Effect of constant and oscillating dietary crude protein concentration on ruminal parameters and microbial protein synthesis in sheep

Introduction: Crude protein is a key element in regulating the diet of ruminants and is considered one of the most expensive nutrients needed by livestock. Feeding management plays an important role in the elimination of nitrogen in the production environment and the emission of greenhouse gases in animal husbandry systems (Avathar et al 2021). When there is excessive discharge of nitrogen in the environment, it causes the ecological balance in surface waters to be disrupted and the groundwater to be polluted (Dijkstra et al 2011). Excretion of these compounds in the environment will increase with fermentation, digestion and inefficient metabolism, and the increase in inefficiency in the rumen is caused by the complex and competitive metabolic pathways in the rumen microbial population. Therefore, increasing efficiency in the use of nutrients, especially nitrogen, to minimize their excretion from the animal's body should be considered. Various methods can be implemented in an animal husbandry unit to improve the use of nitrogen in ruminants. One of these solutions is to reduce the crude protein in the diet, but it has been reported that this method can have a negative effect on animal production (Chibisa and Motswangwa 2013). Another solution is nutrient synchronization or synchronizing the fermentation rate of dietary protein and energy sources, which has led to limited success (Yelchi et al 2020). Another solution is to create oscillating in the concentration of crude protein in the diet and feed it intermittently at intervals of one to three days, and previous reports show that this method has improved nitrogen retention and its utilization in the body compared to eating constant amounts of crude protein daily in sheep and cattle (Rach et al., 2021). This study was conducted in order to investigate the effect of constant and intermittent feeding of dietary crude protein on ruminal parameters and microbial protein synthesis in sheep.
Material and Methods: Three diets were adjusted with 12, 14 and 16% crude protein but with the same metabolizable energy and protein. 8 male sheep were used in metabolic cages in a 4x4 Latin square design (four treatments with 2 squares and 8 replications). The first treatment received a diet with a crude protein level of 14% constantly. The second, third and fourth treatments received diets with 12 and 16% crude protein at intervals of 24, 48 and 72 hours, respectively. Nitrogen balance and retention, rumen and blood parameters, volatile fatty acids and microbial protein synthesis were measured.
Results and Discussion: The lowest total excreted nitrogen (via urine and feces) was observed in the treatment with 48-hour raw protein consumption interval (P=0.031). The retained nitrogen showed a significant increase in this treatment as compared to the constant crude protein feeding treatment (P=0.045). It has been reported that the increase in nitrogen retention in ruminants that received diets with oscillating crude protein levels is due to increased urea recycling in the rumen (Cole, 1999). It seems that the transfer of urea from the blood to the rumen during the consumption of diets with low crude protein level (times 24, 48 and 72 hours) in oscillating diets increased and it compensates for the shortage of nitrogen in the rumen. In this condition, nitrogen excretion through urine also decreases and the efficiency of nitrogen use increases. The highest rumen liquid ammonia nitrogen was observed in the treatment with 48-hour crude protein consumption interval, but the pH and concentration of volatile fatty acids did not show any significant difference among the experimental treatments. Comparing the treatment of constant crude protein feeding (14%) with the treatments with intermittent consumption of crude protein (12 and 16%) in terms of the concentration of volatile fatty acids in the rumen fluid of the experimental sheep, including acetate, propionate and butyrate, as well as total volatile fatty acids, there is not a significant difference. The same ratio of concentrate to fodder and the similarity of the feed ingredients in the diets can be a reason for the lack of significance between the experimental treatments, which is in line with the results of previous researches (Khattab and Abdulwahid, 2018). A significant difference was observed between the experimental treatments in terms of allantoin, total purine bases excreted, as well as nitrogen and microbial protein production, and the treatment with oscillating consumption of raw protein at a time interval of 48 hours had the highest amount. Albumin and total blood protein measured between the experimental treatments showed no significant difference, but blood urea nitrogen was the highest in the treatment with 48-hour crude protein consumption interval (P = 0.005). Microbial protein synthesis in the rumen is affected by many animal and dietary factors, including nitrogen amounts, nitrogen sources, carbohydrate and nitrogen degradation rates, type and amount of carbohydrates in the diet, dry matter consumption, stability of fermentation in the rumen, and synchronization between nitrogen and energy (Hall 2013). It seems that in diets that cause fluctuation in the entry of protein sources into the rumen, the recycling of nitrogen to the rumen is done more effectively through the blood vessels and the liver, which in addition to reducing the excretion of nitrogen from the body also increases the production of microbial protein, in other words, the efficiency of using feed nitrogen increases, which is evident in the results of this research.
Conclusion: The results showed that using the strategy of oscillating dietary crude protein concentration in sheep's diet improves nitrogen retention, rumen fermentation activities and microbial protein synthesis.