生猪生产组织化是推动传统的家庭养殖向规模化、集约化和专业化养殖经营转变的重要举措。非洲猪瘟疫情冲击下，生猪产业化经营组织加快发展步伐，生猪企业的纵向边界向养殖环节不断延伸，大体形成了两类农业产业化经营模式：一是以“公司+农户”为代表的契约生产模式，二是以雇佣关系为代表的垂直一体化模式。事实上，目前中国生猪养殖主体仍是传统且分散的农户，并且受制于劳动力成本上升、环境保护政策趋紧、疫病防控压力增大等多方面的现实约束，普遍面临成本攀升和利润挤压的发展困局。因此，研究生猪养殖户契约生产行为及绩效，对于促进畜牧业转型升级、推进畜牧业高质量发展具有重要意义。

理论上，契约生产模式能够有效提高农户绩效，但任何经典的组织模式具体落实到实践层面时，均需要一个“本土化”的过程。在现实生产中，由于各类农产品属性、农户禀赋和契约安排的不同，以及地区间制度安排、发展阶段和市场条件的差异，导致我国农产品契约出现大量违约现象，契约生产模式对农户绩效的影响效应也大相径庭。具体到生猪契约的实践情况，可以发现，生猪契约与其他农产品契约存在显著异质性。首先，由于农产品的自然属性和产业链紧密程度的差异，不同农产品契约的形成和发展并非同步。与其他农产品契约相比，生猪契约生产模式的形成发展较晚；其次，相较于其他农产品契约下农户通常采用的全环节生产方式，生猪契约生产模式下的农户往往从事单一育肥环节生产；最后，公司与农户利益对立的天然属性，叠加一次性博弈的契约安排，使得农产品契约具有内在的不稳定性。但与其他农产品大量违约现象并存的另一事实是：在非洲猪瘟构成养殖业长期疫病威胁、市场价格风险冲击、生猪流通环节不确定性上升的背景下，生猪契约生产模式的合作关系仍较为稳定。

基于上述分析，有理由认为相比于其他农产品契约，生猪契约生产模式在契约形成、农户参与动力、契约运作机制和农户绩效等方面存在特殊性，尤其是在非洲猪瘟冲击所引致的产业环境不确定性情况下，究竟是哪些因素影响了公司和养殖户契约参与行为？契约生产模式对生猪养殖户绩效的影响效应及内在机制是什么？以及在外部环境不确定性上升时，公司和养殖户会采取哪些策略性行为来保持合约期内的关系稳定性？这对契约生产模式与农户绩效关系会产生何种影响？这些均是当前生猪产业转变生产方式面临的重要现实问题。鉴于此，本文综合运用制度变迁理论、分工理论、交易成本理论和不完全契约理论，对上述问题展开理论分析，并依次进行实证检验。首先，利用省级面板数据，采用连续DID模型，实证分析非洲猪瘟冲击对生猪契约生产公司发展的影响；接着，利用江苏省978户生猪养殖户调查数据，采用Probit模型和内生转换模型，实证分析养殖户契约参与的影响因素、契约生产模式对养殖户绩效的影响效应及内在机制，以及不确定性上升对契约生产模式与农户绩效关系的影响机制。本研究的主要结论是：

第一，非洲猪瘟冲击对生猪契约生产公司发展具有促进作用。具体而言，生猪疫病强度每增加1%，地区内生猪契约生产公司数量增长约3.2%。此外，非洲猪瘟冲击效果存在一定滞后性，且持续性减弱。从关联主体的作用机制来看，非洲猪瘟疫情冲击加剧生猪市场分割程度和提高了养殖规模化程度，公司与养殖户合作的潜在利润增加和组织成本减少，从而促进了生猪契约生产公司的发展。异质性分析表明，非洲猪瘟疫情冲击对重点发展区和约束发展区企业数量增长具有促进作用，而对潜力增长区和适度发展区企业数量没有显著影响。

第二，外部环境不确定性是影响生猪养殖户契约参与行为的重要因素。不确定性上升，养殖户参与契约生产的概率增加。生猪市场风险和生产不确定性每增加1%，养殖户参与契约生产模式的概率分别增加15.1%和18.5%。异质性分析表明，生产不确定性对养殖户契约参与行为的影响没有异质性，而市场风险冲击对养殖户契约参与的影响存在异质性。具体而言，市场风险冲击对风险偏好型偏、中小规模、非苏北地区养殖户契约参与行为没有显著影响，而对风险中立和风险规避型、大规模、苏北地区养殖户契约参与行为具有促进作用。此外，年龄较大、受教育程度较高、从事生猪养殖家庭劳动力数量人数越多、养殖经验越丰富的养殖场主倾向独立经营。

