Managing a sustainable dairy farm requires balancing phosphorus (P) imports and exports that enter and leave through the farm gate. Over the long term, P surpluses will elevate soil-test P concentrations above crop requirements through routine land applications of manure. The objectives of this study were aimed at Virginia dairy farms (a) to determine P mass balances, (b) to define potential guidelines for a sustainable and feasible zone of operation based on P balance and P use efficiency, and (c) to assess risk factors driving P surplus and P use inefficiencies. Data on farm-gate P imports and exports via feed, manure, crops, fertilizers, bedding, animals, and milk were collected for 58 dairy farms in Virginia. There was no relationship between farm P balance and milk production, indicating that a P surplus was not necessary for good milk productivity. A feasible P balance limit was calculated below which 75% of farms could operate, and this was 18.7 kg P ha-1 . Two risk factors were identified for farms having a P balance above this limit: (a) land application of poultry litter and (b) excessive import of P through feed. Combined dairy and beef operations generally had more land and a lower P balance, whereas having combined dairy and poultry did not raise the P balance as long as poultry litter was exported. Dairy farms in Virginia can operate with a sustainable P balance as long as they avoid using excessive poultry litter and pay attention to P imported through purchased feed.

中文翻译：

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弗吉尼亚州 58 个奶牛场的磷平衡状态

管理一个可持续的奶牛场需要平衡进出农场大门的磷 (P) 进口和出口。从长远来看，P 过剩将通过常规土地施肥将土壤测试 P 浓度提高到作物需求之上。本研究的目标针对弗吉尼亚奶牛场 (a) 确定 P 质量平衡，(b) 为基于 P 平衡和 P 使用效率的可持续和可行的操作区定义潜在指南，以及 (c) 评估导致磷过剩和磷使用效率低下的风险因素。收集了弗吉尼亚州 58 个奶牛场通过饲料、粪便、作物、肥料、床上用品、动物和牛奶的农场门口磷进出口数据。农场磷平衡与牛奶产量之间没有关系，表明 P 盈余对于良好的产奶量不是必要的。计算出一个可行的 P 平衡限制，低于该限制 75% 的农场可以运营，即​​ 18.7 kg P ha-1 。对于磷平衡高于此限值的农场，确定了两个风险因素：（a）家禽垫料的土地应用和（b）通过饲料过量进口磷。乳制品和牛肉的联合经营通常有更多的土地和较低的磷平衡，而乳制品和家禽的联合经营并没有提高只要家禽垫料出口的磷平衡。弗吉尼亚州的奶牛场只要避免使用过多的家禽垫料并注意通过购买的饲料进口的磷，就可以在可持续的磷平衡下运营。对于磷平衡高于此限值的农场，确定了两个风险因素：（a）家禽垫料的土地应用和（b）通过饲料过量进口磷。乳制品和牛肉的联合经营通常有更多的土地和较低的磷平衡，而乳制品和家禽的联合经营并不会提高磷平衡，只要家禽垫料是出口的。弗吉尼亚州的奶牛场只要避免使用过多的家禽垫料并注意通过购买的饲料进口的磷，就可以在可持续的磷平衡下运营。对于磷平衡高于此限值的农场，确定了两个风险因素：（a）家禽垫料的土地应用和（b）通过饲料过量进口磷。乳制品和牛肉的联合经营通常有更多的土地和较低的磷平衡，而乳制品和家禽的联合经营并没有提高只要家禽垫料出口的磷平衡。弗吉尼亚州的奶牛场只要避免使用过多的家禽垫料并注意通过购买的饲料进口的磷，就可以在可持续的磷平衡下运营。