https://www.katahdins.org/pdfs/FAQ.pdf 

The above source's "Glossary of Terms" will be used to define the following words in this information - Commercial, Registered, Recorded, Grade, Crossbred, Purebred, Full-blood, Percentage, Up-grading and Grading  Up.  

IF/WHEN you are ram shopping - shop the genotype (pedigree/papers) as well as the phenotype (physical apperance). A faster growing (larger than the rest) ram lamb is more typically a single born lamb getting more milk than a twin.  And of course, triplet and quad born ram lambs are often the smallest, even when given supplemental milk/feed. There is also an inverse relationship to rapid growth and carcass traits per NSIP.  A ram born as a trip/quad can have limited growth due to limited milk and birthweight but this should not influence his offspring - environmental vs genetic. 

Permanent environmental effects — Environmental effects that result in permanent effects on the phenotypic expression of a trait. For example, severe mastitis during lactation may have a permanent effect on milk production and litter weaning weight for an animal in subsequent litters.

 A ram lambs ability to transfer his genetics (Penetrance refers to the probability of a gene or trait being expressed) is unknown until he produces offspring.  And remember, your ewe flock is still 50% of your genetic base and doing most of the work (gestation, lambing, nursing, grazing) and more of your interaction is with your ewes.   

And most of all remember - no matter how great the genetics are, you can ruin it by environmental means - improper nutrition, unsanitary conditions, lack of shelter, disease and stress.  

Using the Katahdin ram as a terminal sire & up-grading 

• Use commercial/recorded/registered Katahdin rams depending on your goals and budget. 
• Add/subtract genetic traits 
• Increase the sales price of your breeding stock and/or butcher stock
• Increase the growth rate and/or quantity and/or weight of lamb crops 
• "your mileage may vary" on cross-breeding and up-grading -  much depends on how you manage your flock and the genetics with which you started. 
•  Katahdin breed standards are rams that weigh from 180 to 250 pounds, HOWEVER, there are shepherds cross-breeding Katahdins with other sheep to produce rams above the breed standard (350 lbs +) and below the breed standard. Consider the size of your ewes and purchase a ram accordingly. 
• Terminal sires are used to create a fast growing hybrid animal, typically for slaughter.  Cattle, swine, sheep and poultry producers use terminal sires to create faster growing offspring.  If you desire the fastest growing lambs from your Katahdin ewes, you would need to breed them to another variety of ram.  The offspring are called F-1 hybrids and "terminal" sire program means they are destined for slaughter - not for sale as purebred breeding stock.  Hybrid vigor is amazing when purebred females are mated to a different breed sire.  You can do your own research, but keep in mind, you will still need to create the purebred replacement females for your ewe flock using a Katahdin ram.  Some shepherds do keep F-1 ewes as replacements, due to maternal heterosis.  That is one reason the Katahdin "open book" registry has been so successful.  Shepherds can breed in traits they need, and maintain a pedigree on all their sheep. 

Many folks in Texas started raising "American" Barbado or Barbados Blackbelly sheep or a cross-bred version. Versions of these popular crosses are called all sorts of names, mainly due to the game hunting ranches selling them as a different animal to go and shoot (Black Hawiian, Painted Desert, Corsican, Texas Doll/Dall)  Crossed with Mouflon and wool sheep, the "Barbado" is a mixed bag.  You just do not know what you have genetically in most cases.  We break them down between the Barbados Blackbelly (the imported polled sheep from the island) and the American Blackbelly (cross-bred with ?, typically with horns, spots, black, white, etc.)

 Blackbelly Barbados sheep -  When using a Katahdin ram as a sire expect lots of hybrid vigor from your F-1 lambs.  They will have better carcass traits and be more docile on average - there are a few strains of docile BB's out there. Select a ram on the smaller/lighter size for you first cross to avoid lambing issues.  Katahdin breed standards are rams that weigh from 180 to 250 pounds. If you use registered Katahdin rams you can eventually "grade up" to registered Katahdin staus. 

https://www.katahdins.org/pdfs/Katadhin_Breed_Characteristics_Standards_9-09.pdf

American/horned Barbado sheep - A Katahdin sire brings better carcass traits, improved docility and you can breed out the horns, unless you happen to find a registered horned Katahdin ram. If your Barbado sheep were a result of crossing with a Mouflon sheep, you will most likely see an increase in fecundity.  Pure Mouflon sheep typically have a single lamb. Barbado sheep without the "black belly" and/or with white/silver "saddle patches" usually have some Mouflon genetics.  Select a ram on the smaller/lighter size for you first cross to avoid lambing issues.  Katahdin breed standards are rams that weigh from 180 to 250 pounds.  By using a registered Katahdin ram for several generations, you can "grade up" to a registered status with your Barbado type ewes. 

