| The Qinghai-Tibetan Plateau is characterized by high attitude, severe cold, low level oxygen, strong UV radiation and short growing season. Under the traditional grazing management, the feed supplied to the ruminants would be inadequate during the cold season in the area every year. The Tibetan sheep is the indigenous breed, which is grazed on the grassland all year round and adapted well to the harsh condition. The Gansu Alpine Fine-wool sheep, a crossbreed that was introduced to this plateau in the 1980s, which is the first fine wool sheep breed that lives in the high altitude area in China. With some concentration supplement in the cold season, the Fine-wool sheep also could adapt well to the grazing management. The different husbandry between the sheep breeds, is probably due to the adaptive abilities to foraging condition. The objectives of present study were to investigate the apparent digestibilities, rumen fermentation, N balance and urea kinetics between Tibetan and Fine-wool sheep, and then provide some basal evidences to support that the Tibetan sheep could adapt much bettter to the low N intakes on the Qinghai-Tibetan Plateau. The main results were showed as below:1. Apparent digestibilities, rumen fermentation and urinary purine derivetives (PD) excretion between Tibetan and Fine-wool sheep under different N content dietsA comparative study of the effect of dietary N content [Low:11.0; Medium-Low (MLow): 16.7; Medium-High (Mhigh):23.1; High:29.2 N g/kg dry matter] on apparent digestibilities, rumen fermentation and urinary PD excretion was conducted in Tibetan and Fine-wool sheep. Eight wethers ([4 Tibetan sheep and 4 Fine-wool sheep,20 to 24 months old; body weight (BW) ± standard deviation (SD) was 50 ± 3 kg]) were used in 2 concurrent 4×4 Latin square designs.Dry matter (DM), organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre (ADF) and nonfiber carbohydrate (NFC) digestibilities were higher (linear interactions, all P<0.05) in Tibetan than Fine-wool sheep when fed the Low, MLow and High N diets, but there was no difference between breed in the crude protein (CP) digestibility. Tibetan sheep had a higher rumen pH than Fine-wool sheep (?<0.05), however, total volatile fatty acids (VFA) were similar between breeds (P>0.10). Molar proportions of acetate were higher (P<0.01) but propionate and butyrate lower (both P<0.05) in Tibetan than Fine-wool sheep. In addition, Tibetan sheep had higher concentrations of ruminal free amino acid-N and soluble protein-N than Fine-wool sheep (P<0.05 or P<0.01), but there was no difference detected in the ruminal ammonia or peptide concentration between breeds (P>0.10).Dietary DM, OM, ADF and CP apparent digestibilities were increased as the dietary N (P<0.01 or P<0.001). Ruminal pH, acetate to propionate ratio and total VFA concentration were not affected by the dietary N (P>0.10). The molar proportions of iso-butyrate, valerate and iso-valerate were increased linearly as the dietary N increased, whereas the acetate was decreased (P<0.05 or P<0.01 or P<0.001). Runimal ammonia-N, amino acid-N and soluble protein-N concentrations were increased linearly as dietary N level (P<0.05 or P<0.001), and pepetide-N concentration was increased quadratically (P<0.05).The urinary total PD excretion was higher in the Tibetan than Fine-wool sheep when fed the Low N diets, but this was adverse on the High N diet, and there was no diference between breeds under the other two diets (linear interaction, P<0.05). Except for the uric acid, the urinary total PD and its franctions elimination were increased linearly with dietary N (P<0.05 or P<0.01). Compared to the Fine-wool sheep, the microorganism of the Tibetan sheep was richer in RNA and lower in protein. With the dietary N increased, the rumen microbial RNA content was decreaed (P<0.05) while the protein content increased (P<0.05).2. The dietary N utilization and urea kinetics in Tibetan and Fine-wool sheep under different N content dietsAn experiment was conducted to characterize and quantify N utilization efficiency and urea metabolism in Tibetan and Fine-wool sheep fed 4 levels of dietary N [Low:11.0; Medium-Low (MLow):16.7; Medium-High (Mhigh):23.1; High:29.2 N g/kg DM] in 2 concurrent 4 x 4 Latin square designs. Urea kinetics were determined using continuous intrajugular infusions of 15N15N-urea, the fecal and urinary samples were collected during 45-56h, and the urea kinetics were calculated according to the isotope enrichment in the outputs samples.Urinary excretions of total-N