| The tillable field of Ningxia irrigation area is the ideal land for alfalfa seed production. It is very important to study alfalfa (Medicago sativa) seed production because alfalfa plays a crucial role for ecology, economy and society while our country advocates developing sustainable ecological agriculture and farmers are getting more and more benefit from alfalfa growing. This paper is aimed to study the relationship between alfalfa seed yield and sowing density; P fertilizer and the factors which affecting seed yield. The results were also analyzed in terms of ecology. There are many factors which influent alfalfa seed yield, such as ecological conditions, alfalfa community density, phosphate fertilizer, soil moisture, pollination, insect, weeds and time of harvest etc. In which, density, soil moisture, pollination and harvest time are most important.1. Study on relationship between alfalfa reproductive production and planting density The experiment was carried out with following density design: 90 cm of row spacing, 10, 15, 20, 25, 30, 35, 42 and 56 cm of distance between plants respectively. The results indicated that actual seed yield, estimated seed yield, racemes per m2, pods per raceme, seeds per pod, thousand grain weight were increased with the reduction of density. After the distance between plants was increased over 35 cm, they decreased gradually. The yield reached the highest at 35 cm, and the actual yield, estimated yield, racemes/m2 are 1560.1 kg/hm2, 11443 kg/hm2, 6753 respectively. Actual seed yield, estimated seed yield, racemes per m2, pods per raceme, seeds per pod, thousand-grain weight are significantly different among density treatments. The importance of effect on yield can be ranked as follows: racemes per m2, pods per raceme, seeds per pod, and thousand-grain weight. Inflorescences per m2 and flowers per inflorescence are little related to density. Plant height and shoots per m2 are positively related to seed yield. The germinating biability and germination rate of seeds performed the best after 2 months storing.2. Study on relationship between reproductive production and fertilizer The effects of fertilizer on alfalfa seed yield were studied and the application rates were 0 (control), 60, 120, 180, 240, 300 and 360 kg/hm2 (P2O5 based). The results showed that the P fertilizer could increase the seed yield, it was increased from 526.7kg/hm2 of control to 1680 kg/hm2 of P fertilizer treatment under 360 kg/hm2 of application rate. There was no significant difference between treatment under 60kg/hm2 of application rate and control. The rest of all treatments were significantly different from control. The effect of interaction between density and P fertilizer on seed yield showed that the actual yield is 1476kg/hm2 under 90cm of row spacing and 30cm of distance between plants, which is the highest among treatments. The seed yield is 180kg/hm2 the lowest under 120kg/hm2 of P2O5 application rate. The effect of density is more than that of P fertilizer In the experiment of NP interaction, the highest actual seed yield is 1606.7kg/hm2 while N=45kg/hm2 and P=90kg/hm2. The lowest actual seed yield was 984.5kg/hm2 while N=135kg/hm2 and P=360kg/hm2. In experiment of NPK interaction, the actual seed yields are 1351.2, 1295.6, 1200.1kg/hm2 respectively .The order is NPK1 ＞NPK2＞NPK3. The actual seed yield reaches the highest when N:P:K=45:120:30. The seed performs a good germination rate under all treatments.3. Studies on quantity features about inflorescence flowers and pod seeds of alfalfa In 2002, the quantity features about inflorescence flowers and pod seeds of alfalfa (Baralfa32 IQ) were studied and the result indicated that the inflorescence flower numbers shown a formal distribution pattern and it could be described as the following model: Y=. Meanwhile, the seed numbers in pod was also shown a formal distribution pattern and it could be described as the following model: Y=. In 2003, the quantity features about inflorescences/shoot,...
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