Influence of fasting and refeeding high forage and all-concentrate diets on beef heifers.

An 88% forage diet (F) and a corn-based all-concentrate diet (C) were fed to compare the influence of prior diet and rumen microbial inoculation on dietary intake and ruminal fermentation of heifers allowed to consume their diet ad libitum after a 4-d fast. Twelve ruminal-fistulated Angus heifers averaging 265 kg body weight were fed F ad libitum for 28 d, fasted for 4 d and refed F ad libitum for 21 d (period 1). After 21 d of refeeding, the heifers were fasted for 4 d and then fed C ad libitum for 60 d (period 2). After 60 d, the heifers were fasted for 4 d and refed C ad libitum for 35 d (period 3). The rumens of six heifers were inoculated (1) at the end of each fast with 2 l of ruminal fluid from two ruminal fistulated donor steers fed the corresponding postfast diet ad libitum. Six heifers were uninoculated (UI). The most severe digestive disturbance associated with any of the treatments was reduced feed intake. During the first week of refeeding the I heifers consumed more feed than UI heifers. Regardless of treatment 2 wk or more were required for heifers to return to before fast intake levels. If ruminal glucose accumulated to 10 mg/100 ml in the rumen for more than 7 h, then ruminal lactate accumulated to higher concentrations and for longer periods of time. Ruminal lactic acid accumulation is considered to be one of the main causes for reduced feed intake. When heifers were switched from F to C, they did not overeat to the point of causing complete feed refusals on subsequent days. However, even at low intakes, a reduction in feed intake occurred.