Research Continues on Lower-Protein Rations
By Bill Mahanna
The 71st Cornell Nutrition Conference convened in October in East Syracuse, N.Y., and included several papers that contributed to the nearly 30-year effort by Cornell researchers to model rumen biology.
Their recent emphasis is on improving the efficiency of milk protein export off the farm while reducing nitrogen excretion to the Chesapeake Bay watershed. New insights into their understanding of rumen microbes (especially protozoa) and successful results from commercial dairy field testing of diets with 0.5-1.5 fewer units of crude protein (CP) are generating considerable interest among consulting nutritionists across the U.S.
Rethinking Model Biology
Updating the biology driving the rumen submodel in the CPM or Cornell Net Carbohydrate & Protein System (CNCPS) computer models is a major focus of Cornell researchers, including modeling the concept that as the protein supply tightens, the cow becomes more efficient at nitrogen recycling.
The current thinking is that CPM version 3 may be underestimating recycled urea-nitrogen by 50-75% (Recktenwald and Van Amburgh, 2007).
In a recent Cornell trial, lactating cows were fed three different diets: one balanced at 14.1% CP to produce a deficiency in ruminal nitrogen (but balanced for adequate microbial protein [MP]), the second formulated to produce a postruminal deficiency of amino acids (but adequate for ruminal nitrogen) and the third to provide adequate levels of both ruminal nitrogen and amino acids.
The cows fed the low-ruminal nitrogen diet recycled more urea nitrogen (32% versus 14-15% in other diets), and the nitrogen was used more efficiently for synthesis of MP. It appears that ruminal nitrogen deficiencies stimulate urea nitrogen recycling and more efficient microbial nitrogen incorporation if the ration is relatively high in fermentable carbohydrate availability (Recktenwald and Van Amburgh, 2009).
The difficulty some midwestern and western nutritionists have with lower-CP rations may be linked to their use of more alfalfa hay and silage compared to the traditionally heavier feeding of corn silage in the East (Schulte, 2009).
Another modeling development is that less emphasis is being placed on the peptide pool estimates in CPM or CNCPS (Van Amburgh, 2009) primarily due to research showing that upwards of 15% of the total protein supply is an endogenous protein contribution to the MP pool (Van Amburgh et al., 2009).
Work is currently underway at Cornell and at The Ohio State University to better quantify urea nitrogen recycling and capture by bacteria and the long-overlooked rumen protozoa.
Protozoa have long been touted as moderators of a postprandial drop in ruminal pH by engulfing starch granules and by their predation on amylolytic bacteria, which reduces the bacterial population and acid production (McAllister, 2001). Protozoa are also linked to 20-45% of rumen amylotic activity and are thought to be more capable than bacteria of degrading starch-encapsulating zein proteins (Van Zwieten et al., 2008).
In the previously mentioned Cornell trial, 16-27% of microbial nitrogen in the rumen was present as protozoal nitrogen. Depending on the relative turnover rates of bacteria and protozoa, 10-40% of the bacteria produced in the rumen were assumed to be preyed on by protozoa.
Though some predation by protozoa is expected, the relative fraction of protozoal protein in digesta flowing from the rumen was previously thought to be quite small because protozoa tend to remain associated with the floating raft of forage in the rumen (Owens, 2009).
It appears that a change in traditional thinking may be needed regarding the role protozoa play in high-producing cows that are fed rations higher in starch and sugar (known to stimulate protozoal growth). These ration are increasingly lower in physically effective neutral detergent fi ber (peNDF), which contributes to more rapid rumen turnover rates.
CNCPS does not currently have a protozoal pool within its rumen submodel, yet various research trials have indicated that 5-20% of the MP leaving the rumen is protozoal protein (Van Amburgh et al., 2009). Protozoa also have a different amino acid profile than bacteria as they are lower in methionine (24.0 g/kg versus 28.4 g/kg of total amino acids) and significantly higher in lysine (121.4 g/kg versus 90.3 g/kg of total amino acids).
Protozoa are also lower in branch-chain amino acids, so work is needed to move beyond lysine and methionine to better understand the amino acid profile in fermented forages and the role of other potentially limiting amino acids like histidine and the branch-chain amino acids (Van Amburgh et al., 2009).
I have discussed the topic of protein levels, especially in high-corn silage rations, in previous columns (Feedstuffs, Feb. 11, 2008) in relation to high protein being a luxury that can no longer be tolerated as soybean meal prices elevate, nitrogen-related environmental concerns mount and more is learned about urea nitrogen recycling and utilization by rumen microbial populations.
