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,3
* Department of Dairy Science, University of Wisconsin, Madison 53706
Agricultural Research Service, USDA US Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706
3 Corresponding author: gbroderi{at}wisc.edu
Ten lactating Holstein cows fitted with ruminal cannulas that were part of a larger feeding trial were blocked by days in milk into 2 groups and then randomly assigned to 1 of 2 incomplete 5 x 5 Latin squares. Diets contained [dry matter (DM) basis] 25% alfalfa silage, 25% corn silage, and 50% concentrate. Rolled high-moisture shelled corn was replaced with solvent-extracted soybean meal to increase crude protein (CP) from 13.5% to 15.0, 16.5, 17.9, and 19.4% of DM. Each of the 4 experimental periods lasted 28 d with data and sample collection performed during the last 8 d. Digesta samples were collected from the omasum to quantify the ruminal outflow of different N fractions. Intake of DM was not affected but showed a quadratic trend with maxima of 23.9 kg/d at 16.5% CP. Ruminal outflow of total bacterial nonammonia N (NAN) was not different among diets but a significant linear effect of dietary CP was detected for this variable. Bacterial efficiency (g of total bacterial NAN flow/kg of organic matter truly digested in the rumen) and omasal flows of dietary NAN and total NAN also showed positive linear responses to dietary CP. Total NAN flow increased from 574 g/d at 13.5% CP to 688 g/d at 16.5% CP but did not increase further with the feeding of more CP. Under the conditions of this study, 16.5% of dietary CP appeared to be sufficient for maximal ruminal outflow of total bacterial NAN and total NAN.
Key Words: dietary crude protein • bacterial protein formation • omasal N flow
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