Starch is the storage form of plant nutrients. Grains contain a large amount of starch, which generally accounts for 20% ~ 40% of the dry matter of the diet. Most starches in grains are found in the endosperm, which contains about 70 %. Starch is a chain polymer composed of many glucose molecules linked together, divided into amylose and amylopectin. The average molecular weight of amylose was 1×105 to 9×106 (Dobladomaldonado et al.,2017), and the average molecular weight of amylose was 1×107 to 1×109, being some of the largest polymers in nature (Tester et al.,2004). During the formation of starch granules, amylose penetrates among amylopectin, forming dense hydrogen bonds, preventing the invasion of digestive enzymes, and reducing starch fermentation (Gómez et al.,2016). The higher the ratio of amylose to amylopectin, the lower the digestibility of starch (Svihus et al.,2005). Therefore, an increase in amylose ratio will make starch degradation more difficult.
The steam sheet is a common heat treatment for grain. The process not only utilizing the grain, but also changes its crystal. The structure makes cereal starches easier to digest in the small intestine. Perez et al.(2010) believed that compared with crushing treatment, steam compression treatment could achieve complete utilizing and increase the utilizing rate of starch. Meyer et al.(1993) reported the effects of varying grain processing methods on the pre-caecal starch digestion of corn in horses. The results showed that whole corn, cracked corn, ground corn, and popped corn pre-caecal digestibility were 28.9%, 29.9%, 45.6% and 90.1%, respectively. A study by Mohsen et al.(2020) showed that the average daily gain and feed efficiency were improved and final body weight was higher in steam-flaked corn grain-fed calves.
In our study, we showed that 2 g starch/kg BW day (DM basis) could improve the body weight gain of foals. In addition to the different sources of starch, the composition and nutritional composition of other feeds were the same, as well as the feed processing method and the breeding and management conditions of foals. Thus, differences in dietary nutritional effects can be attributed to the source of starch. Among the three kinds of starches, the composition of starches is different, among which the proportion of amylose and amylopectin was different. A study showed that the ratio of amylose to amylopectin could effect growth performance and pork quality in pigs. Potter et al.(1992) suggested that the upper limit of starch digestion in the equine small intestine was between 3.5 and 4 g starch/kg body weight per feeding. However, others have suggested that in order to limit starch bypass to the large intestine that starch intake should not exceed 2 g/kg body weight per meal (Meyer et al.,1995; Kienzle et al.,1992). In this study, no digestive abnormalities were observed in the foals, indicating that the increased starch content in this study met the needs of the foals. The total weight gain and daily weight gain of the barley fed group was significantly higher than that of the corn and oat fed groups. The uniform and stable release of glucose by the body is a necessary condition for the absorption and utilization of glucose in the small intestine to promote the growth and development potential of young animals. Only when the release of glucose in the intestine is consistent with the demand for glucose by the body tissues and organs can the performance potential of young animals be fully achieved. Results of Weurding et al.(2001), showed that a continuous, uniform and slow-release mode of glucose provided energy supply for the balanced intestinal tract of piglets, and allowed their growth to reach the optimal level. A study by Nasir et al.(2015) showed that low-quality or high-quality barley can fully replace wheat grain in diets for starter pigs and achieve equivalent or better growth performance. In this study, the significant weight gain in the barley fed group may be related to the speed of starch release from barley; releasing glucose more in line with the growth pattern of the foals.
The grain in the diet was used mainly to increase the energy requirements of the horse. Corn, oats and barley were the primary energy feed for horses, which contain similar amylose and amylopectin. However, due to the different proportion of starch polysaccharides and the different size of starch granules, their digestion and effects on the intestinal tract of horses are other (Kong et al.,2003; Svihus et al.,2005). Studies have shown that there were differences in the digestibility of grain particles in the small intestine of horses. Starch was not digested in the small intestine was degraded by microbial fermentation upon arrival in the small intestine (Defombelle et al.,2004).
The starch that is not digested by enzymes in the horse's small intestine is transported to the hind intestine for microbial fermentation. It has been estimated that the utilized energy of starch produced by post-intestinal microorganisms is 35% − 40% less than that absorbed in the form of glucose digested by enteroglycans (Kienzle et al.,1994; Black et al.,1971). Also, starches not digested by intestinal amylase arrive in the intestine. They are fermented by starch-breaking bacteria, causing fundamental changes in the structure of the posterior intestinal flora and increasing concentrations of volatile fatty acids and lactic acid in the caecum and colon (Garner et al.,1978).
