feed by 6.25. This figure is used because most proteins contain 16% nitrogen. Nonprotein nitrogen (NPN) substances in the feed thus enter into the calculation as if they were true protein, but this is not a serious flaw because the rumen microbes can transform NPN into protein. The previous NRC for goats (1981b) computed protein requirements from a simple calorie-to-protein ratio. Thus, the tables in this publication required 32 g of total protein for each Mcal of digestible energy. Efforts to express requirements in terms of digestible protein (apparently digested protein, determined by sub­tracting fecal nitrogen from dietary nitrogen) are not believed to be more accurate because metabolic fecal nitrogen (i.e., digestive enzymes, sloughed epithelial cells, etc.) comprises a variable proportion of the total fecal nitrogen. Also, ammo­nia released in the rumen in excess of what the microbial population can use for synthesis is absorbed into the blood­stream. This ammonia is converted by the liver to urea, some of which is recycled into the rumen by diffusion or via saliva so that it may be used between meals when the rumen ammonia levels fall, but some is lost in the urine.

When high production is desired it is necessary to refine the protein requirements beyond crude protein and digest­ible protein. Ruminants need both rumen degradable and rumen bypass protein (Waldo and Glenn 1984; Eastridge 1990). Newer systems for goats specify protein requirements in terms of metabolizable protein (MP). MP is defined as the true protein, derived from a combination of dietary and microbial protein, that is digested postrumi- nally and from which the constituent amino acids are absorbed from the intestine (NRC 2007).

CP = MP / ((64 + (θ.16 ? %UIP)) / 1θθ)

Beyond the protein required for maintenance (expressed on a metabolic weight basis), goats need additional protein for growth (0.29 g MP/g average daily gain for dairy and indigenous goats, 0.40 g MP/g daily gain for meat goats). Each g of milk protein requires 1.45 g MP, while each g of clean mohair fiber requires 1.65 g MP. Further protein is required for growth of the mammary gland, uterus, and fetuses during pregnancy.

The 2007 NRC composition tables specify CP, MP, and often UIP for each feedstuff. For forages, actual values determined by laboratory testing typically give a better approximation of protein content than do book values. Overheating that occurs when hay is stored too wet or silage is stored too dry results in complexing of sugars with amino acids that reduces the digestibility of nitrogen. This nitrogen ends up in the acid detergent fiber (ADF) fraction during forage analysis (Eastridge 1990). Some laboratories determine available protein (or adjusted crude protein) on forages to reflect this loss in feed value.

The amount of microbial protein formed with each feed actually depends on whether energy or nitrogen is the lim­iting factor for microbial reproduction and growth. Thus, with most concentrates there is more than enough energy in the feed to fix ammonia released from rumen digestion into microbial protein; the nitrogen supply is what limits bacterial growth.

French composition tables give two MP figures for each feedstuff (Morand-Fehr and Sauvant 1978; INRA 2007). One indicates the sum of digestible protein escaping the rumen and of the microbial protein that could be formed if all of the nitrogen released in the rumen could be con­verted into protein (PDIN in French usage, i.e., if energy were not limiting). The other (PDIE) is the sum of alimen­tary protein digested in the intestine and of the microbial protein that could be formed if all of the energy available for the rumen organisms could be used for their growth (i.e., if nitrogen were not limiting). When a single feed is fed, it is the smaller of the two values that represents the MP. However, when feeds are combined, the ruminant (and the nutritionist) can add up all the PDIE values to give a global PDIE for the ration and can likewise obtain a global PDIN. If PDIE exceeds PDIN, it may be possible to add NPN to make use of the excess available energy, assum­ing that this can be done in such a way that the ammonia supplied and the energy released from digestion of carbo­hydrates are present simultaneously in the rumen. If PDIN exceeds PDIE, the diet could be better balanced by adding more energy or removing some nitrogen, depending on which of these options gives better economic results. Protecting some of the dietary protein from degradation in the rumen (as by heat or formaldehyde treatment) so that it can be digested in the intestine is one way of avoiding nitrogen losses associated with excess PDIN.

Milk Urea Nitrogen

Dairy cattle nutritionists pay close attention to milk urea nitrogen (MUN), the portion of the measured milk protein that is derived from blood urea nitrogen (BUN) rather than casein or whey proteins. Ammonia produced during digestion that is not converted into microbial protein is absorbed from the rumen and converted to urea in the liver.

