EFFECT OF EXTENDED UNDERFEEDING ON DIGESTION AND NITROGEN-BALANCE IN NONLACTATING COWS

The effect of extended underfeeding on digestion was studied in dairy cows. Four nonlactating cows (BW = 747 kg) were first fed a forage-bas ed diet at a level above energy maintenance requirements for 7 wk (9.4 kg DM/d) then were restricted at a low level of intake of the same di et for 5 mo (5.2 kg DM/d), then refed at the first level. Digestion me asurements were made before and after the underfeeding period, and at 1, 5, 9, and 19 wk of underfeeding. Organic matter digestibility decre ased with underfeeding then increased (62.7, 56.2, and 61.5% before, 1 wk after, and 19 wk after underfeeding, respectively). Differences in ruminal apparent OM digestion were nonsignificant (P > .05). This lac k of difference was related to the absence of variation of ruminal par ticle passage rate and DM degradability measured in situ. However, the decrease in feed intake decreased ruminal (P < .01) and intestinal (P < .05) liquid dilution rates, ruminal DM pool size and DM content (P < .01), and protozoa concentration (P < .01). The decrease (P < .01) i n N retention with underfeeding was followed by an increase (P < .05) during the underfeeding period, due to a decrease in fecal and urine N losses. This experiment has shown an unusual and temporary response o f digestion to underfeeding. Knowledge of adaptation of digestion to l ow intakes needs to be improved.