Environmental issues related to nutrient management, runoff control, odor, and dust are major challenges facing the beef feedlot industry. Diet manipulation is pivotal in management of nutrients released into the environment. The focus has been primarily on phosphorus (P) and nitrogen (N), which are important nutritionally, environmentally, and economically. Phosphorus is a concern primarily when entering surface waters where it can lead to premature aging (eutrophication), algal growth, and oxygen depletion. Nitrogen is a concern when nitrate (NO₃^-) enters groundwater (potentially from manure application in cropping areas) and when ammonia (NH₃) volatilizes into the atmosphere. Ammonia is a concern due to odor and because it may be deposited in surface water from rainfall or form particulate matter with other N compounds and potentially harm human health and/or ecosystems. The losses also appear to be large and may be a regulated nutrient in the future.

Phosphorus

The key management challenge with P is to decrease the amount fed in feedlot diets, while still providing adequate P to optimize performance. Therefore, knowing the requirements is vital to addressing this issue. Based on Nebraska research, it appears feedlot diets consisting of grain or grain byproducts (usually corn) contain adequate P. Requirement studies with yearlings (Table 1) and calves (Table 2) illustrate the P requirement is quite low (< 0.16% of diet DM). The 1996 NRC is not accurate in predicting P requirements for feedlot cattle greater than 600 lb. The NRC recommendations are between 0.2 and 0.3% of the diet as P. The range illustrates the change predicted as “days on feed” increase, i.e. body weight increases. Phytate (organically bound) P in grains has been shown to be completely available to ruminants. Since corn contains 0.32 + 0.04% P (NRC, 1996) and studies suggest no performance or bone changes when P is supplemented above 0.16%, then P supplementation is not needed in feedlot diets.

Based on Nebraska research, it appears the use of supplemental P could be discontinued in feedlot rations. This approach makes good environmental and economic sense. Decreasing dietary P decreases P excretion and the amount of acres needed to appropriately spread feedlot manure. The P excreted in manure needs to be distributed at agronomic rates (~30 lb P per acre). Feedlot managers should work with area crop producers to gain access to enough acres to ensure sustainable production. In Nebraska (and some other areas), the resources are adequate to handle this challenge if manure is distributed appropriately.

Nitrogen

The reason nutrition plays a critical role in emissions from feedlots is that 60 to 80% of the N excreted by finishing cattle is in urine, primarily as urea. The urease enzyme (excreted by microorganism in the environment) breaks apart urea, releasing ammonia. Therefore, urinary N can contribute to the “pool” of ammonia volatilizing from open, outdoor feedlots. Since protein is a major contributor of nitrogen to the diet, the amount fed will influence ammonia release into the environment. As protein intake increases, more urea is excreted, leading to more ammonia production. To reduce environmental release of ammonia, cattle should be fed to only meet protein requirements. One method that will help decrease ammonia release is adoption of the metabolizable protein (MP) system proposed in the 1996 NRC. Balancing diets on an MP basis versus the crude protein (CP) system will allow for more accurate feeding of protein to cattle.

The MP system separates protein needs into degradable intake protein (DIP) needs by the rumen microorganisms and MP needs by the animal. The MP is the sum of digestible microbial true protein and digestible undegraded intake protein (UIP). The challenge is to just meet both requirements without overfeeding. The DIP requirement remains relatively constant as DIP is a function of diet composition and intake (Figure 1). The UIP requirement decreases as body weight increases due to a slightly larger supply of bacterial crude protein and from a lower requirement because the composition of gain is increasingly more fat and less lean (Figure 1). The overall MP requirement, expressed in grams per day, for the animal does not change significantly with time on feed. The reason is that the MP needed for gain decreases while the MP needed for maintenance increases. However, the MP requirement as a percentage of diet DM does significantly decrease because cattle generally consume greater amounts of DM as the feeding period progresses.

Figure 1 was developed with performance parameters from calf finishing research at Nebraska from 1994 to 1997 and assumes a 90%-concentrate dry-rolled corn-based diet with alfalfa as the roughage source. Because the type of protein needed (DIP vs UIP) to meet the MP requirement changes with days on feed, a single finishing diet fed throughout the feeding period is inadequate, being deficient up to body weight for which it was balanced and excessive from that point on. Therefore, a series of finishing diets fed in sequential order to meet, but not exceed both the DIP and UIP requirements throughout the feeding period (phase-feeding), should be beneficial. As a general rule, protein has been overfed during the finishing period and the majority is UIP. In corn-based diets, particularly dry-rolled corn (DRC) and steam-flaked corn (SFC), UIP is markedly overfed during the majority of the feeding period.

