Influence Of Supplementary Starch On Associative Effects In Ruminants And Approach To The Mechanisms

A series of trials were conducted in this work to study the influences of supplementary starch on associative effects in ruminants and approach to the mechanisms. In vitro gas test was employed firstly to compare the influence of sources of starch, forage, protein and starch to forage ratio on associative effects, when the proportion of protein was fixed. Animal feeding trial was then carried out with a series of diets to analyze the influence of starch on associative effect both in digestion and metabolism, and to approach the possible mechanism.Trial 1. Three types of starch (cornstarch, wheat starch and potato starch), forages (rice straw, rye grass and peanut vine), and protein sources (soybean meal, rapeseed meal and cottonseed meal) were used to make different mixtures for incubation in vitro. Proportion of protein feed was fixed at a level of 30%, and starch to forage ratios were designed to be at 0:70,10:60,20:50 and 30:40, respectively. Gas production after 24 hours of incubation was used for calculation of associative effects. Source of starch, forage and protein all affected associative effect. Highest positive associative effect was most readily to happen when low quality forage was cultured with slowly-degrading protein source, while the increment of starch proportion tended to lower positive effect. Among three kinds of starch, cornstarch showed the lowest negative effect. When the mixed culture consisted of 30% of rapeseed meal and different ratio of cornstarch and rice straw, a series of highest positive associative effect were detected.Trial 2. Cornstarch, rice straw and rapeseed meal were used in a feeding trial, based on the in vitro gas production trial. Thirty-six male Hu lambs were divided into four equal groups according to body weight. All animals were fed on rice straw ad libitum plus 160 g/d of rapeseed meal, had free access to water,,and supplemented with cornstarch at levels of 0 (control),60,120 and 180 g/d, respectively. The trail consisted of 15 days for adaptation and 45 days for measurements. Body weight change, feed intake and apparent digestibility of nutrients were measured for evaluation of associative effect. Rumen fermentation parameters, carboxymethyl cellulase activity and relative populations of main cellulolytic bacteria species were measured. Average daily gain of body weight was increased by cornstarch supplementation compared with control (P＜0.05), with the optimal performance at level of 120 g/d. Increment of supplemental cornstarch showed little effect on rice straw intake, but increased total DM intake. Increment of supplemental cornstarch decreased apparent digestibility of neutral detergent fiber and crude protein linearly. Ruminal fermentation parameters were little affected by cornstarch supplementation, while carboxymethyl cellulase activity and relative populations of cellulolytic bacteria were reduced with increing cornstarch (P＜0.05). These results indicat increment of starch in diet would result in negative associative effect or reduce positive associative effect. The changes in cellulase activity and populations of cellulolytic bacteria account for these effects. Animal growth performance is inconsistent with the change in associative effect at digestive level, suggesting the necessity to do studies at metabolic level.Trial 3. After the feeding trial, slaughter trial was carried out to analyze the associative effect between cornstarch and rice straw-based diet at metabolic level. The parameters measured included blood biochemical indexes, growth-related hormones, carcass traits and gene expression of key enzymes for gluconeogenesis in liver. Blood glucose level was increased significantly by supplemental cornstarch (P<0.05). Blood protein indexes were not affected by cornstarch, while urea nitrogen was significantly decreased (P<0.05). Serum concentration of T3 and insulin was increased by starch supplementation (P＜0.05), while T4 and IGF-1 were not impacted by cornstarch. Expression of hepatic IGF-1 mRNA was increased when moderate amounts of starch were supplemented, whereas, overfeeding of energy showed no effect. Expression of hepatic PEPCK-C mRNA was increased with moderate starch supplementation. Expression of PEPCK-C mRNA was not affected by starch supplementation.These results indicat that the influence of energy as cornstarch on associative effect at metabolic level may mainly includ two aspects: glucose metabolism in the liver and protein utilization in the tissues. On glucose enhance gene expression of PEPCK, while insulin will depress its expression. These two factors work together to control the level of glucose in blood. As to protein utilization, animal growth, carcass traits and blood urea N all indicat enhanced protein deposition with increasing energy supply, but the mechanism remains unclear.In summary, supplementation of starch showed little influence on forage utilization, while high level of supplementary starch may strongly inhibit fiber digestion. The change in cellulase activity and populations of cellulolytic bacteria were responsible for this negative effect. Positive associative effects of starch with rice straw-based were found at metabolic level, and glucose metabolism in the liver and protein utilization in the tissues may account for these effects.