第三，相比自繁自养的独立养殖户，契约养殖户仅从事单一育肥环节，能够获得专业化生产带来的分工经济，以及缓解信贷约束带来的规模经济，进而提升养殖户生产效率。基于反事实假设，契约养殖户若未参与契约生产，其料重比和育肥猪死亡率分别上升25.80%和62.77%。此外，契约生产模式通过技术采纳机制提升养殖户生产效率。异质性分析表明，契约生产模式扩大了养殖户间料重比的差距，缩小了死亡率的差距；养殖规模越大，契约生产模式对养殖户育肥猪死亡率的抑制作用越大；并且契约生产模式对主产区养殖户料重比的抑制作用明显，而对主销区养殖户育肥死亡率的抑制作用更为明显。以上反映出契约生产所隐含的优势只是潜在的，在实践中并非可以无条件地发挥。

第四，相比完全市场交易模式，契约生产模式通过提升生产效率和降低交易风险，增加养殖户的经营收入。反事实的假设下，契约养殖户若未参与契约生产，其养殖净收入下降69.34%。异质性分析表明，契约生产模式并不能促进小规模和主销区养殖户实现增收，对大规模和主产区养殖户的净收入具有促进作用，反映出在实践中存在有些养殖户采纳契约生产模式能够成功而有些却未见效的现象。可能的原因是：中小规模以及主销区养殖户从自繁自养转为契约育肥生产所诱发的交易成本大于契约生产所带来的收益。

第五，不确定性对契约生产模式与农户绩效关系的影响存在异质性。具体影响效应及机制为：（1）在市场风险冲击下，公司倾向采用延迟收回育成猪、压低生猪价格隐性违约行为，从而弱化了契约生产模式的增产效应和增收效应。并且“一批一签”的合作方式，市场风险冲击下，公司倾向延长契约养殖场空栏期，从而弱化契约生产模式的增收效应；（2）当生产不确定性上升时，契约的风险共担机制会导致双方更加注重要素的配置效率，公司并不会进行隐性违约行为，但公司也不会改变利益分配机制，从而导致契约生产模式对养殖户料重比的负向影响增强，而对死亡率和净收入的作用没有显著影响。此外，当市场分割程度增加时，公司为减少生产者福利损失，可能会通过调整结算体系中物料价格和生猪价格将部分成本转移至农户，从而弱化了契约生产模式的增产效应和增收效应。

基于上述研究结论，本研究提出以下政策启示：一是强化对生猪龙头企业的支持力度，加快推进生猪全产业链的建设；二是因地制宜、合理有序地引导不同规模养殖户进行适宜的经营方式；三是规范公司与养殖户之间的契约合作关系；四是健全对龙头企业支持政策的监管和考核机制。

相较于已有研究，本研究可能的创新点如下：其一，本研究基于制度变迁理论，剖析非洲猪瘟疫情冲击对生猪契约生产公司发展的影响机制，并从外部环境不确定性视角分析生猪养殖户契约参与行为，有助于更深刻的认识农业契约组织形成发展的内在机制；其二，关于契约生产模式对农户绩效影响的研究，学界并未给出一致结论，本研究从生猪特征和分工理论出发，揭示契约生产模式对生猪养殖户绩效影响的内在机制，丰富了农业契约组织合作绩效的研究内容；其三，相比于忽略外部环境条件进行的抽象分析，本研究不仅在研究结论上更为客观，也为学界全面理解农产品契约关系提供重要依据。

The organization of pig production is an important measure to promote the transformation from traditional family breeding to large-scale, intensive and specialized farming. Under the impact of the African swine fever epidemic, the pig industrialization management organization has accelerated the pace of development, and the vertical boundary of pig enterprises has continued to extend to the breeding link, and two types of agricultural industrialization management modes have been formed: one is the contract production mode represented by "company + farmer", and the other is the vertical integration mode represented by the employment relationship. In fact, at present, the main body of pig breeding in China is still traditional and scattered farmers, and is subject to many practical constraints such as rising labor costs, tightening environmental protection policies, and increasing pressure on disease prevention and control, and generally faces the development dilemma of rising costs and profit squeeze. Therefore, it is of great significance to study the contract production behavior and performance of pig farmers for promoting the transformation and upgrading of animal husbandry and promoting the high-quality development of animal husbandry.

