Effect of grain processing and fat supplementation on ruminal pH dynamics of cows grazing a tropical pasture
The objective of this experiment was to investigate the effects of grain processing and fat supplementation to dairy cows grazing a tropical pasture on ruminal pH dynamics and incidence of acidosis. Four rumen cannulated lactating cows (165 ± 32 DIM) were used in a Latin square design and subjected to the following treatments: a) fine ground corn without fat (FGC); b) fine ground corn + 400 g calcium salts of palm oil cow-1 d-1 (FGCCS); c) steam-flaked corn without fat (SFC); d) steam-flaked corn + 400 g of calcium salts of palm oil cow-1 d-1 (SFCCS). The fine ground corn had a particle size of 1.3 mm and steam-flaked corn a density of 360 g/L. Treatment periods were 24 d in length and cows grazed paddocks of Pennisetum purpureum and received 6.0 kg cow-1 d-1 (DM) of concentrate twice daily. Ruminal pH was continuously measured using a LRC pH measurement system. The indwelling electrode measured and recorded the ruminal pH every 10 minutes over the measurement period. Each electrode was standardized using pH 4.0 and 7.0 standards at the beginning and end of each period. The pH data were first summarized by day and then averaged across each measurement period as mean, maximum, and minimum pH and time in which pH was below 6.2, 6.0, and 5.8 as an index of severity of ruminal acidosis. Data were analyzed as repeated measures using a mixed model with period and animal as random effect and grain processing, fat supplementation and grain processing × fat supplementation as fixed effects. The means were compared by Tukey test at 5%. The mean pH, minimum pH and maximum pH were not affected by treatments. The mean pH was 6.63, 6.73, 6.68 and 6.64 for FGC, FGCCS, SFC and SFCCS, respectively. The minimum pH was 6.02, 6.10, 6.02 and 6.15 and the maximum pH was 7.04, 7.12, 7.02 and 7.06 for FGC, FGCCS, SFC and SFCCS, respectively. No pH measurements below 5.8 were observed. The time that the pH was below 6.0 was greater in steam-flaked compared with fine ground corn (5.2 vs. 2.6 h/d), and the time that pH was below 6.2 was also greater in steam-flaked diets (14.0 vs. 11.4 h/d). In conclusion, steam-flaked corn diets decreased the ruminal pH, but not below 5.8, which suggest no acidosis occurrence in dairy cows grazing a tropical pasture.
Keywords: steam-flaked corn, fat, tropical pasture