Modelling energy metabolism of Friesians in Kenya smallholdings shows how heat stress and energy deficit constrain milk yield and cow replacement rate
Modelling energy metabolism of Friesians in Kenya smallholdings shows how heat stress and energy deficit constrain milk yield and cow replacement rate
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DOI：
10.1079/asc200689
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发表时间：
2006-10-01
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影响因子：
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通讯作者：
Wathes, C. M.
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作者：
King, J. M.;Parsons, D. J.;Wathes, C. M.
2002年可持续发展问题世界首脑会议重点关注发展中国家的农业可持续性和生物多样性。这些目标与肯尼亚的畜牧业生产有关，肯尼亚的发展机构鼓励资源贫乏的小农户收购大型、奇异、高产的奶牛，尽管它们的表现很差，最近在高原和沿海进行的调查显示。奶牛的表现毋庸置疑。争论涉及到病因的诊断、治疗和生产系统的预后。为了更好地理解这一系统的动态，我们使用了适应热带地区的营养和能量模型，测量了6头纯种或杂交种的弗里赛牛在平均哺乳期为54(S.D.21.6)天，以单独的零放牧单位计算。四个小农场位于炎热潮湿的海岸，两个位于较凉爽的高地。模型的产奶量与奶牛的产奶量、产犊间隔和产奶量有关。该模型显示，热负荷在白天对所有奶牛造成中等压力，在高原的阳光下变得严重，但夜间气温从27摄氏度下降到13摄氏度，耗尽了体温的增加。在夜间温度保持在24摄氏度以上、相对湿度在0.85%以上的沿海地区，产奶量超过11 L/天的奶牛的热应激没有缓解。所有奶牛的哺乳曲线从最初的峰值下降到低调的5 L/天。初产在20 L/日以上的两个地区的奶牛产犊下降幅度最大，产犊间隔时间最长(457-662天)。它们具有最大的哺乳产量和最低的每升直接成本，但它们糟糕的繁殖记录减少了扑杀销售，增加了更换成本，提高了每升总成本。相比之下，日产奶量和哺乳量最低的母牛直接成本最高，但每升总成本最低，因为她每隔317天生产两头小母牛。这些案例研究提醒人们，在热负荷抑制食欲或劣质食物不能支持哺乳的情况下，能量短缺和压力会降低奶牛的繁殖力、健康和寿命，因此她无法在缩短的生产寿命内繁殖出替代小母牛的奶牛。该模型预测，受热应激抑制的食物摄入量，不足以支撑沿海地区每天14 L和每年30001的牛奶产量，以及高原地区每天22 L和每年50001的牛奶产量。因此，尽管在短期内看起来有利可图，但目前推广产量高于气候和生产系统所能支持的奶牛的政策是不可持续的。这一结果对肯尼亚的小农户奶业发展政策有影响，可能对热带其他地区也有影响。相反，人们应该重新考虑较小的外来品种和与改良的本土奶牛杂交，这些奶牛50年前的产奶量、繁殖力、寿命和耐病能力都好于肯尼亚小农场的现代荷斯坦-弗里西亚奶牛。
The 2002 World Summit on Sustainable Development focussed attention on agricultural sustainability and biodiversity in developing countries. These goals are relevant for livestock production in Kenya, where development agencies encourage resource-poor smallholders to acquire large, exotic, high-yielding dairy cows, despite their poor performance, revealed in recent surveys in the highlands and at the coast. The performance of the cows is not in question. The debate relates to the diagnosis of the causes, their treatment and the prognosis for the production system. To improve our understanding of the dynamics of the system, models of nutrition and energy, modified for the tropics, were used to measure the thermal responses over 24 h of six pure or crossbred Friesians at a mean lactation of 54 (s.d. 21.6) days, in separate zero-grazing units. Four smallholdings were on the hot, humid coast and two in the cooler highlands. The output of the model runs was related to the subsequent lactation, calving interval and profitability of the cows. The model showed that the thermal load caused moderate stress for all cows during the day, which became severe in the sun in the highlands, but the drop in air temperature, from 27 degrees C to 13 degrees C, at night dissipated the gain in body heat. At the coast, where the night temperature remained above 24 degrees C and relative humidity above 0.85, there was no remission of heat stress for cows with a milk yield greater than 11 l/day. The lactation curve, in all cows, declined from an initial peak to a low profile of 5 l/day. Cows in both regions with initial yields above 20 l/day had the steepest decline and longest calving intervals (457 to 662 days). They had the largest lactation yields and lowest direct cost per litre, but their poor breeding record reduced cull sales and increased replacement cost, raising the total cost per litre. By contrast, the cow with the lowest daily and lactation yield had the highest direct cost, but lowest total cost per litre, because she produced 2 heifer calves at an interval of only 317 days. These case studies serve as a reminder that, where heat load depresses appetite or poor food cannot support lactation, the energy deficit and stress reduces cow fertility, fitness, and longevity, so that she fails to breed a heifer replacement during her shortened productive life. The model predicted that food intake, depressed by heat stress, would not support a milk yield above 14 l/day and 30001 per annum at the coast, and 22 l/day and 50001 per annum in the highlands. Therefore, although appearing profitable in the short term, the current policy of promoting cows with higher yields than the climate and production system can support, is unsustainable. This outcome has implications for smallholder dairy development policy in Kenya, and probably elsewhere in the tropics. Instead, one should reconsider smaller exotic breeds and crossbreds with improved indigenous dairy cows whose overall productivity from milk yield, fecundity, longevity, and disease tolerance 50 years ago was better than that of the modem Holstein-Friesian in a Kenya smallholding.