| For the sake of economy and environment,people are interested in reducing nitrogen emission and improving production performance of ruminants.In the case of shortage of high-quality protein feed in China,improving the utilization of nitrogen nutrients i n feed has become a key factor to break through the bottleneck of breeding industry.Therefore,it was urgent to find and develop a safe and effective feed additive to improve the utilization of nitrogen nutrients in ruminants.N-carbamylglutamate(NCG)is a structural analogue of N-acetylglutamate(NAG),which could improve the nitrogen utilization in dairy cows.However,effect of NCG on nitrogen utilization of beef cattle was not very clear.Therefore,in order to develop a nutritional feed additive,this study analyzed the optimal dosage of NCG in the diet to promote the utilization of nitrogen nutrients in Holstein bulls,explored the feeding effect and slaughter performance of NCG on Holstein bulls fed with urea as nitrogen source diet for the first time,and analyzed the regulatory role of NCG on the utilization of nitrogen nutrients in the body from the molecular level for the first time.In order to improve the utilization of nitrogen nutrients and reduce nitrogen emissions of beef cattl e to provide a theoretical basis.Experiment 1: effects of different doses of NCG on growth performance,apparent digestibility,nitrogen metabolism,rumen fermentation and plasma metabolism of Holstein bullsIn order to understand the feeding effect of NCG on Holstein bulls and determine the best dosage,the effects of different dosage of NCG on growth performance,apparent digestibility,nitrogen metabolism,rumen fermentation and plasma metabolism of Holstein bulls were studied.24 healthy Holstein bulls(body weight,408 ± 21.9 kg;age,450 ± 6.1 d)were randomly divided into 4 treatment groups with 6 bulls in each group.The control group(CON)was fed with basal diet;the low-dose group(L)was fed with basal diet supplemented with 20 mg/kg NCG per day;the medium dose group(m)was fed with basal diet supplemented with 40 mg/kg NCG per day;the high-dose group(H)was fed with basal diet supplemented with 80 mg/kg NCG per day.The feeding experiment lasted for 9 weeks.The feed intake and body weight of each bull were recorded,and the relevant indexes such as the residual material,feces,urine,blood and rumen fluid samples of each bull were collected and analyzed.The results showed that the addition of NCG increased the average daily gain(P < 0.05),feed conversion rate(P < 0.05),apparent digestibility of crude protein(P <0.10),nitrogen deposition(P < 0.05)and nitrogen utilization rate(P < 0.05).Whereas the contents of fecal N(P < 0.05)and urinary N(P < 0.05)in NCG-fed bulls were decreased compared with those in CON bulls.The nitrogen deposition and utilization rate of bulls in group M were the highest,while the contents of fecal nitrogen and urine nitrogen were the lowest.Compared with the control group,an increase trend in the rumen microbial protein content(P < 0.10)were observed.A quadratic decrease in plasma urea concentrations(P < 0.05)and ammonia(P < 0.05)were observed with increasing doses of NCG,with the nadir at M.Adding different doses of NCG to the diet significantly increased the plasma nitric oxide concentration of bulls(P < 0.05).Plasma NO concentration(P < 0.05)increased in a quadratic fashion with increasing doses of NCG,with a peak at M.In addition,the plasma concentrations of arginine(P < 0.05),isoleucine(P < 0.05),valine(P< 0.05),alanine(P < 0.05),glutamic acid(P < 0.05),serine(P < 0.05),total nonessential amino acids(P < 0.05)and total essential amino acids(P < 0.05)were significantly increased by adding different doses of NCG(P < 0.05).All of them increased linearly with increasing doses of NCG(P< 0.05),and M treatment group was the highest.Experiment 2: effects of NCG supplementation in urea diet on growth performance,apparent digestibility,nitrogen metabolism,plasma metabolism,rumen fermentation,microbial diversity and economic benefits of Holstein bullsBased on the feeding effect and optimal dosage of NCG in Holstein bulls,urea was used as the nitrogen source to replace soybean meal in the diet.The effects of NCG in urea diet on growth performance,apparent digestibility,nitrogen metabolism,plasma metabolism,rumen fermentation,microbial diversity and economic benefits of Holstein bulls were studied.24 healthy Holstein bulls(body weight,490 ± 29.0 kg;age,540 ± 6.1 d)were selected in a completely