گروه مهندسی مکانیک ماشین آلات کشاورزی

The term "Precision Agriculture" is here defined as "Variable Rate Application" (VRA)" of

cultivation inputs as e.g. seeds, fertilizers, pesticides and in principle also number and/or intensity

of soil tillage operations according to the variable demand within the field. It has always

been obvious to farmers, advisers and researchers that there is an infield variation in the

need of different input factors, and that VRA technology should have both agricultural and

environmental advantages. Around 1990 a new positioning technique, GPS, made it possible

to practice VRA. Investigations and research on the field variation in soil and plant parameters

were intensified in the early 1990's and VRA technology was improved and became

cheaper for the farmers. But even if the concept of VRA is obvious and the technique is available,

the commercial use today is very limited. That is the case in Denmark, Germany and

other North European Countries and it seems to be the case also in the rest of the world. In

Denmark use of VRA is limited to application of lime (approximately10 percent of the area

limed) and VRA of nitrogen is limited to application of fertilizer by 5-10 spreaders equipped

with the Yara-N-sensors. The use of VRA for liming seems to be significantly reduced from

2002 to 2004 (Pedersen, 2005). It seems that we have a situation, where we have the VRA

technology, the concept has obvious advantages from both an agricultural and an environmental

point of view, but only few farmers are using it and the use even seems to decrease. As

responsible for giving advice on the use of fertilizers we need to evaluate, if the farmers here

miss an opportunity for improving their profit and/or to reduce the environmental impact because

we have not been able to convince the farmers of the benefits of the concept. The reason

for this might be, that we have not been able to prove that the technique is profitable, provide

the correct technical solutions or the needed support to the farmers. The reason might

also be that the potential benefits are simply insignificant with the current technology and

knowledge.

Several investigations have been made to find the yield differences between uniform and

VRA of fertilizer. Most of the studies show no or a very limited yield response for VRA, if

the average level of application of nutrients is the same with both uniform and VRA. The Danish

Agricultural Advisory Service have since the beginning of the 1990's made a number of

such studies with nitrogen application trying to find the potential yield response and to test

different systems for VRA e.g. the Yara N-sensor. Most of the trials have been carried out in

fields with a high infield variation. The conclusion has been that the potential of VRA is typically

very limited and probably below 0.1 ton per ha and year in cereals. The conclusion of

our work is also, that the conditions in Denmark with limited N-quotas does not increase the

potential yield response. Some reasons for this relatively low potential yield gain for VRA are

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Normally the response curve for nutrients inclusive nitrogen is increasing until a plateau is

reached and yields will only start decreasing again with excessive over fertilization. If the environmental

impact of fertilization e.g. nitrogen leaching is included in the calculations, the

profit of VRA must be higher. We still have not made investigations of how VRA could be

used to reduce the environmental impact of fertilizations, but from a theoretical point of view

there should be a potential. If we look at the historical cereal yields for the western European

countries, the yield increase per year in the 1980's and 1990's was about 0.1 ton grain per ha

and year. If we look at the yield increase in the 2000's it has been much lower and for some

countries it has nearly stopped. We have been much concerned with this in Denmark in relation

to the fixed nitrogen quotas imposed here. No doubt the restricted nitrogen quotas partly

explain the missing yield increase. Another explanation could be that there has been much focus

on rationalization of fieldwork and to reduce costs because of low grain prices. This

means that the labor input per ha is much lower now than 10 years ago. And the farms have

increased significantly in size, and it is very common now to cultivate 2-300 ha per man. Also

professional farmers have bought neigh-boring farms and are now cultivating what used to be

5-10 independent farm units.

It gives a much higher area per field and more fields per farm. To rationalize we have seen the

problem that the farmers do not want to change fertilizer or pesticide type or even amount of

fertilizer between the fields. So in practice this is the opposite of Variable Rate Application.

The need for rationalization will also in the future be very important for the farmers growing

bulk crops like cereals. If VRA should be used more extensively in the future, is must be designed

as a system that simplify fieldwork and not like today give extra work for the farmer.

It must be in all researchers' and advisers' interest to develop such systems because the concept

of VRA is right both from an agronomic and environmental point of view, and we must

regard it as a defeat if we are not able to implement the technique in practice.