Full Lactation Response on Restricted vs. ad Libitum Roughage Diets with Liberal Concentrate Feeding

The effect of replacing one-half of" the expected roughage consumption with equivalent TDN from concentrates was determined in full lactations with 20 pairs of high producing cows. Control cows (Group I) were fed alfalfa hay and corn ~ilage ad libitum plus concentrates at an average ratio to FC~[ of ]:2.4. Paired cows restricted on roughage (Group II) were allowed roughage at the rate of 3.0 kg hay and 7.5 kg silage per 500 kg body weight plus 3.8 kg concentrates, in addition to the concentrate allowance for FCM at 1:2.4. In 294-day lactations Group I averaged 6,349 kg FC.~I and Group II, 6,137 kg, a nonsignificant difference (P > .10). Fat tests averaged 4.01 (I) and 3.89% (II), with the difference between groups from the 12th to the 22rid wk statistically significant (P < .01). Differences in body weight gain, total dry matter consumption, estimated TDN eonsmnption, solids-not-fat tests, and milk fat yield were not significant. The average TDN/FCS~ ratio was significantly (P < .05) higher for Group II, 0.33, than for Group I, 0.31. Indications were that the Group II diet, which averaged 67.6% concentrate, was used less efficiently than the Group 1 diet, which averaged 51.1% eoneentrate.