|dc.description.abstract||Two groups of 10 Border Leicester ram lambs ( LGR lambs: 10 weeks old; 22.7 ±0.59 kgLW) were grazed on irrigated and N fertilised pastures of Grasslands Puna chicory (Puna) and Wana cocksfoot (Wana) over a six week period. Both pastures were maintained as vegetative, pure species swards and offered in weekly subdivisions at high herbage allowances of 12.5 to 15 kgDM/lamb/day. Liveweight of the lambs was recorded weekly and pasture intake was estimated in week 3 from dilution of two faecal markers, chromium oxide (Cr₂O₃) and ytterbium (Vb) acetate, and in vitro digestibility measured from oesophageal extrusa samples.
Another two groups of 6 lambs each, each lamb fitted with rumen and duodenal cannulae (SOD lambs) were co-grazed on Puna and Wana from weeks 4 to 5 to measure digesta flow and nutrient supply at the duodenum. Chromium ethylene diamine tetra-acetic acid (Cr-EDT A) and Yb acetate were used as solute and particulate phase digesta markers, respectively (Faichney, 1975; Siddons et al., 1985). The digesta markers were infused continuously into the rumen using portable battery powered peristaltic pumps and sampled following the procedure of Faichney (1975, 1980) to measure digesta passage and site of nutrient digestion.
LWG (g/d) of LGR lambs grazing Puna (273 ± 24.3) was significantly greater (p<0.05) than for lambs on Wana (205 ± 9.9) over the six week period. Herbage ingested was about 90% leaf on both pastures and the chemical composition (%DM) was 81.5 and 89.4 OM, 34.9 and 48.1 NDF and 2.6 and 3.6 N, and the in vitro OM digestibility was 72.6 and 74.2% for Puna and Wana, respectively. However, the digestible OM intake (DOMI) and the nitrogen intake (Nil was lower (p<0.05) on Puna (16.1 and 0.71 g/kgLW/d) than Wana (26.7 and 1.39 g/kgLW/d), and did not explain the difference in the LWGs observed. The original DOMI of Puna was equivalent to 6.7 MJ ME/d, which is inadequate even for maintenance (zero LWG) of a 25 kg lamb at pasture (Geenty and Rattray, 1987). The LWG of lambs and ME intake (11.2 MJ/d) on Wana compares well with the standard requirements (14.5 MJME/d) for growth rates of 200-300 g/d for 25 kg lambs grazing pasture (Geenty and Rattray, 1987).
The low ME intake on Puna suggested that pasture intake may have been underestimated. OM intake of Puna measured using faecal output from dilution of Cr₂O₃) (29.8 g/kgLW/d) was similar to that of Vb acetate (31.0 g/kgLW/d). Recovery of both markers was high within pasture treatments (Puna: Cr 93.8%, Yb = 107.8%; Wana: Cr = 97.1%, Yb = 92.6%) and were corrected for 100% recovery to determine faecal output. The other possible source for error in intake estimation was the use of a single in vitro digestibility value obtained from oesophageal extrusa samples (McManus, 1981), that the in vitro OMD of extrusa sample was > 10% age units lower compared to the most edible parts (leaves and stem above 50 cm: 83% OMD) of Puna. The original Puna data (viz. DOMI) was revised by correcting for the low in vitro digestibility of the oesophageal extrusa sample. The difference between the revised DOMI of Puna (680 g/d) and Wana (716 g/d) was less than 10%, and the former was equivalent to an ME intake of 10.6 MJ/d, acceptable for good lamb growth rate (Cruickshank, 1986).
Notwithstanding possible error of the original intake estimate, data on digestion characteristics and site of digestion suggested a higher post-ruminal digestion of Puna than Wana, which is consistent with high growth rate in lambs grazing pasture (Cruickshank, 1986), Digestible OM apparently digested in the rumen (DOMADR) for Puna was 0.51 ±0.018, significantly lower (p<0.05) compared to Wana (0.70±0.042). The revised data for Puna however showed a similar DOMADR (0.70) as for Wana, suggesting a better nutrient supply for Puna associated with a higher outflow (p<0.05) of the liquid fraction of digesta (20.2 ±4.4 lid) compared to Wana (8.5 ± 1.22 lid). Thus although NI of Puna was lower than Wana, the duodenal non-ammonia N (NAN) flow was similar on both pastures (0.951 and 1.098 gNAN/kgLW/d, respectively), perhaps due to a more efficient N use and microbial protein yield on the Puna diet (Ushida et al, 1986). Subsequently, the N loss (asammonia-N across the rumen) for Puna was only up to 23% of N intake (ie. revised; 2% original, much lower compared to a 39.0% N loss on Wana which was expected of fresh temperate pasture (Poppi et al., 1988). The greater loss of dietary N on Wana was consistent with a high mean rumen ammonia-N concentration (312.8 mg NH3-N/I, twice as high (p<0.01) compared to Puna (155.9 mg NH3-N/I). Site of digestion estimates from the SOD lambs were applied to apparently digestible OM (DOMI) and N (Nil intake of LGR lambs to estimate nutrient supply in terms of balance of protein to energy supply to explain the growth of the latter mob of animals.
The significantly higher DOMI (p<0.01) and duodenal NAN supply (p<0.05) on Wana (716.16 and 22.8 g/d, respectively) compared to Puna (original estimates of 428.8 and 18.7 g/d, respectively) should have resulted in a higher LWG on Wana, or at least an equal LWG as on Puna. Even the revised DOMI of Puna was lower than Wanat however the revised NAN supply (30.7 g/d) of Puna was greater by 30% compared to Wana (22.8 gNAN/d). The ratio of NAN:DOMI of Puna (45.1 g/kg, revised) and the corresponding lamb LWG (273 g/d) therefore compares with a balance of nutrient supply and lamb growth performance on fresh grasses (41.0 g/kg and 227-230 g/d, respectively) and legumes (45.0 g/kg and 308-321 g/d, respectively) previously evaluated at Lincoln (Cruickshank, 1986). The ratio of NAN:DOMI on Wana (31.9 g/kg) suggest that of this basis, its nutritive value is lower compared to other grasses for high post-weaning lamb growth rate.
The low LWG relative to DOMI or ME intake (11.2 MJ/d) on Wana compared to Puna, suggests a low efficiency of ME use for lamb growth on Wana pasture. Energy required for conversion of ammonia (equivalent to the difference between NI and NAN flow at the duodenum of LGR lambs) and its excretion as urea, represented a loss of available ME (MJ/d) by 0.84% on Puna and 2.52% on Wana. The high energy cost for removal of excessive ammonia production could have contributed to the low efficiency for LWG of lambs grazing good quality Wana pasture. Otherwise, the ratio of gNAN:kgOMADR of Wana (45.5) was only 0.6 that of Puna (77.1) and was the lowest value for grasses reported in literature to date, indicating that efficiency of rumen microbial protein synthesis for Wana was probably lowest among pasture herbages.
It was concluded that the superior lamb growth rate on Puna over Wana was related to a higher flow rate of the liquid digesta fraction and subsequently a more efficient capture of dietary Nand a higher NAN supply at the duodenum relative to DOMI. This was indicative of a better balance of nutrient supply (protein:energy) and a higher efficiency of ME use for lamb growth on Puna. A recommendation that future work should include assessment of body composition changes may further elucidate the effect of site of nutrient digestion and supply on efficiency of ME use for lamb growth on Puna and Wana pasture.||en