Interesting comments, I have seen this trend in Brazil (soybeans, State of Parana) and it can be more or less pronounced depending on the region. However, part of the "random perturbation" can also be due to other factors such as availability of credit for growers to buy and apply inputs at the right time.

 

Clyde Fraisse



	-----Original Message----- 

	From: Roger Rivero [mailto:[log in to unmask]] 

	Sent: Tue 5/7/2002 2:54 PM 

	To: [log in to unmask] 

	Cc: 

	Subject: About Actual Trends In Yields

	

	



	Dear friends:

	

	In published literature about yield forecasting in middle (temperate) latitudes it has become a tradition (so it seems to me) to decompose yields time series in a linear or nonlinear mean trend plus a random perturbation that alternates in sign about this mean trend.

	

	The mean trend is almost always an upward one and have been related to the steady progress in technology (cultivars, management, etc.) while the random perturbation have usually been assigned to climate variability. This random component have been the subject of forecasts methods based in statistics, longe range weather forecasting, biophysical models or whatever.

	

	Though I don´t have enough data to make any assertion about it, I would like to know if any of you have been able to documentate such an upward trend in (actual) yields for low (tropical) latitudes.

	

	It seems to me that a trend in yield could be not only the result of technological development but of other factors as climate change. But what could be worse, an upward trend may not be present in lower latitudes because of a negative impact of climate change and unsustainable land management leading to progressive degradation of agricultural ecosystems in spite of technological improvements as irrigation, more productive cultivars and fertilizers.

	

	I would like to know if some of you have been able to documentate these trends in lower (tropical) latitudes.

	

	Roger E. Rivero Vega