Different starch sources and other intakes of starch would affect the number of starch decomposition bacteria. The amount of grain the foals were fed determined the amount of starch that they consumed. When fed a low-content grain diet, the starch intake was low, and the number of starch decomposition bacteria increased in horses fed with corn and wheat, but not in horses fed with oats (Harlow et al.,2016). When the amount of grain was high, the number of starch-breaking bacteria increased in horses fed with oats and corn, while the increase of starch-breaking bacteria was more significant in horses fed with a high corn diet. Such a massive difference in microbial digestion of corn starch and oat starch is directly related to the low sensitivity of corn starch to enzymic digestion in the small intestine of horses (Rosenfeld et al.,2008). As alpha-amylase in the small intestine of horses showed low activity in the digestion of corn starch, a large amount of undigested corn arrived in the intestine. This was fermented by microorganisms, increasing starch-breaking bacteria (Harlow et al.,2015). Therefore, the varying starch sources could explain the influence of different starch sources on the microbial diversity of the hindgut, especially on the total starch decomposers. The starch source has been shown to affect the extent of the starch bypass to the equine hindgut (Radicke et al.,1991; Defombelle et al.,2004; Rosenfeld et al.,2008). The starch source would also affect changes to the gastrointestinal microbial community that are induced when the grain is added to a forage-based diet. Ren et al.(2019) used 16S rRNA sequencing technology to reveal the potential mechanism of steam-pressed corn to improve the production performance of ruminants, and fed steam-pressed corn to cows. They subsequently found Firmicutes and mutants in the rumen microbial community. The relative abundance of the Proteobacteria tended to increase or increased significantly, succinic acid vibrios (Succinivibrio) and rothia (Roseburia) and slaughter bacteria genera (Blautia) the relative abundance of the starch decomposition microbes such as add, reduce the relative abundance of cellulose decomposition microbes.
In this study, alpha diversity indices showed that fed grains could change the structure and diversity of bacteria in foal’s faecal matter. The sample from barley fed foals significantly increased Shannon and Simpson indexes and altered the community structure of bacteria in foal’s faecal matter. The sample from oat fed foals had a significantly increased Chao1 index and changed the diversity of bacteria in the foal’s faecal matter. The relative abundance of faecal bacteria results showed that Actinobacteria, S treptococcaceae, Lactobacillaceae, Streptococcus, Lactobacillus and Actinobacillus were significantly increased when corn was fed. The relative abundance of Tenericutes, Rikenellaceae and Lactobacillus in faecal matter was significantly increased when oat was provided. Lachnospiraceae and Agathobacter were significantly increased when barley was fed. A study by Defombelle et al.(2001) looked at the effect of changing the diet from 100% hay (100:0) to 50% hay and 50% barley. The results showed that concentrations of lactate-utilizing bacteria were not significantly altered in the caecum and colon of ponies. However, the concentrations of lactobacilli and streptococci were significantly increased 5 hours after altering the diet, and then significantly decreased after 29 hours.
Ruminococcaceae bacteria have previously been identified as fibrinolytic bacteria (Daly et al.,2012). In this study, the LEfSe result showed that Ruminococcaceae amounts were significantly different in faecal matter from barley fed foals. A study by Bulmer et al.(2019) demonstrated that even a small addition of starch to the diet was enough to reduce this bacterial Ruminococcaceae population. Our research showed that the relative abundance of Ruminococcaceae in the corn, oat and barley group was: Ruminococcus_sp_HUN007 (0.16%,0.13%༌0.47%, respectively) and Ruminococcus flavefaciens (0.01%, 0.02%, 0.17%, respectively). Research has shown that the extent of the alterations to faecal microbiota with the addition of starch to the diet can be influenced by the source of starch feed (Harlow et al.,2016). Our study showed that Lactobacillales, Lactobacillaceae, Lactobacillus and Lactobacillus hayakitensis (14.19%, 18.09% and 7.11% in faecal matter when fed corn, oat and barley, respectively) were significantly different in the oat fed group compared to the corn and barley fed group. An increase in lactic-acid producing bacteria has also been reported to be coupled with a corresponding decrease in fibrolytic bacterial abundance (Daly et al.,2012). The extent of starch digestion in the small intestine can be affected by processing (Potter et al.,1992; Meyer et al.,1995), so the amount of starch that will result in bypass to the large intestine is likely to vary with both starch source and processing. In this study, the grains were treated by steam pressure, and the processing method was consistent. The reason for the difference in bacterial flora may be related to the structure and proportion of cereal starch depending on the source.
The results of functional prediction showed that long-term feeding of three different grains had varying effects on digestive physiology and health of foals. Results from Bulmer et al.(2019) showed that the dietary change resulted in alterations in behaviour and faecal microbiota. The diet could change faecal microbial community composition and relative abundance. Increased starch in the current study had an undesirable effect on behaviour and gut microbiota; it made the ponies more reactive in their behaviour and moved the microbial community composition of the gut towards dysbiosis. However, the opposite was true of the high fibre diet. In this study, we didn’t observe the abnormal behaviour and faecal bacteria in foals when fed corn, oat or barley for 60 days. Intestinal microorganisms have specific effects on immune function, nutrient absorption, and even enzyme metabolism (Martin et al.,2010). The Tax4Fun prediction was that the diet of corn, oat or barley could affect digestion and metabolism in foals, and the effects of the three grains supplementation on microbial function prediction were different.