Feedlot performance of Dorper lambs on Opunti: based diets with different nitrogen sources
Shiningavamwe, Katrina Lugambo
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Three feedlot diets were evaluated with Dorper wether lambs at Bergvlug Experimental Farm, Khomas Region, Namibia. Bergvlug is located about 35 km east of Windhoek. The three treatment diets consisted of a conventional feedlot diet (treatment diet T0) and two Opuntia-based treatment diets (T1 and T2) containing different additional nitrogen sources, namely a non-protein nitrogen (NPN; feed grade urea) or natural protein (sunflower oilcake meal). Treatment diet T0 was based on coarsely ground lucerne hay, yellow maize meal, feed grade urea and molasses meal. The Opuntia-based treatment diets (T1 and T2) were reformulated and part of the lucerne was replaced by sun-dried and coarsely ground Opuntia cladodes at levels of 330 or 300 g/kg. In treatment diet T1 additional nitrogen was included as feed grade urea (non-protein nitrogen; NPN) and for treatment diet T2 the additional nitrogen was included as sunflower oilcake meal (a natural protein). The feed intake and water intake, the growth performance and carcass characteristics of the Dorper wether lambs were evaluated in the feedlot. Forty-five newly weaned Dorper wether lambs, weighing on average about 22 kg, were randomly allocated to the three treatment diets. The 15 Dorper wether lambs per treatment diet were further subdivided into three subgroups or replicates of five lambs each. For the duration of the trial the lambs were kept in small pens in a shaded area (open-sided roofed shed). The Dorper wether lambs were fed the treatment diets until a target average slaughter weight of 35 kg per treatment diet was reached. During the feeding period in the feedlot, one replicate of five Dorper wether lambs per treatment diet was moved from the feedlot pens to metabolism cages for a week every third week to determine their individual daily feed and water intake and apparent digestibility of the three treatment diets. The daily urine and faecal excretions were also monitored. Chemical analysis of the three treatment diets used in this study showed that acid-detergent fibre (ADF), neutral-detergent fibre (NDF), organic matter (OM) and gross energy (GE) have decreased with inclusion of sun-dried and coarsely ground Opuntia cladodes, which is ascribed to the lower ADF, NDF, OM and GE content of the Opuntia cladodes. On the other hand, the ash and lipids increased with inclusion of sun-dried and coarsely ground Opuntia cladodes in the treatment diets. The inclusion of sun-dried and coarsely ground Opuntia cladodes at 330 or 300 g/kg in the treatment diets in general had little or no effect on the feed intake and digestibility of the treatment diets by Dorper wether lambs. Exceptions were observed for the intake and apparent digestibility of ADF and NDF as a result of the difference in fibre content of the treatment diets. Similar water intake and urine excretion were observed for Dorper wether lambs fed any one of the treatment diets during the three cage feeding periods. The results of the study confirmed that the feed intake and apparent digestibility of the treatment diets for Dorper wether lambs were not affected by: (1) the inclusion of sun-dried and coarsely ground Opuntia cladodes at 330 and 300 g/kg; or (2) two nitrogen sources used (NPN or natural protein) in the Opuntia-based diets. Considering the results of the Cage Periods 1 to 3, it can be summarised that the daily intake and apparent digestibility of DM and other chemical constituents of the Dorper wether lambs increased as the trial progressed, regardless of the treatment diets. It suggests that the reticulo-rumen of Dorper wether lambs were getting better adapted over time to the diets and consequently digestibility improved. The average daily gain and feed conversion efficiency of Dorper wether lambs fed the Opuntia-based diet supplemented with natural protein were comparable to those fed the conventional feedlot diet. The Dorper wether lambs fed the Opuntia-based diet supplemented with NPN had a lower growth rate than those fed the conventional feedlot diet and the Opuntia-based diet supplemented with natural protein. Thus, although feed conversion efficiency was not significantly (P>0.05) different among treatment diets, Dorper wether lambs fed the conventional diet and the Opuntia-based diet supplemented with natural protein required less feed to gain weight than those fed the Opuntia-based diet supplemented with NPN. This suggests that supplementing an Opuntia-based feedlot diet with a natural protein source will markedly improve feed efficiency and average daily gain of lambs. This may reduce the feeding period required to reach the target slaughter weight and increase the economic benefit associated with the use of sun-dried and coarsely ground Opuntia cladodes in feedlot diets. Most of the carcass characteristics considered were not significantly different (P>0.05) among treatments. It suggests that carcass quality or grading is not markedly affected by inclusion of sun-dried and coarsely ground Opuntia cladodes in feedlot diets (up to 330 or 300 g/kg) for Dorper wether lambs or by the nitrogen source used to balance the diets. The carcasses of the Dorper wether lambs fed the three different treatment diets fetched very similar prices per kg. However, the Dorper wether lambs fed treatment diet T1, the Opuntiabased diet with the inclusion of feed grade urea (an NPN source), did not reach the average target slaughter weight of 35 kg, even after 91 days in the feedlot. Therefore, their lighter carcasses and poorer carcass grading at slaughter fetched a lower total price per carcass. The results of this study, the fourth study under the auspices of the UFS, opened the prospect of formulating affordable Opuntia-based diets for specific application to ruminant species of different ages and production classes. However, more research is needed to evaluate the growth performance, carcass characteristics and profitability of other small stock breeds and ruminant species fed sun-dried and coarsely ground Opuntia cladodes in feedlot diets, balanced with different nitrogen sources.