STRIKING A BALANCE : PROTEIN FEEDING AND PERFORMANC E by Mary Beth Hal Department of Dairy and Poultry Sciences University of Florida Gainesville, Florida Introduction One of the largest stumbling blocks ever set for dairy cattle nutrition was allowing people bragging rights on the basis of their feed's protein content. Ask someone about their forage, ration, or grain mix, and the first and sometimes only number named is its protein percentage. Protein is an important nutrient, but it does not cure all production problems by itself, and it ca n even cause a few, particularly as we deal with the issue of nitrogen balance of a farm with th e environment. In ration formulation, we need to find balance between feeding adequate protein t o meet needs, and avoiding excesses or deficiencies that will cheat us of production. What We Feed What a Cow Uses Crude protein (CP) is the value used to formulate rations for cattle, but cattle use onl y true protein. True protein is composed of the chains of amino acids that are found in enzymes and plant and animal tissues. Crude protein is an estimate of protein, calculated as the feed' s nitrogen content multiplied by a factor of 6.25. 6.25 is used because true protein in feeds wa s estimated to average 16% nitrogen (1/6.25 =.16). The problem with this approach is that C P contains true protein, but also includes an array of non-protein nitrogen (NPN) sources such a s urea, ammonia, nucleic acids (genetic material), amines, etc. The cow can use the true protein and amino acids to meet her requirements, but she cannot use the NPN....unless the microbes convert it to true protein first. Rumen microbes can greatly modify the CP in a feed (Figure 1). Protein that i s degradable in the rumen includes both true protein and NPN. The true protein can be used directly by the microbes to make more microbes, or it may be fermented and converted to ammonia (NPN). Microbes can manufacture true protein from ration and ruminal NPN sources if adequate energy from rumen digestible carbohydrates is available. Ammonia that is not capture d as microbial protein is absorbed out of the rumen into the blood stream. It travels to the liver where it is converted to urea at an energy cost to the animal. The urea may be recycled to th e digestive tract, or excreted in milk or urine. Rumen "escape" or "bypass" protein is not degraded by rumen microbes, and may be digested by the cow in the small intestine. Rumen microbes have their own set of protein requirements. Bacteria that digest fiber need NPN, whereas bacteria that digest sugars and starches can use NPN or peptides (short chains of amino acids). To be most efficient, rations should be formulated to consider the NPN and peptide needs of the microbes. 15
Feed Crude Protein Figure 1. Fates of crude protein in the rumen and cow. Both true protein that a cow absorbs in excess of her requirements and ammonia no t captured as microbial protein suffer the same fate: their nitrogen is converted to urea in the liver, and it is recycled, or excreted in milk or urine. The levels of urea nitrogen in blood (BUN) o r milk (MUN) give an index of how well the cow is utilizing the crude protein she is being fed. High urea levels (> 16 mg/dl) suggest overfeeding of protein or inadequate energy/carbohydrat e relative to protein. Low levels (<12 mg/dl) suggest underfeeding total protein or inadequat e protein relative to energy. Since the ration most affects UN levels, average cow group value s should be examined by feeding group basis. So, we feed crude protein, but the cow's protein requirements are actually met by the tru e protein from feed that escapes rumen fermentation, and the microbial protein that is produced i n and passes from the rumen. That means that the amount of protein truly available to the cow i s affected by not only the amount of crude protein in the ration, but also by the amount of energ y available from carbohydrates in the rumen. Most Limiting Nutrient A cow does not produce because of a single nutrient, but by a balance among them all. The "limiting nutrient" is that nutrient that is most below the level required by the cow. It restricts production to the level that it will support. For instance, if ration protein is fed to support 9 lb of milk, but energy only supports 71b, milk production will fall to 7 lb when co w energy resources are depleted and she has to rely strictly upon the ration. The issue of limiting 16
nutrients becomes complex as energy, protein, and fiber interact in the ration so changes in on e alter other nutrients the cow receives. Like when feeding more fiber improves production by increasing protein and energy available from rumen fermentation, although ration CP and energ y had appeared to be adequate before. Or where a change in carbohydrate increases production b y supporting more microbial protein production to meet the cow's true protein needs. A study which illustrates the issue of limiting nutrients examined the effect of increasing levels of cottonseed meal (CSM) in the ration of lactating dairy cows. As CSM was increased, ration CP increased, and corn and barley (energy sources for rumen and cow) were removed t o make room for the CSM. The first increase in CSM resulted in an increase in milk. Subsequent increases in CSM increased protein intake, but did not significantly change production (Figure 2). Note that as protein increased, nonstructural carbohydrates in the ration decreased (Figure 3),. If the cows were capable of higher production, it is likely that the lack of further response t o increasing CSM was due to energy becoming the limiting nutrient, as the energy sources wer e removed from the ration. The cows did not respond with increasing production when protein wa s apparently not limiting, or was in excess of requirements. Feeding CSM did increase the essentia l amino acids in the blood, suggesting that it did increase true protein available to the cow. However, plasma urea nitrogen also increased, suggesting that the added protein was in excess o f requirements, was converted to urea, and was excreted (Figure 4). Y 14 16 18 2 22 24 26 Ration Crude Protein % of D M Figure 2. Milk production of cows fed increasing levels of cottonseed meal relative to increasin g levels of ration crude protein (points with different numbers of "*" differ, P <.1)(Cmngs et al, 1991). 17
