Fourteen multiparous midlactation dairy cows were used in a change-over experiment with 3 periods and 3 diets to evaluate the effects of fodder beets and potatoes on N metabolism, microbial protein production, and milk production. A basal ration of alfalfa/grass silage offered ad libitum, 1 kg of grass hay and 1 kg of heat-treated rapeseed cake was supplemented with 5 kg DM of either rolled barley/raw potatoes 80:20 (BAP), fodder beets/raw potatoes 80:20 (BEP) or rolled barley (BA). Urine and feces were collected quantitatively from 8 cows and ruminal samplings, and evacuations were performed on 4 cannulated cows. Intake and production did not differ between BAP and BA, but the BEP diet lowered intake of both silage and total ration by 0.9 kg DM. Daily yield of energy-corrected milk (ECM) was decreased by 1.7 and 2.3 kg compared with BAP and BA, respectively. Milk urea concentration was 1 mM lower with the BEP diet. The proportion of feed N recovered in milk was 20 to 21% for all diets. With the BEP diet, urinary N amount and proportion were reduced correspondingly to the lower total N intake. Fecal N amount remained unchanged, and hence nitrogen apparent digestibility decreased by 5 percentage units with the BEP diet. Microbial protein production, assessed by allantoin excretion, tended to be highest with the BAP diet. Acetate proportion of VFA was lowered by the BEP diet, while proportions of propionate and butyrate both tended to increase. Different fermentation patterns, probably related to differences in rumen microbiota, could explain why changes in energetic efficiency and milk composition reported in the literature did not occur in the actual experiment when roots replaced barley. Compared with barley, roots appeared to have a greater negative effect on silage intake in conjunction with a prewilted silage with high intake potential allowed ad libitum and this decreased milk production by a magnitude corresponding to the lower intake of ME.

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A number of factors affect predictions of rumen degradability of feedingstuffs when using thein saccotechnique (Lindberg, 1983). One factor which exerts a great influence is the bag cloth aperture (Lindberg & Knutson, 1981; Lindberg & Varvikko, 1982). Without doubt part of the difference in degradation between pore sizes can be explained by differences in particulate matter losses (Lindberg & Knutsson, 1981; Lindberg & Varvikko, 1982). It has, however, also been suggested that the differences between pore sizes are due to the combined effect of differences in liquid exchange between the bags and the rumen contents and on the selection of microbes entering the bags (Lindberg & Varvikko, 1982).

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