"This drop is a result of expected lower average yields in the south where fields have been significantly damaged by drought in recent weeks," AgRural said in a report.
The southern soya region accounts for 34 percent of Brazil's total soyabean output.
AgRural is the latest in a series of forecasters that began lowering estimates for Brazil's soya crop a couple of weeks ago due to the lack of rain in the south, principally in No 3 soyabean state Rio Grande do Sul.
Private analysts Safras e Mercado said on Friday that drought in Brazil's southern soyabean belt had shaved 2.4 million tonnes off the forecast for the 2004/05 crop, to 61.16 million tonnes from the 63.57 million forecast on December 10.
On Friday, Fecoagro co-operatives federation in Rio Grande do Sul said it had reduced its forecast for the states output to 7.5 million tonnes from 9.4 million seen initially because of the lack of rain.
On February 9, the US Department of Agriculture pared back its estimate for the Brazilian soya crop to 63 million tonnes from its previous estimate of 64 million.