2-12 14 16 18 2 22 Crude Protein % of Ration D M Figure 3. Changes in non-structural carbohydrate and crude protein pounds consumed wit h increasing ration crude protein (things et al., 1991). 24 26 12 25 E E Q a w 1-8 - 6-4 - 2 - --- EAA -a- PUN 2-5 I I I I I I 12 14 16 18 2 22 24 Crude Protein % of Ration D M Figure 4. Changes in plasma essential amino acids and plasma urea nitrogen with increasin g ration crude protein (things et al., 1991). Protein: Performance and Excretion The relationship between protein feeding and milk production has been perceived a s positive, but the true picture is not so clear cut. Graphs of information on protein feeding an d production taken from 25 studies in the Journal of Dairy Science (128 treatment means from 68 7 cows) give a clearer picture. 18
9 teit! t s4aa 4 ri 4 - AA A A A 3 i I I 1% 12% 14% 16% 18% 2% 22% 24 % Ration Crude Protein, % of D M Figure 5. Relationship of crude protein percentage in the ration and 3.5% fat-and proteincorrected milk production. The crude protein percentage of the ration was not well related to milk production (Figure 5). Even pounds of protein consumed did not necessarily translate into pounds of milk (Figure 6). At an intake of 6.5 lb of protein, cows produced 45 to 8 lb of milk. Cows producing 8 lb of milk did so when consuming 6.5 lb to 13.75 lb of protein. At low protein levels, there is the risk tha t changes in feedstuffs will result in underfeeding cows. At the high end of protein feeding, it i s unlikely that protein is the limiting nutrient. It is quite possible that feeding high levels of protei n 1 9 8 7 Milk, lb/day 6 5 4 3.5% Fat- Protein- Corrected /1' IIPPII!ISS w Wrenn %rmmcmwm. imlowkwamm s 5 6 7 8 9 1 11 12 13 1 4 Crude Protein Intake, lb/da y Figure 6. Crude protein intake and milk production. (Squares: National Research Counci l recommendations, diamonds: Journal of Dairy Science treatment means). 19
takes up space in the ration that would be better used by nutrients that are more likely limiting. Formulating rations to produce more milk with less protein has the advantage of reducin g nitrogen excretion by the cow. By and large, in lactating dairy cows, if protein does not mak e milk, it makes manure. The amount of nitrogen that is excreted in the manure is easily estimated as nitrogen in feed minus nitrogen in milk (Tomlinson et al., 1996). There is a very direct relationship between nitrogen (crude protein) consumed and nitrogen excreted (Figure 7). As nitrogen or crude protein intake increases, so does nitrogen excretion. The relationship between milk production and nitrogen excretion is not nearly as strong (Figure 8), suggesting that hig h production is not related to high levels of excretion. 2. >., 1.6 -- Milk N o Excreted N 1.2 a) C,.8 Z.4. I Mead"4.et," I i I..5 1. 1.5 2. 2.5 Intake Nitrogen, lb/day Figure 7. Excreted nitrogen and milk nitrogen as related to nitrogen intake of lactating dairy cows. 2. 1.8-1.6-1.4-1.2-1. -.8 -.6 -.4 -.2 -. 4 Y e ', : 4~ Z : AAi, A, r- 4 5 6 7 8 3.5% FPCM, lb/day 9 Figure 8. Excreted nitrogen and 3.5% fat- and protein-corrected milk production. 2
on cows with low intake should be, "How do we get them to eat more?", rather than simply tryin g to increase the nutrient density. In an effort to achieve the protein balance to meet microbial and cow needs, general recommendations have suggested feeding approximately 3% of the protein as soluble protei n (rapidly degraded degradable protein/npn), 65% degradable, and 35% undegradable. The recommendations will vary with the levels of different carbohydrates and fats in the ration. Our challenge with protein is to provide adequate amounts to support production without overfeeding. The idea that feeding more protein is cheap insurance for production looks only at one side of the risks. On the other side, feed dollars are needlessly spent on nutrients the cow doesn't need, that she'll have to expend energy to excrete wasted nitrogen, and that the amount o f nitrogen excreted into the environment will rise above the levels that crops and the environment can reasonably absorb. Finding the middle ground where we adequately meet the cow's needs i s the best option. References Grings, E.E., R.E. Roffler, and D.P. Deitelhoff. 1991. Response of dairy cows in early lactation t o additions of cottonseed meal in alfalfa-based diets. J. Dairy Sci. 74:258. National Research Council. 1989. Nutrient requirements of dairy cattle. 6 h revised edition update. National Academy Press, Washington, DC. Tomlinson, A.P., W.J. Powers, H.H Van Horn, R.A. Nordstedt, and C.J. Wilcox. 1996. Dietary protein effects on nitrogen excretion and manure characteristics of lactating cows. American Society of Agricultural Engineers, 39:1441.