Evaluation of the use of chromic oxide to estimate the feed intake of dairy cows

In each of 3 trials, the digestible organic matter (DOM) intake of 32 cows was determined by direct measurement and by a chromic oxide technique. Each trial involved either 2 or 3 21-day periods during lactation. Half of the cows had high Breeding Index (BI) and half had low BI. Cows were individually housed and offered pasture either ad libitum or 75070 ad libitum. Dosing with chromic oxide (10g Cr 2 0 3 in a gelatin capsule) and sampling of faeces from the rectum occurred twice daily. Organic matter (OM) digestibility of the feed was obtained by in vitro determination, and both actual and estimated intakes are expressed as kg DOM per cow per week. Breeding Index had no significant effect on the relationship between estimated and actual intake. The chromic oxide technique consistently overestimated actual intake, but there were significant differences between weeks within and between periods in extent of overestimation. Mean overestimation was 14%. The variation in intake between cows of similar BI was greater for the chromic oxide technique. This resulted in the chromic oxide technique being less sensitive in detection of real differences in intake between the BI groups. Of the 33 comparisons in food intake of the 2 BI groups, significant differences were found on 31 and 18 occasions with the direct measurement and chromic oxide techniques respectively.