Little research has been done relative to interpretation of MUN in goats. Brun-Bellut et al. (1991) proposed a normal range of 28-32 mg/dL. A study comparing Alpine goats fed alfalfa hay or pasture and varying levels of concentrates found a range of MUN values of 18-22 mg/dL (Min et al. 2005). When varying amounts of tallow were added to the diet in another study, the MUN varied from 24 to 26 mg/ dL (Brown-Crowder et al. 2001). In a long-term study that compared a forage-based diet with one formulated with sunflower seeds, cassava, coconut meal, and cottonseed, the MUN was lower for the forage diet (approximately 15 mg/dL) than for the non-forage diet (approximately 30 mg/dL), even though milk yield was similar (Bava et al. 2001). It seems apparent that MUN values in goats are generally higher than those reported for dairy cattle. In a series of metabolic studies evaluating various diets in dairy goats in Italy, the mean MUN was 34.2 mg/dL, while the range was 11.9-67.5 mg/dL (Rapetti et al. 2014). MUN was not correlated with milk yield.

How nutrition might influence true milk protein content in dairy goats has been reviewed by Chamberlain (1997).

Urea Toxicosis

As is commonly done with dairy cattle, urea can be fed to goats as an NPN source because rumen organisms can con­vert it to microbial protein if adequate energy sources are available (Harmeyer and Martens 1980). Much of the urea that reaches the plasma (absorbed from the gastrointesti­nal tract or produced in the liver) is recycled to the rumen via saliva.

Pathogenesis and Epidemiology

Toxicosis is expected to occur if 30-50 g urea/100 kg bw is consumed in a single meal by an unadapted or hungry ruminant. This is because the urea is degraded to ammonia and excessive non-ionized ammonia in the rumen diffuses out into the bloodstream. Toxic levels are reached when the liver's ability to convert ammonia back into urea is over­whelmed. The citrate cycle is inhibited by the ammonia (Lloyd 1986).

Improperly mixed feed and urea fertilizers are potential sources for toxicity. NPN is particularly dangerous when animals are fed high-fiber diets lacking readily digestible carbohydrate. In one instance, a goat and other livestock at a fair died after drinking water contaminated by trans­port in a tanker that usually hauled a liquid fertilizer composed of urea and ammonium nitrate (Campagnolo et al. 2002).

Clinical Signs

Toxicosis in small ruminants (Fujimoto and Tajima 1953; Obasaju et al. 1980; McLennan 1987; Ortolani et al. 2000) typically occurs within one hour after ingestion, and death after a few hours. Signs include muscle and skin tremors, salivation, frequent urination and defecation, incoordination, dyspnea, loud bleating, bloat, tetany or convulsions, and death. Laboratory findings include ele­vations in blood packed cell volume, potassium, phos­phorus, and urea nitrogen. Sometimes, but not always, the rumen fluid pH exceeds 7.5 (Lloyd 1986), and ammo­nia concentration in rumen fluid exceeds 500 parts per million (ppm) (Ortolani et al. 2000; Campagnolo et al. 2002).

Treatment

The best antidote available is 0.5-1 L of cold vinegar (typi­cally 5% acetic acid), which may be given by stomach tube to decrease rumen pH (Osweiler et al. 1985). A potentially equivalent vinegar dosage is 10 mL/kg. At a lower pH, the ionized ammonium ion diffuses less readily through the rumen wall into the circulation, and free ammonia already in the blood may actually return to the rumen. If the goat is already in tetany, emergency surgery to empty the rumen has been suggested as the one method that might possibly save the animal's life (Bartley et al. 1976). Intravenous lac­tated Ringer's solution (for its acidifying effect) and B vita­mins have also been proposed, among other treatments, as beneficial in treating animals with chronic consumption of excess urea (Hazarkia et al. 2002).

Prevention

In the absence of PDIN/PDIE values for dietary compo­nents used by French nutritionists, other guidelines may be used to predict a safe level of urea use. For instance, in American literature there is often a statement that urea should not exceed 1.5% of a finished (total) ration (Adams 1986). It has also been recommended that urea supply no more than one-third of the total CP in forage or roughage diets and that it be no more than one-half of the CP in the concentrate part of the ration (NRC 1981b; Fernandez et al. 1997). At least three weeks are required for the rumen microflora to adapt to such a high level of urea feeding, and therefore gradually increasing the concentra­tion of urea in the feed is advisable. Not all goats are willing to eat grain containing so much urea (Skjevdal 1981).