We have studied the heat tolerance of Senepol cattle at the Subtropical Agricultural Research Station (USDA-ARS in cooperation with the University of Florida) near Brooksville, Florida, for many years and have published data showing that Senepol calves, heifers and cows are equal in heat tolerance to Brahman calves, heifers and cows. We have also determined that this heat tolerance is able to be passed on to F1 crossbreds of Senepol with cattle of temperate Bos taurus breeds such as the Angus and the Hereford. Senepol and Senepol F1 crossbred cattle with short, sleek haircoats are able to maintain rectal temperatures about 0.5° C lower than those of unadapted temperate breeds of cattle when they are under some degree of heat stress. They may maintain somewhat lower temperatures when they are not under heat stress. This may be concern under periods of cold stress.
The question of why Senepol and Senepol crossbred cattle maintain lower body temperatures has not yet been determined. Previous studies in Australia have demonstrated that sleek, dense coats are associated with low body temperatures and high growth rates while deep, wooly ones are associated with high body temperatures and low growth rates. This relationship apparently results from a dense flat coat being able to provide greater resistance to heat transfer to the skin; its smooth surface being able to reflect more radiation. The haircoat of the Senepol apparently is of this type.
I have long felt that the short, sleek, haircoat of Senepol and Senepol crossbred cattle is responsible for the heat tolerance of Senepol cattle and that it may be controlled by a single gene. Heat tolerance in other breeds of cattle has been determined to be a quantitative trait (one controlled by many genes) and has been shown to have a low to moderate heritability. Identification of a single, major gene in cattle that would reduce the effects of heat stress and its subsequent incorporation into temperate breeds such as the Holstein could have a major impact on cattle production in warm climates, particularly through increased embryo survival and greater milk production during periods of heat stress in dairy and dual-purpose cows. Cattle that are more heat tolerant should graze more during periods of heat stress and may be more productive as a result of greater feed intake under grazing management. This trait may become even more important to the U.S. dairy industry if environmental regulations force an increase in the use of grazing to reduce the concentration of large numbers of cows in confined areas as is commonly practiced in most U.S. dairies and certainly is of importance in tropical dual-purpose systems where cows must graze for all or nearly all of their nutrition.
Despite the fact that major genes with important impacts on productive traits have only rarely been identified in cattle, evidence from several sources encourages us regarding the existence of a hair length/heat tolerance gene. Criollo cattle, the cattle of the Americas descended from the first cattle brought to the new world by the Spaniards, are frequently extremely shorthaired ("slick") in tropical areas of Central and South America (Criollo Rio Limon of Venezuela and the Chino Santandereno, Blanco Orejenegro, and Romosinuano of Colombia), but never so in more temperate areas in both North (Texas Longhorn) and South America (Argentine Criollos). Thus, it appears that there was a selective advantage for this very short hair in the lowland tropics of the Americas. Secondly, the ease with which this slick hair coat and heat tolerance was incorporated so uniformly into the Senepol breed, as well as into the Carora of Venezuela, a Brown Swiss x Milking Criollo composite, by breeders untrained in genetics suggests a simple mode of inheritance. The facts that animals with normal hair coats occasionally segregate out from short, slick-haired Senepol and Carora parents and that several Senepol bulls have been identified that sire 50 percent normal-haired and 50 percent slick-haired progeny when mated to Angus or Holstein cows provide strong additional support for the existence of a major gene responsible for hair length and its related heat tolerance. Additionally, hundreds of progeny of Carora sires and Holstein dams in a large herd in Venezuela are either slick or normal-haired and not intermediate in hair length between the parental breeds; this also argues for the existence of a major gene that is segregating in the Carora breed. Finally, the discovery of upgraded Holsteins in Puerto Rico with the same slick haircoats as Senepol and Senepol crosses in 1997 seemed to confirm that a single gene was responsible for the slick hair coat.
While variation in heat tolerance between breeds and breed crosses has been studied for many years, relatively few efforts have been directed toward increasing our understanding of the mode of inheritance involved in heat tolerance and we are not aware of previous studies of a single gene with a significant impact on this trait. Confirmation that a major- gene exists in heat tolerant Bos taurus cattle that instills heat tolerance and is dominant in mode of inheritance would be the first step in the incorporation of such a gene into dairy cattle and other breeds of beef cattle that are adapted to temperate climates. In recent years we have studied the inheritance of the hair coat type of Senepol cattle in a number of different crossbred populations. First we looked at the progeny of Senepol x Hereford Fl crossbred cows when bred to Angus bulls. The calves segregated approximately 50:50 for the hair coat type I have described as "slick" and for normal Bos taurus hair coats. This is what would be expected if the F1 cows were heterozygous for a dominant gene responsible for slick hair. Also, under periods of moderate heat stress, the slick-haired calves maintained approximately a 0.5° C lower rectal temperature than the normal haired ones. In addition, after weaning the slick-haired calves gained faster during a period of grazing during the fall months. More recently, we have examined the progeny of Senepol x Angus Fl crossbred cows that have been bred to Charolais bulls over a period of years. These calves also segregate out at close to a 50:50 ratio of slick to normal-haired calves. The weight data on these did not show an advantage for the slick-haired calves. In fact, it is my current opinion that heat tolerance in beef calves is not important even in the southern regions of the U.S. prior to weaning as long as their dams are heat tolerant and they have access to plenty of good grazing and shade.