Theoretically, contract production mode can effectively improve the performance of farmers, but any classic organization mode needs a "localization" process when it is implemented in practice. In actual production, due to the different attributes of various agricultural products, farmers' trait and contract arrangements, as well as the differences in regional institutional arrangements, development stages and market conditions, a large number of agricultural contract defaults occur in China, and the impact of contract production mode on farmers' performance is also very different. Specific to the practice of live pig contract, it can be found that there is significant heterogeneity between live pig contract and other agricultural products contract. First, due to the differences in the natural attributes of agricultural products and the tightness of industrial chains, the formation and development of contracts for different agricultural products are not synchronized. Compared with other agricultural contracts, the production model of live pig contract was formed and developed later. Secondly, compared with the full-link production mode usually adopted by farmers under other agricultural products contracts, farmers under live pig contract production mode are often engaged in single fattening link production. Finally, the natural nature of opposing interests between companies and farmers, superimposed one-time game contract arrangement, makes the contract of agricultural products inherently unstable. However, another fact coexisting with a number of defaults of other agricultural products is that the cooperative relationship of live pig contract production mode is still relatively stable under the background that African swine fever poses a long-term disease threat to the breeding industry, the impact of market price risk, and the uncertainty of live pig circulation link.

Based on the above analysis, it is reasonable to believe that compared with other agricultural contracts, the live pig contract production mode has its particularities in terms of contract formation, farmers' participation motivation, contract operation mechanism and farmers' performance, etc. Especially in the case of industrial environment uncertainty caused by African swine fever, what factors affect the contract participation behavior of companies and farmers? What is the effect of contract production mode on the performance of pig farmers and its internal mechanism? What strategic actions will companies and farmers take to maintain the stability of the relationship during the contract period when the uncertainty of the external environment increases? How will this affect the cooperative performance of the contract production mode? These are the important practical problems facing the transformation of production mode in the current pig industry. In view of this, this paper comprehensively uses the theory of institutional change, the theory of division of labor, the theory of transaction cost and the theory of incomplete contract to carry out theoretical analysis of the above problems, and then carry out empirical tests in turn. Firstly, using the provincial panel data and the continuous DID model, the impact of African swine fever on the development of live pig contract production companies was empirically analyzed. Then, using the survey data of pig farmers in Jiangsu Province, the Probit model and endogenous conversion model were used to empirically analyze the influencing factors of farmers' contract participation, the effect of contract production mode on farmers' performance and its internal mechanism, as well as the influence mechanism of rising uncertainty on the cooperation performance of contract farm. The main conclusions of this study are:

First, the impact of African swine fever has a promoting effect on the growth of the number of live pig contract production companies. Specifically, for every 1% increase in the intensity of pig disease, the number of pig contract production companies in the region increased by about 3.2%. In addition, the impact effect of African swine fever had a certain lag and continued to weaken. From the perspective of the mechanism of the related entities, the impact of the African swine fever epidemic has intensified the segmentation degree of the live pig market and improved the scale of breeding, and the potential profits of the cooperation between the company and farmers have increased and the organizational costs have decreased, thus promoting the development of the live pig contract production company. Heterogeneity analysis showed that the impact of African swine fever had a promoting effect on the number of enterprises in key development areas and restricted development areas, but had no significant effect on the number of enterprises in potential growth areas and moderate development areas.

Second, the uncertainty of external environment is an important factor affecting the contract participation behavior of pig farmers. With the increase of uncertainty, the probability of farmers participating in contract production increases. For every 1% increase in pig market risk and production uncertainty, the probability of farmers participating in contract production mode increases by 15.1% and 18.5%, respectively. Heterogeneity analysis showed that production uncertainty had no heterogeneity on farmers' contract participation behavior, while market risk impact had heterogeneity on farmers' contract participation. Specifically, market risk impact has no significant effect on the contract participation behavior of farmers with higher risk preference, small and medium-sized scale and non-northern Jiangsu region, but promotes the contract participation behavior of farmers with risk-neutral and risk-averse type, large scale and northern Jiangsu region. In addition, farmers who are older, more educated, more engaged in pig breeding and more experienced tend to operate independently. The greater the degree of regional market segmentation, the greater the probability of farmers participating in contract production.