Four finishing trials, two with calves and two with yearlings, were conducted. These trials evaluated the impact of more accurate formulations (phase-fed) to conventional feeding practices on N losses from manure. The conventional diet was kept constant throughout the feeding period and contained 92.5% concentrate and 13.5% crude protein (CP). Phase-fed diets were also 92.5% concentrate and formulated to match DIP, UIP, and MP requirements throughout the feeding period. Phase-fed steers excreted less N than control steers (Table 3). Nitrogen excretion to the pen surface was reduced by 22% with yearlings, while total N lost was reduced by 32% for the phase-fed diets compared to the conventional diet. Nitrogen excretion to the pen surface was reduced by 13% with calves, while total N lost was reduced by 15% for the phase-fed diets compared to the conventional diet.  Differences in N losses between the yearlings and calves are likely due to cooler temperatures during the calf-finishing studies (November to May) compared to the yearling-finishing studies (May to October). Losses were greater (60 to 70%) in the summer than in the winter (40%). The amount of N lost in lb was markedly decreased by lowering N intake. Presumably, the observed decrease in N losses is directly related to decreasing the urinary N excreted.

Protein (DIP) Requirements of Feedlot Cattle

As previously mentioned, the majority of feedlot diets contain excess UIP from the basal (corn, roughage) ingredients. However, DIP or degradable protein is almost always deficient. Therefore, DIP sources such as urea are needed in feedlot diets. Surprisingly, few data exist that adequately address the optimum amount of DIP required. Another critical point is that the amount of DIP required is a function of microbial growth, not the animal. Therefore, DIP requirements change as diets change depending on the amount of energy used by rumen microbes. As energy use increases by microbes (i.e. more intense processing of corn), DIP requirements increase. The DIP requirements have been evaluated with DRC-based diets. The conclusion is the requirement for DIP is ~6.8% of diet DM.

Another trial evaluated DIP requirements with DRC, high-moisture corn (HMC), and SFC-based diets. As predicted, more intense corn processing increased DIP requirements. It appears DIP requirements are 6.3, 10.1, and ~9.0 for DRC, HMC, and SFC, respectively. Optimizing DIP in the diet appears to be necessary to optimize cattle performance. Presumably, inadequate DIP decreases energy utilization of finishing diets. The dilemma is that while protein may need to be supplemented in finishing diets (DIP sources), UIP is in excess. This illustrates how adopting the MP system will allow nutritionists to continue to “fine-tune” diets to optimize performance while meeting environmental goals.

Corn Processing and Feeding Byproducts

Corn byproducts such as wet corn gluten feed (WCGF) and wet distillers grains+solubles (WDG) are important feed products for many feedlots. The availability of these products is expected to increase. In a review of research studies, it was concluded that WCGF contains between 100 to 105% of the energy value of corn and is dependent on the ingredient composition, which varies from plant to plant. Based on a review of nine studies, WDG contains 124% of the energy value of corn when calculated as the difference between 17 and 40% of the diet. Clearly, feeding wet corn byproducts has positive attributes for cattle feeding.

One benefit relates to control of acidosis when starch from corn is fed to cattle. When corn is processed, total extent and rate of starch digestion increases. The rapid rate of starch digestion results in acid production from volatile fatty acids (VFA) in the rumen during fermentation. The large amount of acid can lower pH and cause cattle to go “off feed,” lower DM intake, reduce performance, and may even cause animal death. Feeding byproducts, which have the starch removed, can dilute the starch amount in the diet, change fermentation, and control these acidosis-related challenges when corn is fed. With that in mind, researchers hypothesized that more intense processing (such as HMC and SFC) may now be more manageable and lead to improved performance when byproducts are fed. Without byproducts and proper feeding management, more intense processing may not always improve efficiency. Because byproducts alleviate many of these challenges, intense processing may be very beneficial.

Over the past four years, researchers evaluated grain processing when byproducts were fed. The data illustrate performance is enhanced when corn is more intensely processed and byproducts are fed. Research currently points to intensely process corn (example, HMC at higher moistures) when feeding byproducts to allow for maximum starch digestion in the rumen, which leads to improved animal performance without acidosis concerns. However, most feedlots are currently feeding lower levels of byproducts than typically evaluated in these studies. If byproduct inclusion is too low, then the benefit of acidosis control may not be fully realized. I generally suggest 20 to 25% WDG or 25 to 35% WCGF both on a DM basis, assuming the economics of inclusion are favorable as an energy source.

 More information on Nebraska beef research can be viewed at www.ianr.unl.edu/pubs/beef/beefrpt.htm.

 References available upon request.