Performance of dairy cows offered isonitrogenous diets containing urea or fishmeal in early and in mid-lactation.

Sixteen Friesian cows were used in Expt 1 to measure the effect of substituting urea-N with fishmeal-N either in early lactation (Part 1) or in mid-lactation (Part 2). In Part 1 (days 15-84 of lactation) the major N constituent of the concentrate was urea (U), urea-N: fishmeal-N in the ratio 2:1 (UF) or 1:2 (FU), or fishmeal (F). In Part 2 (days 84-175 of lactation) only urea (UM) and fishmeal (FM) were used. Replacement of urea-N with fishmeal-N significantly (P less than 0.05) increased yield of milk protein both in early and in mid-lactation. At both stages of lactation the cows were, by calculation, in positive energy balance. In mid-lactation replacement of urea-N by fishmeal-N significantly depressed (P less than 0.001) the concentration of fat in milk. Blood urea concentration decreased with increasing fishmeal inclusion (P less than 0.05) from U to FU. In Expt 2 the diets used in Expt 1, Part 1, were offered at a maintenance level of feeding to non-pregnant, non-lactating heifers in a 4 X 4 Latin square design experiment. Digestibility of dry matter, organic matter and cell-wall constituents increased progressively (P less than 0.05) with the first two increments of fishmeal inclusion. A major effect of replacing urea-N with fishmeal-N was to increase digestible organic matter intake (DOMI) and differences in DOMI between treatments in Expt 1, Part 1, accounted for observed differences in performance.