and urea-N increased linearly (P<0.001) with dietary N and were not different between breeds (P>0.10). Fecal N excretion increased with dietary N for Tibetan sheep but not for Fine-wool sheep (linear interaction, P<0.05). Nitrogen retention (both amount per day and percentage of N intake) increased with increasing dietary N concentration (P<0.001), and the rates of increase were greater in Fine-wool than in Tibetan sheep (linear and cubic interactions, P<0.05). In Tibetan sheep, N retention as a percentage of intake was greatest for diets containing 16.7 g N/kg DM, whereas it was maximal for Fine-wool sheep when the diet contained 23.1 g N/kg DM.Urea-N entry rate (UER), [gastrointestinal tract (GIT) entry rate, GER], and urea-N returned to the ornithine cycle (ROC) were greater in Tibetan than Fine-wool sheep for the 11.0g N/kg DM diets but greater in Fine-wool than Tibetan sheep for the diet with 29.2 g N/kg DM (linear interaction, P<0.05). Urea-N excreted in feces (UFE), both amount and fraction of GER, was less in Tibetan than Fine-wool sheep for 11.0 and 16.7 g N/kg DM diets but similar for diets with 23.1 or 29.2 g N/kg DM (linear dietary N×breed, P<0.01). For the lowest protein diet, the fraction of urea-N production recycled to the GIT was greater in the Tibetan than Fine-wool sheep (88% vs.82%), but for the diet with 29.2 g N/kg DM it was greater for Fine-wool than for Tibetan sheep (46% vs.39%; linear dietary N×breed, P<0.05). Plasma and saliva urea-N concentrations were higher in Tibetan than Fine-wool sheep (linear interactions, both P<0.05) when supplied with the Low N diet. The urinary creatine excretion was higher in Tibetan than Fine-wool sheep (P<0.05). Urea tubular load and the amount and percentage of urea reabsorbed by the kidney were greater in Tibetan than Fine-wool sheep (P<0.05). The urea-N used for anabolism (UUA) as a fraction of GER and the glomerular filtration rate both were trended to be greater in Tibetan than Fine-wool sheep (P<0.10).As the dietary N increase, UER, GER, UUE, ROC and UUA all linearly increased (P<0.01), where as UFE, UFE:GER and GER:UER decreased (P<0.05). Urea tubular load and reabsorption by the kidney were increased (P<0.05) as the dietary N content while the reabsorption rate was decreased (P<0.001). Urea pool size was increased (P<0.001) when dietary N increased, however, turnover time decreased (P<0.001).3. The effect of the oat hay feeding level to the N balance and urea kinetics in Tibetan sheepA 4 x 4 Latin square design was used with 4 levels of oat hay intakes (0.3,0.5,0.7, and 0.9 x voluntary intake), to determine urea kinetics, N balance and requirements for maintenance in five growing Tibetan wethers (BW was 42 ± 2.3 kg). There was a linear increase in fecal N excretion, urinary N elimination and N retention (P<0.01) with an increase in feed intakes. There was a good linear regressionbetween N retention (g/d) and N daily intake (g/d):N retention= 0.547 N intake-4.566 (R2= 0.998, n= 20, SE= 0.287). Based on this equation, the estimated N requirements for maintenance were 0.50 g/kg BW0.75 per day, that is, only 66% of the amount recommended by NRC for growing sheep of its size. UER, GER, ROC and UFE all were increased linearly (P<0.01) with an increase in feed intakes.The ratio of UER to apparent digestible N intake increased linearly from 1.53 to 2.99 with a decrease in feed intakes. The proportion of UER entered the gut did not differ (P>0.10) among intakes and ranged between 0.52 and 0.61. GER used for anabolism decreased (P<0.01) from 0.45 to 0.28, whereas GER to ROC increased (P<0.01) from 0.51 to 0.68 with increasing N intakes. GFR increased (P<0.05) with increasing N intakes, but urinary creatinine excretion and plasma urea-N concentrations remained constant (P>0.10). The proportion of renal urea-N reabsorbed increased linearly with a decrease in N intake (P<0.05) while GIT urea-N clearance was always higher than kidney urea-N clearance.In general, Tibetan and Fine-wool sheep both adapted well to the harsh foraging condition on the Qinghai-Tibetan Plateau. However, when the animals fed with the low N diets, Tibetan sheep appeared to be more efficient in feed digestion, dietary N utilization and urea metabolism than the Fine-wool sheep. These results primarily indicated that the Tibetan sheep have evolved some mechanisms to match the low N intakes, which provided some important data and technology to improve the sheep management on the Qinghai-Tibetan Plateau. |
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