4.1. Effects of NFE on Growth Performance and Nutrient Digestion of Cashmere Kids in the High-Concentrate Diet
Rumen, as a unique digestive organ of ruminants, plays an important role in protein and energy supply of the animal body. In this experiment, the HC diet increased the final body weight and ADG, decreased the FCR, and improved the growth performance of cashmere kids compared with LC kids. This was probably related to the change in rumen fermentation pattern. The concentration of propionate in HC kids was higher than that in LC kids, and the A/P value was lower than in LC kids. Although the concentration of propionic acid in HN kids was not significantly different from that in LC kids, the A/P value in HN kids was lower than that in LC kids. In ruminant rearing, propionate is more effective in promoting growth than acetate [
33]. Therefore, the change in rumen fermentation pattern in HC and HN kids probably promoted the improvement of production performance.
Some studies have shown that the apparent digestibility of organic matter increases with the increase in the ratio of concentrate [
34]. In this trial, the increased digestibility of DM and EE in kids of HC and HN during the late period, and the increased digestibility of NDF in the early period were also consistent with this statement. The increase in EE digestibility provided more energy for rumen microorganisms and cashmere goats, which was beneficial to improve the growth performance of HC kids. While the digestibility of EE increased in HN kids, the digestibility of CP also increased, which ensured the carbon and nitrogen balance of nutrient metabolism in the body. This was likely to be one of the reasons why HN kids showed better growth performance and feed conversion efficiency compared with HC and LC kids. At present, there are many studies on noni fruit and its extracts used to improve the growth performance of fish and poultry [
6,
9,
10,
11], but there are still few data reports on its application in ruminants. Limited data suggested that noni fruit meal could be used as a feed ingredient for Holstein dairy cows. The rumen degradable protein in noni meal was higher than that in wheat bran in vitro, and diet supplemented with noni fruit meal up to 1.5% could increase C18:1 fatty acid level in milk [
35]. In addition, feed conversion efficiency of weaned beef calves increased linearly with the increasing noni pulp addition, but the supplementation of noni pulp had no significant effect on DMI of weaned cattle [
36]. This is also similar to the results of this experiment. Similar results have also been obtained from studies of other plant extracts in ruminants. Diet of lactating dairy sheep supplemented with polyphenol-rich extract from grape seed could increase the ADG of their kids [
21]. Supplementation of astragalus root extract in the diet of weaned calves could increase ADG and final weight, and decrease the DMI and DMI:ADG [
37].
Apparent digestibility can not only indicate the utilization of nutrients by the animal body but also reflect the gastrointestinal function of the animal. The NDF digestibility of cashmere kids in HC and HN was greater than that of LC kids at the early period of this trial. While at the late period, the NDF and ADF digestibility of HC kids was lower than that of LC kids, and there was no significant difference between kids of HN and HC. The normal digestibility in the early period of various nutrients in HC kids was probably due to the adaptation mechanism of rumen barrier function in ruminants to HC diets in a short period [
38]. The decrease in fiber digestibility in HC kids during the late period in this study was probably due to the decrease in pH value, which inhibited the growth and reproduction of fiber-degrading bacteria [
39], and ultimately reduced the degradation rate of structural carbohydrates. In addition, Ivan et al. (2005) [
40] showed that with the increase of NDF content in the diet, the flow of NDF in the digestive tract decreased and the NDF digestibility in rumen increased. This is also consistent with the results of NDF digestibility in the late period of this trial. It is not difficult to find that the difference in NDF content between the HC diet and the LC diet was widened in the late period. Interestingly, NDF and ADF digestibility in HC and HN decreased significantly in the late period than in the earlier period, but DM digestibility increased significantly in late period. This was probably because the proportion of NSC in the HC diet increased in the later period [31.25% vs. 46.35% (=48.33% increase)]. NSC are easier to digest than structural carbohydrate. Therefore, although the DM digestibility in the HC and HN kids during the late period was greater than that in the early period; the digestibility of NSC were mainly improved. Additionally, dietary NSC increase in a certain range could cause the decrease of fiber digestibility [
41]. At the same time, HC diet caused a significant decrease in rumen pH, which affected the ability of the cellulolytic bacteria to break down fiber [
42]. In addition, the digestibility of ADF and NDF in HN cashmere kids was greater than that in HC group in different levels, indicating that NFE supplementation mitigated the decrease of fiber digestibility in HC diet. Some studies have shown that mulberry leaf flavonoids could improve the relative abundance of cellulolytic bacteria in rumen fluid [
43], which might be the reason why NFE mitigated the decrease in ADF digestibility of HC goats in this experiment. The abundance of the ruminal bacterial community was not measured in this trial, so the mechanism of NFE increasing fiber degradation needs further investigation.
