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Introgression of genes conveying resistance to heat stress into cattle populations using the “Slick” genetic variant as a model
There are ~270 million dairy cows globally and over 75% of these are found in hot climates and most of these have not undergone intensive, genetic selection. The ability to integrate genetically-improved cows into tropical cattle populations will underpin improvement of production performance.
The main impediment to the introduction of genetically-improved, high producing dairy cattle, typically from temperate countries, into tropical climates is the relatively poor heat tolerance and low tick resistance of the cattle breeds common in temperate climes. The concept of using gene introgression to improve heat tolerance and tick resistance was given momentum more than 15 years ago following the identification of “slick”, a major, dominant, gene for heat tolerance (and likely, tick resistance), segregating in the Senepol beef breed.
Our discovery, in 2013, of the “slick” causal mutation, a truncation of the prolactin receptor, provided the impetus to embark on crossbreeding of NZ dairy cattle with Senepol in order to enhance dairy performance in the tropics. A further advantage of the Senepol is that it is a Bos taurus breed, avoiding the potential disadvantages of Bos indicus crossbreds, which includes poor milk let down and late age at first calving.
The “slick” mutation is an enabling genetic variation which provides the necessary physiological traits (notably sweating ability) to improve animal welfare and performance in hot climates. The characteristics of NZ dairy breeds provide fertility, grazing ability and lactation performance on high roughage diets.
The primary objective of the breeding program is to produce homozygous “slick” bulls that have 75% NZ dairy genetics. The (5 year) breeding program requires crossbreeding to produce 50% dairy F1 daughters; using these animals as egg donors through JIVET to produce 75% dairy F2 offspring and then inter-crossing the lines to generate homozygous sires. Gene editing of the “slick” mutation directly into dairy sires is not a practical option in NZ at the current time.
Further improvements in tropical dairy cow development are possible if more genetic variations associated with heat tolerance are found, although introgression of additional variations (for example, coat color) will be challenging in a breeding program such as that outlined here.
Keywords: Senepol, thermoregulation, Dairy