Hi.
A little update.
After a bit research, a load cell seems like the best solution. I just have to find one cheap enough 
My plan so far is to in each beefarm have one control box(master), and several scales(slave) that will communicate with each other using wireless transcivers.
The control box will be powered by a lead acid battery, and contain an microcontroller, a GSM module, a RTC, a wireless transceiver, sensors for measurment of temperature, humidity and rainfall and maybe a load cell with a fixed load for using as a referance.
For the hive scales I was tinking of using a single load cell made for platform scales. For the op-amp I was thinking about the ina125 http://www.ti.com/lit/ds/symlink/ina125.pdf since it got an onboard voltage referance and a sleep mode.
For communication I have looked at various NRF24L01+ transceiver module since it is cheap and have a low power consumption.
To power the scale I was thinking of using a small solar panel to charge up a supercapasitor. when the supercapasitor is charged up, the atmega will wake up from sleep, measure the weight of the hive, transmit it to the master, and then go back to sleep. I only need 5 or 6 samples during the day, so the atmega and radio, will be in sleep mode for 99% of the time.
Does this sounds manageable solution? I do not have much experience with op-amps, and I have never used wireless transceivers. So if someone has any opinions regarding to these, I would be very grateful