Figures from Scotland’s chief statistician suggest initial estimates for 2015 reveal a 15 per cent, which followed a 9 per cent drop in 2014.
It is only the second time since the 1990s incomes have dropped in two consecutive years.
The country’s rural chiefs said the decline emphasised the sector’s reliance on European subsidies, which are also decreasing.
NFU Scotland’s director of policy Jonnie Hall said: “Worryingly, [the] fall in Scottish farming’s total output is as much to do with decreased production as it is to do with decreased prices.
“These figures back up our argument that farmers need monies available through all support schemes as soon as possible, and justifies the pressure we have been putting on Scottish Government in recent months to deliver payments on time to cash poor businesses.”
Although not all the data is in, TIFF for 2015 may have fallen back to about £667 million, making it the second lowest in the past decade.
Agriculture was worth £777m to the Scottish economy in 2014, down from £837m in 2013, with potatoes and barley all seeing big falls.
The dairy sector suffered from the drop in price, with the poultry-meat industry also falling back, now having lost half its value in two years.
The beef industry saw a small decline in 2014 followed by a partial recovery in 2015.
Output from slaughter or sales of cattle amounted to an estimated £732m in 2015. However, the opposite pattern was seen by sheep farmers, with a ten per cent increase in 2014 followed by an estimated four per cent fall in 2015 to £202m.
Likewise the pig industry saw an increase in 2014 followed by a fall in 2015 to £85m.
Cereals fell 13 per cent both years, with barley now worth an estimated £198m and wheat £119 million.
The average milk price fell 23 per cent in 2015, resulting in a drop of 21 per cent in the overall value, to £364 million.
Eggs increased an estimated 14 per cent during 2015 to £94 million.
Potatoes saw two drops, of 24 per cent and 12 per cent, now down at an estimated £167 million.
Vegetables saw a steady 2015 after a 14 per cent fall in 2014, and now stand at £115 million.
Fruit was one area that saw growth in 2015, with increases in volume and price leading to an estimated 39 per cent increase in value to £128 million.
Feed costs are estimated to have fallen as much as £70 million in 2014, and may fall further in 2015.