Effects of plant vegetative stage and field drying time on chemical composition and in vitro ruminal degradation of forage soybean silage

Abstract

The aim of this study was to assess the chemical composition and rumen degradability of ensiled soybean forage harvested at three plant vegetative stages (i.e., R4, R5, R6) and wilted for 20 or 28h. The wilted forages were chopped from 1 to 2cm and then manually pressed into 6kg bags of fresh material (two bags for each combination of maturity stage and wilting duration) prior to storage under anaerobic condition for 90 days. The soybean silages were analysed for fermentative parameters and manually separated into leaves, stalks and pods. Whole forage and separated plant components were chemically analysed and the in vitro neutral detergent fibre (NDF) ruminal degradation (NDFD) was determined. Only whole plant silages were assessed for in vitro gas production (GP) and in vitro crude protein degradability (CPD). The dry matter (DM) content of the ensiled whole plants increased (P=0.005) from 454 to 485 and 518g/kg from the R4 to R5 and R6 vegetative stages and was higher for forages wilted at 28h than those at 20h (528 vs 444g/kg, P<0.001). Advancing plant maturity increased (P<0.001) the crude protein (CP) and ether extract (EE) content from 164 to 199 and from 18 to 53g/kg DM, respectively, but reduced ash (P=0.011) from 71 to 63g/kg DM. The NDFD increased with plant maturity from 0.319 to 0.465 (P<0.001). The GP did not differ among maturity stages of forages or wilting lengths, while CPD increased from 0.391 to 0.548 (P<0.001) with advancing maturity and slightly decreased with the longer wilting duration (P<0.008). The DM content of the pods and stalks increased (P<0.01) with the vegetative stage and CP content increased in pods and decreased in leaves. Results indicate that harvesting soybean forage at an advanced maturity stage (e.g., from R4 to R6) greatly increases the protein, the fat and the degradable NDF contents. Moreover, the wilting soybean forage has to be targeted to achieve a DM content of ensiled forage of about 440g/kg (between about 410 and 480g/kg, according to the maturity stage), and a further wilting determines no fermentative improvements at ensiling or relevant modification of silage nutritional contents.