randomized block design.They were randomly divided into 4 groups,6 in each group.They were: basal diet group(CON)fed basal diet;basal diet supplemented with 40 mg/kg body weight NCG(CON + NCG);non protein nitrogen diet group(Urea)fed urea diet;urea diet supplemented with 40 mg/kg body weight NCG(Urea + NCG).The feeding experiment lasted for 9 weeks.The feed intake and body weight of each bull were recorded,and the relevant indexes such as the residual material,feces,urine,blood and rumen fluid samples of each bull were collected and analyzed.The results showed that compared with the basal diet,the average daily gain(P < 0.05)of bulls fed urea diet was decreased.And the content of urine nitrogen was increased(P < 0.05).No difference(P > 0.10)was observed in fecal nitrogen,nitrogen deposition and nitrogen utilization rate of bulls fed urea diet.However,NCG supplementation in two different diets could significantly improve the average daily gain(P < 0.05),feed conversion rate(P < 0.05),DM(P < 0.05)apparent digestibility,CP(P < 0.05)apparent digestibility,nitrogen deposition(P < 0.05)and nitrogen utilization rate(P < 0.05),and reduce the contents of fecal nitrogen(P < 0.05)and urine nitrogen(P < 0.05).The plasma urea(P< 0.05)and plasma ammonia(P < 0.05)concentrations of bulls fed urea diet were significantly increased in bulls fed basal diet,while the plasma urea(P < 0.10)and plasma ammonia(P < 0.05)concentrations of bulls fed with NCG in both diets were decreased.Meanwhile,an increase trend in the plasma nitric oxide concentration of bulls fed NCG(P < 0.10)were observed.No difference in essential and non essential amino acids between the two diets(P > 0.10).The addition of NCG to the two diets could significantly increase the concentration of plasma arginine(P < 0.05),leucine(P < 0.05),isoleucine(P < 0.05)and tyrosine(P < 0.05).The concentration of valine had an increase trend(P < 0.10).In addition,the addition of NCG could increase the content of microbial protein(P < 0.05),and reduce the concentration of ammonia nitrogen(P < 0.05),acetic acid(P < 0.05)and the ratio of acetic acid to propionic acid(P < 0.05).Bulls fed NCG had a decrease trend for total volatile fatty acids(P < 0.10).The relative abundance of Prevotellaceae,Lactobacillaceae,Erysipelotrichaceae and Prevotella＿1 in the rumen of bulls fed urea diet was significantly lower than that of the basal diet group(P < 0.05).The relative abundance of Spirochaetaceae and Treponema＿2 decreased significantly(P < 0.05).Bulls fed NCG significantly increased the relative abundance of Prevotellaceae in the rumen and decreased and the decreased relative abundance of Ruminococcaceae and Treponema＿2(P < 0.05)in the rumen.Compared with the basal diet,the feed cost of bulls fed with urea as nitrogen source diet was significantly lower(P < 0.05).The feed cost of each bull was increased by adding NCG to the two diets.Moreover,the addition of NCG increased the weight gain benefit of per bull.Therefore,bulls fed NCG increased the economic benefit of per bull.Experiment 3: effects of NCG supplementation in urea diet on slaughter performance,meat quality,intestinal nutrient uptake and m TOR signaling pathway expression of Holstein bullsThe effects of NCG on nitrogen utilization of Holstein bulls were studied at molecular level.The design,formula and sample collection and determination were the same as those in Experiment2.The results showed that compared with the basal diet,urea diet significantly reduced the slaughter rate,live weight before slaughter,hot carcass weight,eye muscle area,drip loss and shear force of meat(P < 0.05).On the contrary,NCG supplementation could significantly improve(P < 0.05)the hot carcass weight,slaughter rate,eye muscle area,coo king loss and shear force of meat.Furthermore,NCG supplementation increased the m RNA expression of IGF-1,SLC7A7 and SLC7A9(P < 0.10),and significantly increased the protein expression of p-m TOR(P < 0.05)Overall,supplementation 40 mg/kg BW NCG to the diet could improve protein digestibility,reduce nitrogen emission and improve growth performance of bulls.The addition of NCG to urea diet could promote the utilization rate of non-protein nitrogen of Holstein bulls,improve the weight gain benefit of bulls and economic benefits of farm.The addition of NCG might activate the expression of m TOR signaling pathway in liver to promote protein synthesis by increasing the expression of IGF-1. |
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