The effect of daily herbage allowance, herbage mass and animal factors upon herbage intake by grazing dairy cows

Abstract

The effects of daily herbage allowance (DHA = herbage mass [HM] x daily offered area [DOA]) and cow characteristics upon herbage intake at grazing were assessed in two experiments. In experiment 1, two DHA (low and medium, 19 and 26 kg organic matter [OM]/cow/day) were compared in a continuous design using two groups of five cows. In experiment 2, three DHA (low, medium and high, 19, 29 and 46 kg OM/cow/day) were compared in a 3 x 3 latin square design using three groups of five cows. Mid-lactating cows (six first lactation per trial) were used. Fat-corrected milk at turnout (FCMt) ranged between 17 to 35 kg and live weight (LW) from 510 to 680 kg. Cows strip-grazed plots of vegetative Lolium perenne and did not receive concentrates. Herbage mass cut to ground level (HM) ranged from 3.5 to 7.1 t OM/ha. In experiment 1, herbage organic matter intake (HOMI) (13.5 vs 14.9 kg/day) and FCM yield (20.6 vs 22.0 kg/day) tended to increase from low to medium DHA but differences were not significant. In experiment 2, HOMI increased in a quadratic manner (13.8, 16.2 and 16.7 kg/day) and FCM increased linearly (P < 0.01) with DHA (20.4, 21.7 and 23.0 kg/day for low, medium and high DHA, respectively). In both experiments, HOMI was consistently lower in first lactation compared to adult cows and large between-cow variations within lactation were noted. From the pooled data, HOMI was related to DHA and cow characteristics: HOMI = 7.9 - 98 DHA-1 + 0.264 FCMt + 0.0073 LW (n = 95, R2 = 0.60, rsd = 1.77 kg). However, splitting DHA into its two components accounted for more of the variance in HOMI: HOMI = -20.4 -115 DOA-1 + 9.63 HM - 0.873 HM2 + 0.266 FCMt + 0.0095 LW (R2 = 0.70, rsd = 1.56 kg). These relationships showed that HOMI was affected by DHA but the original sward herbage mass/structure does have an independant effect in regulating intake. Moreover, voluntary intake increases with the potential of milk yield and this increase could account for the two-thirds of the supplementary energy requirements.