Investigation of aerobic stability in extruded silages

Abstract: In dairy farming, feed cost represents the highest single expense. To maintain a high milk production, feed intake must be maintained, and forage is the major feed for dairy cows in many parts of the world. However, due to constraint on feed intake, forage alone cannot meet the demand of a high producing dairy cow because of its large particle size and high fibre content. Feed processing and preservation aiming at particle size reduction has shown promising results in increasing intake. Extrusion is a mechanical process where screws shear and mix lignocellulosic material for particle size reduction and cell wall disintegration. During extrusion temperature rise in the processed biomass. Rise in temperature and mixing are potential risk factors for re-duced aerobic stability in silage. It is well known that aerobic deterioration is an im-portant source of feed waste in silage related products. It is therefore necessary to study and gain insight in how to avoid aerobic deterioration in silages. The aim of the present study was therefore to investigate the effect of extrusion on aerobic stability. Silage samples were collected from a bunker silos at Lövsta, (SLU research station). A mixture of Timothy grass/red clover and whole crop barley (WCB) were processed using a Bio-extruder in two Trials. In Trial 1, silage of good hygienic quality used while in Trial 2 silage of poorer hygienic quality was used. A study was conducted using the aerobic stability test unit at the forage lab, SLU. The unit was connected to a computer for recording temperature every two hours until temperature in aerated silage had increased by +5o C above ambient temperature. This experiment employed a factorial design with two factors to consider; types of silage and treatment of silage. In Trial 1 there were two types of silages (Timothy grass/red clover and WCB) col-lected from the centre of the silo, and three treatments (extruded silage that was kept warm after extrusion, extruded silage that was cooled off and untreated silage as con-trol). In Trial 2, the same types of silages as in Trial 1 was used but collected from the periphery of the silo and for the grass/clover even from a different silo. In Trial 2, the same treatments as in Trial 1 were used, but a fourth treatment was added (extruded silage that was kept warm with inclusion of silage additive). Samples were collected and processed on 4 consecutive days in each trial resulting in 24 samples in Trial 1 and 32 samples in Trial 2. These samples were incubated in the aerobic sta-bility unit and temperature of each sample was recorded continously over 7 days. After incubation, the fermentation residues were used to produce juice for pH meas-urement. In Trial 1, all incubated silages displayed a similar pattern in the development of temperature over time. That means temperature of the aerated silages did not in-creased by +5° C until measurement stopped at 164 hours. In Trial 2, silages dis-played different patterns in the increase in temperature over time (hours). Generally, in Trial 2 it took less time for temperature in the silage to rise by +5° C. This differ-ence between the trials indicates that the silage in Trial 1 was of better quality (more stable) compared to Trial 2. In Trial 1, results showed that the effects of silage type on temperature and pH before aerobic stability test were significant (P<0.001) meaning that temperature was higher in WCB and pH lower, compared with grass/clover silage. The treatment effects were significant on temperature (p<0.001) with increased temperature after extrusion. The interaction effect between treatments and silage type was observed to be significant only on temperature (p = 0.01) with a greater difference in temperature before and after extrusion in grass/clover silage. In Trial 2, the results showed that the effect of silage type was significant on pH before aerobic stability test (higher for WCB, p = 0.007) and after aerobic stability test (higher for grass/clover, p = 0.017), as well as in number of yeast CFU (greater for WCB, p = 0.003) and moulds CFU (greater for grass/clover. p = 0.009). Signifi-cant treatment effect was observed for yeast CFU (reduced after treatment, p = 0.01). The interaction effect between treatments and silage type was significant only on temperature (p = 0.015) with a greater increase in WCB compared to grass/clover. The findings of this study have shown that the process of extrusion with different silage treatments did not have a significant effect on the aerobic stability of the silage types. However, pH, temperature, microbes (yeasts and moulds) of the extruded si-lage were found to be important parameters determining silage quality.