SITE AND EXTENT OF MINERAL ABSORPTION IN LACTATING COWS FED WHOLE-CROP CEREAL GRAIN SILAGE OR ALFALFA SILAGE

The site of apparent absorption of Na, K, Ca, P, Mg, and S in lactatin g dairy cows fed whole-crop barley, oats, triticale, or alfalfa silage s was studied. Eight ruminally and duodenally cannulated Holstein cows with ad libitum access to a total mixed diet were assigned to one of four treatments as a replicated 4 x 4 Latin square design. All diets c ontained the same concentrate (50%, DM basis) plus one experimental si lage. The concentrations of Na, K, Ca, P, Mg, and S in the concentrate were .84, .71, .85, .78, .27, and .38%, respectively. Dry matter inta ke was higher (P < .05) for cows fed alfalfa and barley silages than f or cows fed oats and triticale silages (19.6, 18.6, 16.7, and 17.2 kg/ d, respectively). Alfalfa silage contained a higher concentration of a ll minerals studied than the cereal silages, except Na. Sodium flow at the duodenum was substantially greater than dietary intake and appare nt total tract digestibilities ranged between 74.5 and 85.2%. Secretio n of P in the forestomach ranged from 34 to 61 g/d and the major site of absorption was in the intestine. The correlation between P intake a nd fecal excretion of P was significant (P < .001, r(2) = .39) and lin ear. Potassium absorption occurred before the duodenum and in the inte stine. Apparent digestibilities of K were lower for cereal silages (ra nge 74.0 to 82.9%) than for alfalfa silage (88.7%). Apparent total tra ct digestibilities of Ca (28 to 32%), P (27 to 34%), and Mg (17 to 24% ) were similar for all diets so that Ca, P, and Mg absorption (g/d) re flected dietary Ca, P, and Mg levels. Data indicate that forage source can influence the site and extent of absorption, fecal output, and ap parent digestibilities of macrominerals.