Production, rumen fermentation, apparent digestibility, and nitrogen balance when fed dried corn distillers grains versus soybean protein as a source of rumen undegraded protein for lactating dairy cows

Past research led to the development of the metabolizable protein (MP) system. This system introduced the terms rumen degraded protein (RDP) and rumen undegraded protein (RUP) to recognize the different fates of protein within the ruminant animal and to separate the protein requirements of the microbial organisms of the rumen from those of the animal itself. The advantage of this system is the improved ability to more precisely determine the protein requirements of the lactating dairy cow. Individual nutrients vital to maintaining health and production can be under or overfed, such is the case with protein.;For a high producing cow, additional RUP in the diet may be required. Past research has compared dried corn distillers grains with solubles (DDGS) as a RUP supplement to soybean meal (SBM). Distillers dried grains with solubles has a greater concentration of RUP than SBM as indicated by the NRC (2001). Distillers dried grains with solubles has been shown to be low in lysine, which may cause a decrease in the synthesis of milk protein compared with SBM. Three studies were conducted: (1) to investigate the attributes of DDGS and its effects on production, digestion, and nutrient excretion and (2) to determine how DDGS manufactured with modern technologies can be fed with respect to the nutritional needs of lactating dairy cows when compared to feeding soy proteins.;The first study investigated the effect of increasing RUP by comparing DDGS with soy protein as a protein source in lactation diets. Dry matter intake and feed efficiency were not affected by source or concentration of RUP. Milk production, milk protein percentage and yield, and milk fat yield increased (P < 0.05) for cows fed diets containing DDGS compared with diets containing soy protein. Diets with the greatest concentration of RUP increased MUN (P < 0.01) regardless of protein source. The second part was an in situ experiment where protein disappearance was estimated in different concentrate sources including DDGS, corn, solvent extracted soybean meal (44% CP), expeller soybean meal (SoyPlus), extruded soybeans, and soyhulls.;The second study investigated the effect of increasing RUP by comparing DDGS with soy protein on rumen fermentation, digestibility, and nitrogen balance. Ruminal pH decreased (P = 0.01) with the addition of supplemental dietary protein. Ruminal ammonia concentrations increased (P < 0.01) with increasing concentrations of RUP. Total amount of VFA in the rumen was greater (P = 0.04) for diets not containing DDGS. Nitrogen intake increased (P < 0.01) with increasing RUP and tended to increase (P = 0.07) when DDGS replaced soy. Total urinary N excretion was not affected by RUP source or concentration; however, urinary urea excretion increased (P < 0.01) with increasing RUP. Milk N excretion increased (P < 0.01) when diets containing DDGS were fed. Milk urea N increased (P < 0.01) with increasing concentrations of dietary RUP. The apparent total tract digestibility of DM was greater (P = 0.05) when cows were fed diets not containing DDGS.;The third study compared lactation performance and nutrient digestion of cows fed diets containing protein sources derived only from soy products, only DDGS, or a combination of soy and DDGS protein. Dry matter intake, milk yield, the yield of milk components, and milk urea N were not significantly affected by dietary treatment. Ruminal pH decreased linearly (P < 0.01) with increasing dietary DDGS. Ruminal acetate proportions decreased (P < 0.01) and propionate proportions increased ( P < 0.01) with increasing dietary concentrations of DDGS. The apparent total tract nutrient digestibility was not significantly affected by dietary treatment. The excretion of urinary N and urinary urea N tended to increase with increasing DDGS.;This research is unique because, unlike previous work, it investigated the effects of feeding DDGS compared with a mixture of soy proteins rather than any single soy product. This research also compared the effects of feeding DDGS with a mix of soy products on nutrient digestion, fermentation, and N excretion. Very little, if any, previous research investigating these parameters exists. The results of these three studies suggest that replacing a combination of soy products with DDGS may not negatively impact the production of milk or milk components. Increasing the CP concentration of the diet may improve the production of milk and milk components regardless of protein source. Altering the dietary RUP source changed ruminal pH, and concentrations of NH3 and VFA's, which may be an indicator of changing fermentation paterns.;Keywords: dried distillers grains with solubles, ruminally undegraded protein, dairy cow.