Field nutritionists have reported success with feeding high-corn silage rations that contain more conservative levels of CP (16.0-17.5%) and rumendegradable protein (8.0-8.5% of dry matter; Drehmann, 2008).
Milk nitrogen efficiency (MNE) was discussed (Chase et al., 2009) as an index that can be used to assess the efficiency of nitrogen use by dairy cows. MNE is calculated as the ratio of the quantity of nitrogen excreted in milk to the quantity of nitrogen consumed and expressed on a percentage basis. MNE values on most commercial dairies range between 20% and 35%, implying that 65-80% of the consumed nitrogen is excreted in urine and manure. As ration CP values increase, MNE tends to decrease.
In a field trial with two New York dairy herds, CNCPS version 6.1 was employed to decrease dietary CP from about 17.5% to 16.7% while increasing dietary starch from 23% to 27%. This resulted in no change in milk yield or composition while improving MNE and increasing income-over- purchased-feed costs by 35-90 cents per cow (Chase et al., 2009).
Nutritional parameters were reported (Table) from 14 herds from Wisconsin, Michigan, Pennsylvania and New York that fed lower-CP (less than 16%) total mixed rations. Most of the herds were feeding more than 55% forage (primarily corn silage) with relatively high nonforage carbohydrates and starch levels. Despite the high production with lower CP levels, there appeared to be an opportunity to refine the amino acid balance in a number of the herds (Chase et al., 2009).
It would have been interesting to also have data reported on the peNDF levels in these herds successfully feeding lower-protein, high-starch and corn silage rations given recent conclusions from the Miner Institute showing maximum feed efficiency when ration peNDF ranged from 21% to 25% (Grant, 2009)
A 2006 survey of New York feed industry personnel indicated that some of the challenges that limit the success of lower-CP diets centered around: (1) consistency of feed mixing and delivery, (2) daily variations in forage quality and dry matter, (3) increased levels of soluble protein in forages and (4) limited availability of milk urea nitrogen values as a monitoring tool (Chase et al., 2009).
The Bottom Line
The use of newer ration software that can track MP production and amino acid intake has allowed nutritionists to balance lactating diets at CP levels of 15-16% while maintaining milk yield and, in many cases, increasing milk protein output.
Research is starting to unveil more of the biology behind nitrogen recycling and levels/sources of MP supply that, when incorporated into future models, should help nutritionists be even more confident in reducing ration protein costs and also decreasing the excretion of nitrogen into the environment.
Chase, L.E., R.J. Higgs and M.E. Van Amburgh. 2009. Feeding low crude protein rations to dairy cows — Opportunities and challenges. Proc. 71st Cornell Nutrition Conference for Feed Manufacturers, East Syracuse, N.Y. Oct. 20-22.
Drehmann, P. 2008. Personal communication.
Grant, R. 2009. From the president’s desk — Fiddling with efficiency and effective fiber. William H. Miner Agricultural Research Institute Farm Report. November.
McAllister, T., A. Hristov and Y. Wang. 2001. Recent advances/current understanding of factors impacting barley utilization by ruminants. Proc. 36th Pacific Northwest Animal Nutrition Conference, Boise, Ida. Oct. 9-11.
Owens, F.N. 2009. Personal communication.
Recktenwald, E.B., and M.E. Van Amburgh. 2007. Examining nitrogen efficiencies in lactating dairy cattle using corn silage based diets. Proc. 69th Cornell Nutrition Conference for Feed Manufacturers, East Syracuse, N.Y. Oct. 23-25.
Recktenwald, E.B., and M.E. Van Amburgh. 2009. Refining nitrogen feeding using current knowledge of recycled urea and microbial nitrogen uptake. Proc. 71st Cornell Nutrition Conference for Feed Manufacturers, East Syracuse, N.Y. Oct. 20-22.
Schulte, K. 2009. Personal communication.
Van Amburgh, M.E. 2009. Personal communication.
Van Amburgh, M.E., T.R. Overton, L.E. Chase, D.A. Ross and E.B. Recktenwald. 2009. The Cornell Net Carbohydrate & Protein System: Current and future approaches for balancing of amino acids. Proc. 71st Cornell Nutrition Conference for Feed Manufacturers, East Syracuse, N.Y. Oct. 20-22.
Van Zwieten, J.T., A.M. van Vuuren and J. Dijkstra. 2008. Effect of nylon bag and protozoa on in vitro corn starch disappearance. J. Dairy Sci. 91:1133-1139.
This article was originally published in the December 2009 Feedstuffs issue, and is reproduced with their permission.