https://www.katahdins.org/pdfs/Katadhin_Breed_Characteristics_Standards_9-09.pdf

Dorper sheep - Dorper sheep are a shedding sheep, not a true hair sheep as they are a cross between the Dorset wool sheep and the Persian Blackhead.  Thus the name Dor-Per using the first few letters of the contributing breeds.  They are not as parasite tolerant as Katahdins, nor do they shed as well as an A coat Katahdin.  Fecundity can also be an issue.  So depending on how pure your Dorpers are, expect better shedding, less de-worming, more lambs per birth and less carcass fat when crossing with a Katahdin ram.  By using a registered Katahdin ram for several generations, you can "grade up" you Dorper ewes to registered Katahdin status.  

 St. Croix - Similar to the pure island version of the Blackbelly, St. Croix have great parasite tolerance and fecundity.  But the carcass traits they lack can be improved using a Katahdin ram.  The St. Croix was/is the founding breed of "African hair sheep" used to create the Katahdin breed.  St. Croix ewes of good quality are good for "upgrading" to registered KHSI status. 

 https://www.katahdins.org/pdfs/KatadhinBreedHistory.pdf

https://www.katahdins.org/pdfs/FAQ.pdf 

The above source's "Glossary of Terms" will be used to define the following words in this information - Commercial, Registered, Recorded, Grade, Crossbred, Purebred, Full-blood, Percentage, Up-grading and Grading  Up. 

Glossary of Terms Note – There are multiple uses of these terms. We in KHSI use definition 1 in each case. But put in the second definition to note other common uses. Many breeders use Registered, Purebred and Fullblood interchangeably. Others distinguish between a Registered and a fullblood.

Commercial- A sheep that is not registered. A commercial Katahdin usually refers to a sheep that is of 100% Katahdin ancestry but is not registered. A second definition includes sheep that are either unregistered Katahdins or crossbred Katahdin.

Registered – A Registered Katahdin is listed in the KHSI flock book as 100% Katahdin. Registered sheep of other breeds are listed as 100% in their flock books. The KHSI Certificate of Registration is white.

Recorded- A Recorded Katahdin is an animal that is listed as a percentage animal by the KHSI Registry & Flock Book. Note: A Recorded Katahdin may have all Katahdin ancestry, but if a grand-dam or dam is not Registered it is can not be registered as a 100% Katahdin. It may be Recorded with the KHSI Registry. Or a Recorded Katahdin may be the offspring of a Registered Katahdin and a Registered Suffolk and be Recorded at 50% Katahdin. The KHSI Certificate of Recordation is blue. The percentage according to the KHSI Registry is listed on the Certificate.

Grade – Used in two manners. Some use this to designate a commercial Katahdin that is not Registered. Others use it to designate a crossbred Katahdin

.

Crossbred- Has parents of two or more breeds. Typically, animals that are 7/8 or greater blood of one breed are not considered crossbred, but of that breed. Some breeds of livestock designate animals with less than 15/16 or 31/32 are crossbred.

Purebred- Used in two manners. KHSI will use the 1st definition. 1) Designates an animal that is of one breed. 2) Some Breed Registries and breeders reserve the “purebred’ designation for only those animals that are Registered and 100%.

Full-blood- Used in two manners. KHSI will use the 1st definition. 1) Designates an animal that is of one breed. 2) Some Breed Registries and breeders reserve the “full-blood” designation for only those animals that are Registered and 100%.

Percentage – Used in two manners. A percentage Katahdin may refer to either 1) a Katahdin that is grading up to 100% in the KHSI Registry or 2) a cross-bred Katahdin that may be in a grading-up program.

“Up-grading” versus “grading up” – Up-grading will be used in this document to indicate that the breeding goal is to bringing in genetics from another breed or composite to improve a genetic parameter in their Katahdin flock or the breed. A breeder who used highly parasite resistant Florida Native ram on his/her Katahdins and then selected only those offspring with superior parasite resistance to record and eventually register is “up-grading”. A breeder who starts with a flock of commercial Katahdins or registered/commercial ewes of another breed and sequentially breeds them and their offspring to Registered Katahdin rams and eventually registering the 7/8 (87.5%) offspring is grading up.  

https://www.katahdins.org/pdfs/FAQ.pdf