Methods for Measuring Moisture Content of Grains and Implications for Research and Industry

Several methods were used to measure “weight loss on drying” and moisture content of 120 samples freshly harvested corn grain containing from 12 to 45% moisture (10 different hybrids; 6 harvest dates; grain separated from the butt and tip section of multiple ears). Moisture content varied with measurement method. Compared with direct measurement of water content by a chemical reaction (the Karl Fischer method), drying whole kernel corn samples for 144 h at 62 C failed to remove approximately 3% of the water, especially from drier samples. Weight loss during oven drying at 105 C for 144 h matched Karl Fischer estimates of moisture content very closely. Rapid moisture measurements by near infrared procedures were almost as accurate as 105 C drying. Capacitance-conductance measurements were slightly less accurate than 105 C estimates, particularly for samples that contained more moisture. Compared with moisture content of kernels obtained from the tip half of the ear, kernels from the butt half of the ear contained up to 5% more moisture. Compared with the samples tested, weight loss on drying would be greater from samples that contain volatile compounds derived from microbial fermentation (high moisture corn; silage; wet distillers grains). For such samples, more accurate prediction of caloric value would be obtained by measuring volatiles and water content separately by appropriate procedures. If properly calibrated, near infrared techniques hold promise for obtaining such measurements. INTRODUCTION Moisture content of a feed usually is calculated as the weight lost by material during application of heat to a sample. Indeed, Thiex and Richardson (2003) proposed that the term “weight loss on drying” should be substituted for the term “moisture” when discussing feeds. Hence, the term “dry matter” probably should be replaced with “100 minus weight loss on drying.” This revised terminology is based primarily on the observation that numerous compounds (organic acids, ethanol, ammonia) in addition to water become volatile and are lost when a feed is heated. But in addition, heating can result in chemical reactions that release water. Of primary concern in the grain trade is the loss of volatile compounds during drying of certain feedstuffs, e.g., silages, high moisture corn, wet distillers products. Loss of volatiles results in an underestimate of dry matter content of the feedstuff; in turn, this inflates the efficiency of feed use (gain to fed dry matter ratio) as an estimate of the true energetic efficiency of fermented crops and wet distillers products. If weight lost during fermentation is combined with the improved gain to feed dry matter ratio, these two errors will partially cancel, but very seldom are these two measurements both measured and combined. Consequently, if one inhibits fermentation so that a fermented product contains a lower concentration of volatile compounds, dry matter recovery increases. Further, apparent digestibility increases because the dry matter fed has fewer volatile compounds that are lost during drying. Perkins (1943) noted that 4 to 7% more dry matter was lost from corn silage by 100 C drying than by toluene distillation. Fox and Fenderson (1978) found that try DM of corn silage was underestimated by 8% and 11% by oven drying at 60 and 100 C, respectively. As discussed by Thiex and Richardson (2003), chemical water content should be measured by the Karl Fischer titration method. By this procedure, the extent of a chemical reaction quantifies the amount of water present. Being considerably more complex, time-consuming, and expensive than drying samples with a forced air, vacuum, or microwave oven, the Karl Fischer titration seldom is used for routine measurement of moisture content. However, even the standard operating procedures for moisture measurement by oven drying will vary with feed type and among laboratories. Standard procedures include vacuum oven drying at 95 to 100 C (AOAC 934.01), forced air oven drying at 103-104 C for 5 h (AOAC 935.29), drying at 135 C for 2 h (AOAC 930.15), and 105 C for 3 h (NFTA 2.2.2.5). Drying samples above 65 C without aeration for rapid removal of moisture will cause formation of Maillard products that will be assayed erroneously as lignin (Van Soest, 1982). As a result, some laboratories use a two-stage procedure; to