Forage Residue Filtration, In Vitro Forage Digestion, and Elemental Solution Concentrations as Influenced by Soil Type and Concentration

Ruminant animals may ingest large quantities of soil at certain times during the grazing season, but the effect of ingested soil on the mineral nutrition of ruminants is not clear. Our objectives were to determine the effects of soil on elemental concentrations in solution in an active in vitro system to estimate matter digestibility (IVDMD), rate and extent of cell wall digestion, and neutral detergent fiber (NDF) concentrations. Elemental concentrations were determined with an emission spectrophotometer immediately after the in vitro incubation of soil (0 to 300 g kg⁻¹ forage) with annual ryegrass (Lolium multiflorum Lam.) for 6 to 48 h. The effect of soil on IVDMD was studied by adding three levels of soil (0, 75, and 150 g kg⁻¹) to ryegrass samples prior to digestion. Four levels of soil (0, 100, 200, and 300 g kg⁻¹) were combined with ryegrass samples to determine the effect of soil on NDF residue filtration. Increased soil levels in annual ryegrass forage samples resulted in linear increases in solution concentrations of Ca, Mg, Fe, Mn, and Zn, but no measurable Al or Ti in solution. Soil type had only small influences on Mg and Ca concentration in solution, with values ranging from 41.9 to 42.9 and 41.7 to 47.4 mg kg⁻¹, respectively. Iron concentrations among soil types ranged from 0.93 to 1.81 mg kg⁻¹, while Mn concentrations among soil types varied by nearly nine-fold, from 0.25 to 2.24 mg kg⁻¹. When soil was not present in the in vitro system, Ca, Mg, and Fe concentrations increased linearly with increased incubation time. However, solution concentrations of each of these elements responded differently to soil additions with increased incubation time. Soil had no apparent effect on the in vitro digestion process itself, but may lead to decreases in IVDMD values by interfering with filtration of sample residues. There was a linear increase in NDF values with increased soil additions to the forage samples, after correction with soil blanks. Neutral detergent fiber was significantly affected (P < 0.05) by filtration method, with mean values of 491 g kg⁻¹ for sintered glass and 475 g kg⁻¹ for glass wool plus sintered glass. The effect of soil contamination on forage quality analyses, resulting from inadequate filtration through sintered glass filters, cannot be overcome by expressing values on an ash-free basis.