Third, compared with independent farmers who are self-breeding, contract farmers are only engaged in a single fattening link, which can obtain the division of labor economy brought by specialized production and the economies of scale brought by the easing of credit constraints, thus improving the production efficiency of farmers. Based on the counterfactual hypothesis, if contract farmers did not participate in contract production, their feed to gain ratio and mortality rate of fattening pigs increased by 25.80% and 62.77%, respectively. In addition, the contract production model improves the production efficiency of farmers through the technology adoption mechanism. Heterogeneity analysis showed that contract production mode expanded the gap of feed to gain ratio and narrowed the gap of mortality rate among farmers. The larger the farming scale, the greater the inhibition effect of contract production mode on the mortality of pigs. In addition, the contract production mode has obvious inhibitory effect on feed to gain ratio of farmers in main producing areas, and has more obvious inhibitory effect on fattening mortality of farmers in main selling areas.

Fourthly, compared with the full market transaction mode, the contract production mode increases the operation income of farmers by improving production efficiency and reducing transaction risks. Under the counterfactual assumption, if contract farmers did not participate in contract production, their net income would decrease by 69.34%. Heterogeneity analysis showed that the contract production model could not promote the income increase of farmers in small-scale and main selling areas, but had a positive effect on the net income of farmers in large-scale and main producing areas, reflecting that in practice, some farmers adopted the contract production model successfully while others did not. The possible reason is that the transaction cost induced by small and medium-sized farmers and farmers in main marketing areas from self-breeding to contract fattening production is greater than the income brought by contract production.

Fifth, uncertainty has a heterogeneous effect on the cooperative performance of contract production mode. The specific effects and mechanisms are as follows: (1) Under the impact of market risks, the company tends to adopt the hidden default behavior of delaying the recovery of bred pigs and lowering the price of live pigs, thus weakening the effect of increasing production and increasing income of the contract production mode. In addition, under the cooperation mode of "batch and signing", the company tends to extend the empty period of contract farms under the impact of market risks, thus weakening the income increasing effect of contract production mode; (2) When production uncertainty increases, the contractual risk-sharing mechanism will cause both parties to pay more attention to the allocative efficiency of factors, and the company will not carry out implicit default behavior, but the company will not change the benefit distribution mechanism, resulting in the negative impact of contractual production mode on the ratio of feed to gain of farmers, while the effect on mortality and net income is not significant. In addition, when the degree of market segmentation increases, in order to reduce the welfare loss of producers, the company may transfer part of the cost to farmers by adjusting the prices of materials and live pigs in the settlement system, thus weakening the yield increase effect and income increase effect of the contract production mode.

Based on the above research conclusions, this study puts forward the following policy implications: First, strengthen the support for leading pig enterprises and accelerate the construction of the whole industrial chain of live pigs; The second is to guide different scale farmers to carry out appropriate management mode according to local conditions and reasonable and orderly; The third is to standardize the contractual cooperation between the company and the farmers; Fourth, improve the supervision and assessment mechanism for supporting policies of leading enterprises.

Compared with the existing studies, the possible innovations of this study are as follows: First, the existing studies on farmers' contract participation behavior lack a targeted analysis of market constraints, and there are few literatures to analyze the factors affecting the development of contract production companies. Based on the theory of institutional change, this study analyzed the impact mechanism of the African swine fever epidemic on the development of pig contract production companies, and analyzed the contract participation behavior of pig farmers from the perspective of external environmental uncertainty, which is conducive to a deeper understanding of the internal mechanism of the formation and development of agricultural contract organizations. Second, regarding the study on the impact of contract production mode on the performance of farmers, the academic community has not given a consistent conclusion. This study, based on the characteristics of pigs and the theory of division of labor, reveals the internal mechanism of the impact of contract production mode on the performance of pig farmers, and enriches the research content of the cooperation performance of agricultural contract organizations. Third, based on the perspective of the company's implicit default behavior, this study explores the impact mechanism of rising environmental uncertainty on the cooperation performance of contract production mode. Compared with the abstract analysis that ignores external environmental conditions, this study is not only more objective in terms of research conclusions, but also provides an important basis for the academic community to comprehensively understand the contract relationship of agricultural products.