EFFECT OF FEEDING CORN SILAGE BASED DIETS PREDICTED TO BE DEFICIENT IN EITHER RUMINAL NITROGEN OR METABOLIZABLE PROTEIN ON NITROGEN UTILIZATION AND EFFICIENCY OF USE IN LACTATING COWS

Due to concern over nitrogen (N) emissions, this study attempted to evaluate dietary approaches to reduce N excretion by dairy cattle. Knowledge about potential N sources that were either unaccounted for or under-predicted by CPM Dairy and the Dairy NRC (2001) was used to formulate rations that were much lower in crude protein (CP) than typically fed to dairy cattle but would potentially not decrease production. Specifically, the three diets fed were predicted to have: (1) positive rumen N and metabolizable protein (MP) balances (Diet P) (2) negative MP balance and positive rumen N balance (Diet N), or (3) negative rumen N balance but positive MP balance (Diet T) as predicted by CPM Dairy version 3. The objective of this experiment was to determine whether, and to what extent, the decrease in predicted ruminally available N and MP supply would affect milk production. Eighty-eight multiparous lactating Holstein cows (83 +/- 20 DIM), were blocked by average daily milk yield to 50 DIM and parity and assigned to three diets differing in N content or predicted rumen degradability of the feed N. The diets were formulated with CPM Dairy V3 using library values for all feeds except corn silage where actual chemical, digestibility, and degradation rate values were determined and used. The diets (DM basis) consisted of approximately 50% corn silage, 2% wheat straw and 48% of a diet specific ingredient mix and were formulated for 22.2 of kg DMI. Actual diet CP levels were 16.7, 14.2 and 14.3% for Diets P, N and T, respectively. The predicted CPM Dairy rumen N balance at the formulated DMI was 29 and 27 g for Diets P and N and negative 39 g/d for Diet T, whereas the predicted MP balance was 263 and negative 145 and 91 g/d for Diets P, N and T, respectively. Monensin was included in the diets at a formulated intake of approximately 300 mg per cow per d and somatotropin was administered per label. Actual DMI for cattle fed these treatments were 25.7, 25.5 and 24.2 kg/d for Diets P, N and T, respectively and were significantly lower for Diet T. Actual milk yield was 45.0, 42.6 and 43.3 kg/d and 3.5% FCM was 38.1, 36.5, and 36.4 kg/d for cows fed Diets P, N and T, respectively and was significantly lower for cows fed Diets N and T. Milk protein percent was not affected by diet; however, milk protein yield was significantly greater for cows fed Diet P due to the difference in milk yield. Plasma urea N concentrations were 11.31, 8.40 and 7.13 mg/dl for cows fed diets P, N and T, respectively and were different and paralleled the rumen ammonia levels of 8.32, 6.58 and 5.84 mg/dl. Milk fat depression (MFD) was observed in all cows and was not affected by treatment, and the average milk fat levels were 2.67, 2.68 and 2.54% for diets P, N and T, respectively. To determine if monensin was partially responsible for the MFD, monensin was removed from the diets of approximately half of the cows on treatment once they had finished the experimental period. Removal of monensin resulted in a 30% increase in milk fat percent, and milk protein content was not affected. Calculated milk N:intake N ratios for the three treatments were 0.31, 0.33 and 0.36 for Diets P, N and T respectively. The results of this study suggest that more productive N is available than currently predicted by either CPM Dairy and the Dairy NRC (2001). Understanding where these differences exist would allow for feeding less CP to dairy cattle and decreasing N emissions to the environment. It may also be a profitable strategy for dairy farmers, as they would be able to reduce their purchase of costly protein feeds, but that was not demonstrated in this study, primarily due to the severe milk fat depression that decreased the economic value of milk. However, ration cost was not a concern for this experiment, and that aspect can be considered when implementing feeding strategies stemming from this research