4.2. Effects of NFE on Ruminal and Colonic Fermentation of Cashmere Kids in the High-Concentrate Diet
Rumen fermentation variables are important indicators to reflect ruminal homeostasis. pH is a prerequisite for the proper function of rumen and its microorganisms. A large amount of carbohydrates contained in the HC diet will be fermented rapidly after entering the rumen, resulting in the rapid production of VFA. Lactate is also a product of the rumen fermentation process. Although the content of lactate is low, the ionization constant of lactate (pKa = 3.9) is much lower than that of VFA (pKa = 4.8), so the contribution of lactate to pH in the rumen is much greater than that of VFA. All the above processes cause a rapid decline in pH and increase the risk of SARA in ruminants. Some studies have suggested that rumen pH below 5.5 can be diagnosed as SARA [
44]. In this study, at weeks 6 and 12, rumen fluid pH at 3 h after morning feeding was significantly decreased in HC cashmere kids, and was also lower than 5.5, and lactate concentration in rumen fluid was significantly increased, both of which were characteristics of SARA. The supplementation of NFE in HN kids alleviated the decrease in pH and the increase of lactate in the HC diet. Similar results were obtained in the study on citrus flavonoids, HC diet supplementation with citrus flavonoid extract decreased the lactic acid concentration and increased the pH, which was related to increasing the relative abundance of lactate-consuming microorganism
Selenomomas ruminantium and
Megaesphaera elsdenii, and its pure component poncirine also decreased the relative abundance of
Streptococcus bovis in vitro [
45]. Another study showed that supplementation with plant extracts containing turmeric, thymol, and yeast cell wall components (mannan oligosaccharides and beta-glucans) could reduce the number of protozoan and fungi, and increased the relative abundance of lactic acid utilizer
Selenomomas ruminantium in rumen [
46]. A recent study has shown that rutin can inhibit the growth of lactate-producing bacteria in rumen fermentation in dairy cows in vitro, thereby reducing the concentration of lactate [
47]. NFE contains 9.24% flavonoids, 9.18% polyphenols and 4.53% polysaccharides. These suggests that these active ingredients probably affect the relative abundance of lactate-producing bacteria and lactate-consuming bacteria in rumen, thereby reducing lactic acid concentration and reversing the pH decline induced by HC diet. In addition, as a precursor of lactate, the decrease of propionic acid concentration in the HN group may also be the main reason for NFE to alleviate the increase of lactic acid and the decrease of pH.