The impact of the slick hair gene appears to be quite dramatic in the grazing dairy in Puerto Rico where the slick-haired Holsteins were observed. When the owner of these slick-haired Holsteins was asked whether they were more heat tolerant than his normal-haired Holsteins, he responded, "I don't know; I only know that they are more productive!" Data (DHI records) from his farm support to his strong statement. The average first lactation herdmate deviation of six slickhaired Holstein cows was over 3,500 pounds. This is slightly more than 25 percent of the herd average! Each of these six cows was classified as an "A" cow during her first lactation. While these data include any slick-haired cows that were currently in this herd and not those that may have been culled due to low production, they do affirm the owner's statement, "I know if they (heifers) are slick-haired that they are going to be good." A possible explanation for the increased productivity of the slick-haired cows in this herd is that it is a grazing dairy; very little supplemental feeding is offered to the cows except for concentrate feed consumed in the milking parlor and a liquid supplement. Apparently the slick-haired cows are willing to graze for additional hours during the day and/ or are more efficient due to their increased heat tolerance and, therefore, are able to produce at a higher level.
Another study of the impact of the slick hair gene has recently been completed in Venezuela. This herd, however, was maintained in a very dry area and under drylot conditions where the cows had access to shade and did not graze. Under these conditions there was not a high level of heat stress and purebred Holstein cows were being utilized. The reproductive rate in the herd, however, was a problem and it was for this reason that the cows were crossbred with Carora. The F1 cows from this cross had a calving interval that was 60 days shorter than that of the purebred Holsteins. About 70 percent of the Carora x Holstein Fl were slick-haired. A comparison of the slick-haired to the normal-haired F1 cows showed that the slick haired cows had over a 0.4°C lower rectal temperature, about a 23day shorter calving interval (first to second calving) and produced slightly more (~700 pounds) milk per lactation than their normal-haired contemporaries. Cows that were 75% Holstein: 25% Carora were also compared in this study. The slick-haired cows from this group maintained rectal temperatures about 0.6°C lower, also had a 23-day shorter calving interval, and produced over 1500 more pounds of milk per lactation than normal haired cows of the same breed composition.
The data that I have reported here indicate the importance of the slick hair gene to the Senepol breed. I have been concerned for some time with the fact that some Senepol bulls are not homozygous for this gene. Three Senepol x Angus F1 cows in our study at STARS were not slick-haired and all were sired by the same Senepol bull who also sired one slick-haired cow. This bull clearly was heterozygous for the slick hair gene that I have symbolized as Sk. The normal-haired allele to the Sk gene is represented by the symbol sk+. Thus, the geneotype of the bull that sired the normal-haired cows would have to have been Sk/sk+. If such a bull is bred to normal-haired cows (sk+/sk+), we would expect him to sire half normal-haired and half slick-haired calves. There are several fairly popular Senepol bulls that I am aware of that are Sk/sk+, that is, heterozygous in genotype. For those of you with some background in population genetics, you might be asking what is the gene frequency of the Sk gene in Senepol cattle, since it is obviously not 1. My guestimate is that it is around 0.9. If one accepts this estimate, the frequency of hairy Senepol calves that would be expected to be about one percent. It would also mean that about 80 percent of Senepol bulls would be homozygous for the Sk gene and thus always be expected to produce slick-haired heat tolerant calves. The question that you might have then is, "Is my Senepol bull homozygous for the slick hair gene?"
We are currently doing research that we hope will result in a DNA test that would answer very quickly the question of homozygosity for the Sk gene. This is not yet available so it could take longer to determine whether or not your bull is homozygous for Sk. First of all, if your bull has EVER sired a SINGLE Senepol calf that had hair like a Red Angus or Red Poll during the summer and while in good health, he is NOT homozygous for Sk. Before you would declare him as heterozygous and perhaps reduce or eliminate his use from your breeding program, however, it would be wise to make sure that he is indeed the sire of the hairy calf through use of a DNA test for parentage. Now if you can say that he has never sired a hairy Senepol calf, this doesn't necessarily mean that he is not heterozygous. This is because about 80 percent Senepol cows would be expected to be Sk/Sk (homozygous) in genotype and thus would always produce slick-haired calves even when bred to Sk/sk+ bulls. A good test to determine if a bull is homozygous is to breed him to ten to 15 Angus, Hereford, Black Baldy, or other normal-haired Bos taurus cows and wait for the calves to be born. Don't get too concerned about the haircoats of the calves at birth, especially if they are born in the winter. It seems that often calves that are Sk/sk+ in genotype are born with some hair. Wait until the calves are five to six months old and the weather has been hot so that the winter coats have been shed out. At this point ALL the calves of bulls that are Sk/ Sk (homozygous) should be slick-haired. The only place other than the switch that there could be a little hair is on the poll, and there shouldn't be much there. Most of the time, about half the progeny of a heterozygous bull (Sk/sk+) will be normal-haired and half slick-haired.
The question might arise, "Should I use a bull in my Senepol breeding program that is heterozygous?" My recommendation would be to not use him unless he is extremely superior for traits in which the breed particularly needs improvement. This is because half of all his calves even when bred to Sk/Sk cows will be heterozygous. If these heterozygotes are bulls and used as sires, say in Brazil or Australia, about half their crossbred calves will be "hairy" and thus may discourage the further use of Senepol bulls.