In vitro fermentation of different ratios of alfalfa and starch or inulin incubated with an equine faecal inoculum
Introduction
Diets high in fibre are required for normal gastrointestinal function in horses (Harris et al., 2016). Meanwhile, diets high in non-structural carbohydrates (NSC), such as starch and fructan, are known to elicit microbial disturbances often leading to clinical disorders like laminitis and colic (Hudson et al., 2001, Bailey et al., 2004).
In diet-induced laminitis lactic acidosis has been described as a consistent characteristic (Garner et al., 1977). Diet has been linked to laminitis for a long time; in fact diet-induced laminitis was first described around 350 BCE as barley disease in reference to the development of laminitis after the consumption of excessive amounts of cereal grain (starch overload). High-starch diets have been associated with hindgut acidosis in horses, and grain overload is often implicated in the onset of this debilitating condition (Garner et al., 1975). In pasture-induced laminitis it is proposed that it is the ingestion of high levels of grass fructan that elicits the onset of laminitis in this instance.
The increase in orchard grass hay intake (from 0.5 to 1.0 kg hay/100 kg BW/day) was shown to cause a dose-dependent elevation of the faecal pH when horses were fed a constant amount of crushed oats (1 kg/100 kg BW/day) (Zeyner et al., 2004). However, another study suggested that the pH of the gastric juice was significantly higher in samples obtained 2 through 5 h after feeding when horses were fed alfalfa hay-grain diet, compared with values when horses were fed bromegrass hay (Nadeau et al., 2000), indicating that forage type is also important. Alfalfa is a legume known to have a high buffering capacity due to its high organic acid and protein levels (Merry et al., 2000). Moreover, alfalfa has been reported to negate the deleterious effects that high NSC diets can have on gastric pH due to protein's buffering capacity which neutralises the ability of the short chain fatty acids (SCFA) to cause acid injury and ulceration. (Nadeau et al., 1998). However, little work has been done to assess the effect of alfalfa on the hindgut of horses fed diets high in NSC. This high-buffering capacity of alfalfa may be exploited to counter the acidotic effects associated with high NSC intakes on the large intestinal environment of the horse; however, no information currently exists to support this premise.
The aim of this study was to assess the effect of substituting starch or inulin with increasing ratios of chopped or ground HTDA used as substrates for in vitro fermentation with an equine faecal inoculum.
Section snippets
Experiment 1: Ground alfalfa and starch or inulin
Three identical series of 160 ml serum bottles were used to assess the fermentation characteristics of ground (to pass through a 1 mm dry mesh screen) high-temperature dried alfalfa (GA) and starch (S: Fisher Scientific, Loughborough, UK: s/7960/50) or inulin (I: Fisher Scientific, Loughborough, UK: 9005-80-5: from Dahlia tubers) added to each bottle in the following ratios; 100:0, 80:20, 60:40, 40:60 and 20:80 to give a total sample mass of 1 g (±0.5%) per bottle.
A further set of bottles
Experiment 1: Ground alfalfa and starch or inulin
Mathematical analysis of gas production curves revealed an interaction (P<0.001) between treatment (+/- SFD) and substrate for all of the in vitro parameters measured, with the exception DML. GP reduced as the ratio of alfalfa to starch or inulin increased (P<0.001: Table 1, Table 2), with values lower overall in bottles containing +SFD alfalfa (P<0.001). MFR was also affected by the ratios of alfalfa and starch or inulin as substrates, as well as SFD treatment. MFR values were, in general,
Discussion
In equids, diets containing high levels of NSC have been associated with the onset of hindgut acidosis, laminitis and colic (Carroll et al., 1987, Clarke et al., 1990, Garner et al., 1977, Rowe et al., 1994). High levels of starch entering the hindgut environments have been seen to elicit detrimental changes to the gastrointestinal tract (Garner et al., 1975) and similar effects have been seen with high levels of fructan (inulin) entering the large intestine of the horse (Pollitt, 2002)
Conclusion
The substitution of inulin/starch with HTDA in vitro appeared to buffer the deleterious effects of the S/I on in vitro fermentation; the substitution of starch/inulin with 40% HTDA maintained pH values within physiological levels encountered in the large intestine of the healthy horse. However, the pH measurements in this in vitro system may not entirely reflect the in vivo situation since the culture medium used in the gas production method is heavily buffered (Theodorou et al., 1994) and due
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2020, AnimalCitation Excerpt :The pH in vitro did not reflect differences between substrates in a similar way to that observed in the in vivo results. Differences between pH levels in vivo and in vitro may also be due to the intestinal absorption of fermentative end products, which takes place in live animal but not in vitro (Garber et al., 2017). Hence, there is a need to assess the in vitro results alongside the in vivo results.