Culling: Soft vs Hard
This is an educational section on an often difficult-to-discuss, controversial subject. If you are easily upset or sensitive, please avoid reading beyond this paragraph. However, the subject of culling is important to include as this is something all breeders do, whether they know it or not. The general definition of culling as stated by the Merriam-Webster dictionary means to "select from a group" and "to reduce or control the size of something by removal of" which is done by pet breeders (whether horse, dog, or rat) by adopting out the animals to pet homes or euthanizing them.
https://www.merriam-webster.com/dictionary/cull
In the rat breeding community, culling is "eliminating animals from a breeding program," and is divided into two types: soft and hard. Soft culling is pet homing out rats that will not be used in the breeding program, and hard culling is euthanasia. All breeders use one form or the other, or both. Those who breed and adopt rats out are soft culling all the babies that go to pet homes. Breeders who hard cull will humanely euthanize by either CO2, cervical dislocation (CD), or freezing if babies are under 7 days old.
CO2 and CD, as detailed by the AVMA:
CO2: "Inhalation of CO2 causes respiratory acidosis and produces a reversible anesthetic state by rapidly decreasing intracellular pH. Both basal and evoked neural activity are depressed soon after inhalation of 100% CO2."
"A gentle death that takes longer is preferable to a rapid, but more distressing death. Gradual-fill CO2 exposure causes aversion in rodents beginning at approximately a 15% concentration and lasting to onset of unconsciousness. If an appropriate gradual displacement rate is used, animals will lose consciousness before CO2 concentrations become painful. A 20%/min gradual displacement produces unconsciousness in 106 seconds at a CO2 concentration of 30%; a slower 10%/min displacement increases time to onset of unconsciousness to 156 seconds at a CO2 concentration of 21%."
"—Carbon dioxide is acceptable with conditions for euthanasia in those species where aversion or distress can be minimized. Carbon dioxide exposure using a gradual fill method is less likely to cause pain due to nociceptor activation by carbonic acid prior to onset of unconsciousness; a displacement rate from 10% to 30% of the chamber volume/min is recommended. Whenever gradual displacement methods are used, CO2 flow should be maintained for at least 1 minute after respiratory arrest."
CD: "Cervical dislocation has been used for many years for euthanasia and, when performed by well-trained individuals on appropriate animals, appears to be humane." I will not quote the details as it may be disturbing for individuals, and the name "cervical dislocation" is quite self-explanatory.
https://www.avma.org/KB/Policies/Documents/euthanasia.pdf
Freezing, as detailed by the NIH Intramural Research Program:
"Fetuses and Altricial Neonates (note NOT Guinea Pigs): Hypothermia is an acceptable method of euthanasia for fetuses and altricial neonates up to 7 days of age as long as direct contact with ice/cold surfaces is avoided. Injectable anesthetic agent overdose can also be used."
https://oacu.oir.nih.gov/sites/default/files/uploads/arac-guidelines/rodent_euthanasia_pup.pdf
https://www.merriam-webster.com/dictionary/cull
In the rat breeding community, culling is "eliminating animals from a breeding program," and is divided into two types: soft and hard. Soft culling is pet homing out rats that will not be used in the breeding program, and hard culling is euthanasia. All breeders use one form or the other, or both. Those who breed and adopt rats out are soft culling all the babies that go to pet homes. Breeders who hard cull will humanely euthanize by either CO2, cervical dislocation (CD), or freezing if babies are under 7 days old.
CO2 and CD, as detailed by the AVMA:
CO2: "Inhalation of CO2 causes respiratory acidosis and produces a reversible anesthetic state by rapidly decreasing intracellular pH. Both basal and evoked neural activity are depressed soon after inhalation of 100% CO2."
"A gentle death that takes longer is preferable to a rapid, but more distressing death. Gradual-fill CO2 exposure causes aversion in rodents beginning at approximately a 15% concentration and lasting to onset of unconsciousness. If an appropriate gradual displacement rate is used, animals will lose consciousness before CO2 concentrations become painful. A 20%/min gradual displacement produces unconsciousness in 106 seconds at a CO2 concentration of 30%; a slower 10%/min displacement increases time to onset of unconsciousness to 156 seconds at a CO2 concentration of 21%."
"—Carbon dioxide is acceptable with conditions for euthanasia in those species where aversion or distress can be minimized. Carbon dioxide exposure using a gradual fill method is less likely to cause pain due to nociceptor activation by carbonic acid prior to onset of unconsciousness; a displacement rate from 10% to 30% of the chamber volume/min is recommended. Whenever gradual displacement methods are used, CO2 flow should be maintained for at least 1 minute after respiratory arrest."
CD: "Cervical dislocation has been used for many years for euthanasia and, when performed by well-trained individuals on appropriate animals, appears to be humane." I will not quote the details as it may be disturbing for individuals, and the name "cervical dislocation" is quite self-explanatory.
https://www.avma.org/KB/Policies/Documents/euthanasia.pdf
Freezing, as detailed by the NIH Intramural Research Program:
"Fetuses and Altricial Neonates (note NOT Guinea Pigs): Hypothermia is an acceptable method of euthanasia for fetuses and altricial neonates up to 7 days of age as long as direct contact with ice/cold surfaces is avoided. Injectable anesthetic agent overdose can also be used."
https://oacu.oir.nih.gov/sites/default/files/uploads/arac-guidelines/rodent_euthanasia_pup.pdf