Determination of rumen fill, retention time and ruminal turnover rates of ingesta at different stages of lactation in dairy cows.

Rumen fill and turnover rates of liquid, grain, and hay in the gastrointestinal tract were measured using rare-earth elements as multiple markers in four rumen fistulated cows during the dry period and throughout lactation. Apparent digestibilities of dry matter, crude protein, neutral-detergent fiber and aciddetergent fiber were also measured. Hay to grain ratios of 82.5:17.5, 45:55, 57:43, and 67:33 (dry matter basis) were fed during the dry period and during weeks 0 to 12, 13 to 24, and 25 to 44 of lactation, respectively. Apparent digestibilities of DM, CP, ADF and NDF were 64.1, 67.2, 68.3, 66.4; 77.2, 70.9, 73.0, 73.7; 41.0, 34.5, 35.6, 38.2;and 51.6, 36.7, 44.0 and 47.1 for the four phases of the experiment. For the dry period, 0 to 12, 13 to 24, and 25 to 44 weeks of lactation, rumen dry matter and liquid content (kg) were 9.6, 62.1; 16.0, 79.4; 16.0, 89.3; and 15.7 and 86.0. Mean ruminal turnover rates across periods for liquid, grain and hay were 8.1, 4.4, and 3.9% per hour. Experimental period had no effect on turnover rate. However, stage of lactation, level of intake and hay:grain ratio all changed with each experimental period. The greatest difference in turnover rate (twofold) was among individual cows. There were small differences noted among stage of lactation periods in total mean retention time of liquid, grain and bay fractions