First it is helpful to think about some of the most basic characteristics of food production systems: their land-, labor-, and capital-intensiveness (with capital including machinery, pesticides, etc.).

In terms of the difference in efficiencies between vegetable and commodity crop operations in the USSR, most of it comes down to one very basic fact of Soviet agriculture as a whole—it was highly labor intensive (output per unit of labor was roughly 25% of that in the US) and capital intensive(with fairly widespread mechanization, although trailing most other industrial nations). This is peculiar because often times labor intensive systems will transition to capital intensive ones when certain labor constraints necessitate a change. The Soviet approach to mechanization was also somewhat of a problem; while there was generally enough equipment for most of the USSR’s croplands, scrap rates were high—most likely because of the limited equipment being worked hard in order to cultivate vast tracts of land.

While I do not know specifically about the political conditions that lead to this structure, I can theorize it may resulted from industrial outputs being allocated on certain parts of Soviet agriculture without adequate consideration for real demand. While at the same time there was pressure to keep rural unemployment low, leading to the retention of rural ag. laborers despite mechanization.

Now, back to your question on efficiency in vegetable production. Labor intensive systems tend to be more viable for the production of vegetables, while commodity crops have shifted to capital intensive production in many parts of the world. Labor intensive cultivation is certainly appropriate in some situations, however it is not ideal when attempting to produce massive quantities of grains—which when combined with other factors (more on that just below) lead to disappointing yields. It is worth noting that the USSR had territory well suited to extensive, capital intensive agriculture—with reasonable land availability and industrial output.

In terms of the other factors mentioned above, from my understanding of Soviet farm collectives, there was a problem with the state intensively managing aspects of the farm operation that would ordinarily be left to those closer to the actual operation. For instance, the state would dictate the what crops would be grown and supplied inputs deemed necessary to produce these yields. From there the state also took responsibility for delivery to consumers. Workers on these farms were either compensated with a fixed monthly wage (with the state retaining the residual income after paying wages) or they were compensated with the residual yield of the farms (after the state took their quota). Considering the value of site-specific knowledge and, in turn, informed decision making in agriculture, this was most likely a serious part of why yields were so disappointing.

Now, there were also some notable crop failures—with the 1970s seeing the USSR import massive quantities of food due to weather related crop failure. This also somewhat ironically paved the way for modern US food policy, but thats a topic for another time.

If you’re interested in this topic, you may also want to research collective agriculture in China and Vietnam. Both of these systems experienced issues similar to the USSR’s, and eventually shifted away from collectivization with favorable results.

There were undoubtably other issues with Soviet agriculture, but hopefully the above gives you a general sense of how the structure of the system was inefficient and allowed for other problems to compound and lead to failures.

References:

Dovring, Folke. “Soviet Farm Mechanization in Perspective.” Slavic Review 25.2 (1966): 287–302. JSTOR. Web.

Ellman, Michael. “Soviet Agricultural Policy.” Economic and Political Weekly 23.24 (1988): 1208–1210. Print.

Griffin, Keith, Azizur Rahman Khan, and Amy Ickowitz. “Poverty and the Distribution of Land.” Journal of Agrarian Change 2.3 (2002): 279–330. Wiley Online Library. Web.