**1. Introduction**

China is the country of origin of Chinese milk vetch (*Astragalus sinicus* L.), and also the country with the earliest utilization and cultivation of Chinese milk vetch and the largest planting area in the world. Chinese milk vetch, also known as grass seed, zi yunying and so on, is one of the main winter green manure crops in paddy fields of central and southern China [1–3]. In the 1970s, the planting area of Chinese milk vetch once exceeded 6.7 million square kilometers, mainly distributed in Sichuan, Hubei, Hunan, Jiangxi, Anhui, Jiangsu, Zhejiang and other provinces to the south of the Yangtze River. It has a strong ability of nitrogen fixation and high utilization efficiency, which can improve soil fertility and protect the ecological environment [4–6]. Currently, the harvest methods of Chinese milk vetch green manure seeds mainly include artificial harvest and mechanical harvest. Artificial harvest is time-consuming and laborious, and the yield of reserved seeds for planting is low. Generally, the seed yield of Chinese milk vetch in a paddy field is 300~650 kg·hm<sup>−</sup>2. There are two common methods of mechanical harvest: the first one is segmented harvest, which uses a rice, wheat, rape or bean swather to harvest Chinese milk vetch, and then, through natural drying, a thresher is used for threshing afterwards; the second one is combine harvest, done by adjusting parameters and changing working components of a traditional

**Citation:** You, Z.; Gao, X.; Yan, J.; Wei, H.; Wu, H.; He, T.; Wu, J. Design and Multi-Parameter Optimization of a Combined Chinese Milk Vetch (*Astragalus sinicus* L.) Seed Harvester. *Agriculture* **2022**, *12*, 2074. https://doi.org/10.3390/ agriculture12122074

Academic Editor: Jin He

Received: 9 November 2022 Accepted: 1 December 2022 Published: 2 December 2022

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grain harvester or rapeseed combine harvester to complete seed harvesting of Chinese milk vetch [7]. The segmented harvesting process is cumbersome and inefficient, and combine harvesting is of high efficiency, which is the development trend of Chinese milk vetch seed harvesting, however, the harvest quality of existing Chinese milk vetch green manure seed combine harvesters is affected by the unreasonable structure configuration of headers, weak separation ability of threshing mechanisms and poor impurity-removal function of cleaning devices. The loss rate of machine harvesting in field tests is in the range from 31.5% to 32.1% [8], which seriously affects the scale promotion and application of Chinese milk vetch.

At present, existing research on seed combine harvesters, both at home and abroad, mainly focuses on food crops such as rice, wheat, maize, as well as commercial crops such as rapeseed, soybean and flax. Wang et al. [9] designed a cutting table to be a stepless speed-adjustable telescopic structure to harvest rapeseed, and the threshing device was designed to be a longitudinal-axis drum with the same diameter and different speed. Wang et al. [10] developed a cleaning device with segmented vibrating screens, whose holes were round so that the cleaning rate and loss rate of maize grain could meet the requirements of national standards for maize grain harvesters. Zhang et al. [11] used the Plackett–Burman test method to study the impacts of vibration screen amplitude, crank revolving speed, fan revolving speed and fan dip angle on cleaning loss ratio and impurity percentage of rapeseed based on a two-roller and air-screen field mobile harvest testbed. Jin et al. [12] used a series of field trials to explore the influence of nine key working parameters on the quality of soybean harvesting operations, and figured out the optimal combination of parameters systematically. Shi et al. [13] designed a track combine harvester for hilly mountain flax, which included a crawler-type walking system, a low-damage header to prevent winding, a transverse-flow beater with the grain rod and rod teeth with small taper and narrow-grid concave plates, but the impurity rate was relatively high. Bruce et al. [14] studied the effect of threshing rotor speed and concave clearance on the threshing performance of shattering rape pods. Mekonnen et al. [15] presented the effect of a cross-flow opening on the distribution of flow along the width of a forward curved, wide centrifugal fan with two parallel outlets, and Computational Fluid Dynamics (CFD) was utilized to study the effect of the addition of a cross-flow opening on the performance of the fan using three fans of similar geometries but different in their cross-flow opening. Shreekant et al. [16] studied the effects of mechanical damage of soybean seeds during vertical bucket lifting, cleaning and grading on seed germination and seed activity.

Existing studies have provided some research on the seed combine harvester, however, there have been much fewer reports on the combined seed harvest of Chinese milk vetch. Therefore, in this paper, the Chinese milk vetch (*Astragalus sinicus* L.) seed combine harvester was developed based on the "World Group 4LZ-5.0E Ryzen Grain Harvester", the structure and movement parameters of the key components, such as the flexible anti pod-dropping seedling-lifting header, the longitudinal rod-teeth-type threshing device and the air-sieve-type layered impurity-controlled cleaning device were newly designed and optimized according to the harvest characteristics of Chinese milk vetch. The factors that had great influence on the seed harvest quality of Chinese milk vetch were chosen to be machine forward speed, rotation speed of the threshing drum, rotation speed of the fan and the scale sieve's opening. A four-factor and three-level response surface test was carried out, the effects of various factors on the evaluation indexes of seed harvest quality of Chinese milk vetch were explored and the ideal combination of parameters was obtained as well. On this basis, a combined Chinese milk vetch green manure seed harvester was developed, which could complete the work of lifting, dividing, cutting, conveying, threshing, cleaning and unloading of Chinese milk vetch at one time. The performance of the machine was verified by field test, which provides reference for the development and application of mechanized seed harvesting technology and equipment for different Chinese milk vetch varieties and other green manures.
