Effect of herbage mass and daily herbage allowance on rumen pH in dairy cows grazing ryegrass

PALLADINO, R.A., O’ DONOVAN, M.A., KENNY, D.A AND MC EVOY, M.

Irlanda

Congreso; Agriculture Research Forum, Ireland; 2008

Irish Grassland and Animal Production Association

Introduction It is commonly accepted that a pH under 5.8 in indoor feeding systems affects fiber digestion. Under grazing production systems, cows can often experience variations in pH in the range of 5.4 to 6.8 and this in turn could be affected by sward characteristics and herbage intake (Stockdale et al., 2005). Bargo et al (2002) reported lower rumen ph (6.40 vs 6.57) for dairy cows grazing at a high pasture allowance compared to a low pasture allowance. The aim of this study was to examine the effect of level of pre grazing herbage mass and daily herbage allowance on ruminal pH and milk production and composition in grazing dairy cows. Materials and Methods The experiment was carried out in Moorepark Research Centre, Fermoy, Co. Cork, Ireland (50°7´ N; 8°16´ W). Eight ruminally cannulated cows were allocated to four grazing treatments in a replicated (n = 2) 4x4 Latin square design and data were collected during 4 periods (one week period) from June to August of 2007. Animals grazed a perennial ryegrass dominated pasture. The treatments were high (HHM) and low (LHM) herbage mass (2200 and 1500 kg DM ha-1), at two levels of daily herbage mass (16 and 20 kg DM cow-1 day-1, LDA and HDA respectively). Rumen pH was measured during two consecutive days at 0700, 1100, 1500 and 2000 h in each period. Milk production was recorded daily during each period and milk fat and protein were determined. Rumen pH, milk production and milk composition were analyzed by ANOVA using PROC MIXED (SAS Institute, 2002). Time was included as an effect in the rumen ph data. Results and Discussion The herbage masses were 2580 ± 517.1 and 1726 ± 303.6 kg of DM ha-1 for HHM and LHM. The daily herbage allowances were 16 and 20 kg of DM cow-1 day-1 (HDA and LDA respectively). Across the treatments, mean rumen pH was 5.87 with a maximun of 7.00 and a minimun of 5.35. Rumen pH was lower for the treatment LHM – LDA (pH 5.77; P<0.05) and tended to be lower at LHM (5.82 vs 5.88 in the HHM; P=0.09).  Additionally, rumen pH was affected by time (P<0.01) decreasing along the day (all treatments dropped pH from 6.33 at 700 h to 5.55 at 2000 h) and by period (P<0.01), being the first period (June) which presented the highest pH values (data not shown). Milk yield, fat and protein content were not different for high or low herbage mass. However, milk yield was significantly higher (P<0.05) for high DHA (Table 1) as consequence of higher dry matter intake. Cows grazing the low DHA treatment recorded the lowest milk yield (-0.6l/day). Milk protein was numerically higher with the higher DHA treatments. Conclusion Wales et al. (2001) found that cows grazing perennial ryegrass at LDA ruminated less but the authors did not find differences in rumen pH. Moreover, Stockdale et al. (2005) found that cows grazing perennial ryegrass at LDA presented a higher rumen pH. Discrepancies between authors may be due to differences in pasture quality.  In this study cows grazing LHM in combination with LDA had the lowest pH and FCM in the current study. Additionally, cows across all treatments had low rumen pH levels (< 6.00) for long periods of the day. It appears that offering a low DHA with a leafy sward (low herbage mass) may reduce rumen pH levels to periodically low levels. In contrast to previous work Bargo et al (2003) offering a high daily herbage allowance and high quality sward did not reduce rumen pH.  Grazing dairy cows appear to have low levels of rumen ph, this study suggests that there no effect on milk production performance.