Technical note: A comparison of methods to determine pH in silages

Abstract

The techniques used to assess pH in silages vary greatly. The aim of this study was to evaluate the effects of water-to-sample ratio, extraction procedure, and standing time on pH determination. Silage samples (n = 20 for each silage) were chosen to represent diverse crops (corn, elephant grass, sugarcane, and forage peanut) to have a varied ensilability index and thus a wide range in final pH. Three water-to-sample ratios and 2 extraction procedures were used to measure pH at 0, 5, 10, 15, 30, 60, and 120 min of standing time. The ratios (undried silage to water) were 9:60, 25:100, and 30:270. The samples with the first 2 ratios were manually extracted, using a glass beaker and a glass stirring rod. The samples with the 30:270 ratio were extracted by using a stomacher blender for 4 min at 200 rpm. An electrode was used to perform pH readings. Dry matter (DM), water-soluble carbohydrates, and lactic acid concentrations were determined. The experimental design was completely randomized using a mixed repeated-measures model. Mean separation was performed using the Tukey test at P < 0.10 using the MIXED procedure of SAS (SAS Institute Inc., Cary, NC). The DM concentrations ranged from 24.5 to 40.2, 15.8 to 25.9, 26.9 to 30.6, and 17.8 to 21.4% for corn, elephant grass, sugarcane, and forage peanut silages, respectively. The lactic acid concentrations ranged from 2.9 to 10.1, 1.8 to 4.4, 0.7 to 11.4, and 0.3 to 1.4% of DM for corn, elephant grass, sugarcane, and forage peanut silages, respectively. The pH values measured by the 9:60 method were greater than other techniques at any standing times. The pH values from the 25:100 and 30:270 methods did not differ for elephant grass and forage peanut silages at any standing times. However, the 30:270 method had greater pH values for corn and sugarcane silages than the 25:100 technique at any standing times. The pH values measured by the 30:270 method did not vary for any of the silages according to standing time. Nevertheless, the pH values of the 25:100 method were greater from 0 to 10 min than for other standing times for all silages. Thus, for this method, there was pH stabilization from 15 min of standing time on. Overall, the 25:100 and 30:270 methods are recommended for assessment of silage pH. Fifteen minutes of standing time should be used for the 25:100 method but the pH reading can be performed immediately after mixing for the 30:270 technique.