HC diets generally have a high starch content, and high concentrations of starch promote protozoan growth and reproduction [
48]. Because of the engulfment of starch granules, the production of lactic acid can be slowed down to a certain extent. Interestingly, although the number of protozoa in HC cashmere kids was higher than that in HN kids, the pH was still lower than that in HN lambs. On the one hand, the reason was probably that the addition of NFE inhibited the lactic acid-producing bacteria in the rumen of cashmere goats as mentioned above, thereby reducing the lactic acid concentration, and alleviating the rise of pH. On the other hand, there might be a niche replacement in the rumen microbiota of HN cashmere goats, which may compensate for the gap in pH buffering in the rumen caused by the decrease in protozoan population by increasing the abundance of other microorganisms that have the effect of engulfment and utilizing starch. A recently study of our team showed that diet supplemented with polysaccharide rich extract of noni fruit could replace protozoa in a niche by increasing the ratio of Firmicutes to Bacteroides, in order to replace its role of fiber decomposition [
29]. However, the rumen microbiota abundance was not determined in this study, so further studies are needed to explore the metabolism of NFE in the rumen and its interaction with rumen microorganisms. Protozoa cannot use NH
3-N, but feed on bacteria [
49], so the increase in the number of protozoa in HC kids in this trial was likely to be the main reason for the decrease in MCP concentration in rumen fluid. At present, there is still a lack of reports on the effect of NFE on ruminal fermentation. Saponins can achieve an antiprotozoal effect by destroying protozoan cell membranes and making intracellular substances extravasate [
50]. Results of UPLC-MS/MS analysis in this trial showed that NFE contained a variety of triterpene saponins [hederagenin-GlcA-Ara, cadambagenic acid, oleanolic acid-3-O-glucosyl(1→2)glucoside, etc.], which may be one of the reasons why NFE has a good antiprotozoal effect.
Due to the smaller granularity of concentrate than roughage, it has a faster flow rate in the rumen [
51], while the small intestine of ruminants has limited digestion of nutrients [
52], which may increase the amount of nutrients entering the hindgut, resulting in enhanced fermentation and even acidosis of the hindgut. A study has shown that after 12 weeks of HC diet (ratio of concentrate to roughage = 70:30), the colonic pH of Boer goats decreased markedly and the concentrations of lactate, acetate, propionate, butyrate, and TVFA in colon increased significantly [
53], which is consistent with the changes of colonic pH, VFA, and lactate in HC goats in this trial. Similar changes in cecal VFA concentration were observed in cows fed HC diets for a long time [
54]. The addition of NFE also alleviated the fermentation disorders of colonic acidosis caused by HC diet with obviously increased VFA concentration and lactate concentration, and significantly decreased pH. Wang et al. (2023) [
15] found that phenolics from noni fruit could alleviate intestinal damage in high fat diet-fed mice by modulating the gut microbiota. Another study has shown that noni juice-fortified yogurt could increase the contents of total flavonoids and total phenols in yogurt, and could also ameliorate colon length and histopathological changes in the ulcerative colitis mice via improving the expression of anti-inflammatory factors and reducing the expression of pro-inflammatory factors in spleen [
14]. NFE contains 18.42% flavonoids and phenolic acids, indicating that the alleviating effect of NFE on colonic fermentation disorders caused by HC was also probably related to the regulatory effect of these active substances on cytokines. However, the cytokine content and their gene expression were not measured in this study, so further verification is needed in subsequent experiments.
In this trial, NFE was added in the form of a solution contained 0.5% NaHCO3. In this test, NaHCO3 was used to dissolve NFE for solubilization purpose. NFE has a certain viscosity, which may cause uneven mixing when mixed directly in feed. Therefore, we chosen dissolve it in liquid and feed it in the form of spraying on the diet. However, the dissolution of NFE in water was not ideal. The commonly used co-solvents are DMSO, glycerol and NaHCO3, etc., NaHCO3 was chosen as the cosolvent because its solution has a weak alkali, which was conducive to the dissolution of ethanol extract. In addition, NaHCO3 is a commonly used feed additive that can be used in HC diets to relieve SARA. Therefore, the introduction of trace amounts of NaHCO3 does not cause toxic side effects in animals. Moreover, NaHCO3 as a cosolvent also has the advantage of low cost. The preliminary test of our team showed that the concentration of NaHCO3 of 0.5% could dissolve NFE better. In order to eliminate the buffering effect of NaHCO3 on rumen pH, the same amount of 0.5% NaHCO3 solution was added to the HC and LC diets. In actual production, NFE can be dissolved with 0.5% NaHCO3 and sprayed on feed for use. Whether there is a better method for its application form needs to go into further detail.