Dairy production in Zimbabwe is principally from exotic (Bos Taurus) strains of cattle. These cattle have been genetically enhanced progressively since their introduction into the nation early this century. The section will explain the status of crossbreeding in Zimbabwe, means through which this genetic improvement has been achieved and describe the breeding programs currently running in the country and offer suggestions as to how these may be strengthened. Traits of economic importance to Zimbabwe dairy farmers and their genetic parameter estimates are also discussed.
According to Smith et al (1996) and Muchenje et al (2007), the smallholder dairy division is led by the Nkone (35%), Jersey (30%) and the crossbreeds (25%) with extra breeds such as Red Dane, Friesian, Ayrshire, Sussex and Tuli. Typically adaptability (35%), availability (25%) and milk yield (20%) were used to select which breed type to use (Muchenje, 2007). Crossbred animals indicated an advantage for deprived resource farmers particularly in Semi-arid areas. This is because of the connection between the level of inputs, environment and breeds (Moyo et al, 1993 and Garwe et al 2001). Garwe et al concluded that native breeds need to be subjected to selection of production traits and the impression of adaptation to be elucidated.
According to Bebe et al (2003), the mainstream of farmers (60%) use commercial bulls, 25% use own bred and 15% purchase bulls. He included that about 70% of the farmers performs uncontrolled breeding. Additionally, the bulls will be of unidentified pedigree, despite knowing generally the genotype showing that there is absence of organized selective breeding.
Due to this, the consequence of unproven bulls and less artificial insermination may enforce hostile effects on production and the degradation of herd genotype in the long run. The use of artificial insermination and proven bulls may have a value to decrease inbreeding.
Garwe et al (2001) also detected that 60% of farmers observed estrus on cows, 75% of farmers separated calves from dams two weeks after birth, 60-70 days after calving of dry period was stated by over 90% of farmers, 40% of farmers cullied their cows at 7th parity, 77% kept milk yields, date of birth (75%), mortality (65%), purchases and sales (55%) and animal health (40%).
Holstein / Friesian, also recognized as the black and white, is by far the supreme leading dairy cattle breed in Zimbabwe, establishing more than two thirds of the country’s dairy cattle population. This is followed by the Jersey, which accounts for about 10% of this population. Guernsey, Ayrshire and the Red Dane breeds occur in fairly small numbers. The performance levels of these breeds in Zimbabwe are below. In addition to the above-mentioned breeds, there are also small numbers of Dexter, Milking Shorthorn and Sahiwal.
Milk recording is the objective and organized measurement of individual dairy cow performance. It is an imperative tool for genetic improvement, as only through performance records can genetic evaluation and accurate selection be carried out.
Official milk recording has been practiced in Zimbabwe since 1929. The milk – recording scheme was run by Dairy Service, a branch of the Department of Research and Specialist Service, until 1993 when The Zimbabwe Dairy Herd Improvement Association (ZDHIA) took over. Through the years the scheme has progressed from a manual and cumbersome system to a state of the art technology.
Since the goal of crossbreeding is to combine two, three, or four different breeds in order to achieve some necessary traits from each different breed, researchers measure crossbreeding’s achievement by quantifying hybrid vigor to get a feeling of what benefit there could possibly be by crossbreeding a herd. Conserving hybrid vigor is ideal when running a successful crossbreeding program. Hybrid vigor is best significantly addressed in the second and later generations where hybrid vigor tends to drop in percentages. Dairymen will usually select purebred sires for crossbreeding their already crossbred cow to maintain hybrid vigor. Another way in maintaining hybrid vigor is by using more than a combination of two breeds. Choosing between a two breed and three breed crossbreeding program can be evaluated by observing the amount of heterosis maintained from generation to generation. In a two breed crossbreeding program heterosis can drop off to 50% in the second generation cross and continue to stay between 50 and 70 percent all the way into later generations and finally leveling off at 67%.
A system of 4 breed’s results in a high heterosis on average, but the amount of breeds will eventually cause the cross to be diluted of desirable traits achieved from each breed (Pro Cross, 2009). According to Creative Genetics, of California, a three way cross is optimal when deciding on how many breeds to use when crossbreeding. The combination of three breeds provides for maintained heterosis in the second generation, and continuing to stabilize at around 285% (Pro Cross, 2009). A three way crossbreeding could prove to be the best option if heterosis does not drop off as much as anticipated, and the result of less dilution of breed traits when equated to a four breed crossbreeding program. Along with breed combinations, selecting the best bulls is the fundamental to a positive crossbreeding program.
Selecting bulls for their feet and legs, udders, or other traits that are lacking in the dam that will be bred are ideal for maintaining hybrid vigor. A crossbreed program capitalizes on traits that can be improved through heterosis by selecting proven breeds and targeting their strengths in order to improve a herd’s profitability.
Heterosis is attributed to intra and inter loci allelic interactions (Lopez- Villalobos et al, 2000) and is greater for breeds which are genetically diverse (Cassell, 2007). In production traits heterosis ranges from 0-10% and 5-25% for fertility traits (Buckley, 2014 and Swan, 1992). Greatest heterosis in the present day was observed for Holstein- Jersey F1 cow. This is not surprising because the breeds are genetically diverse (Sorensen et al, 2008). Presently, heterosis for Holstein-Friesian F1 is least because the breeds are genetically very similar (Sorensen et al, 2008).
The heterosis estimates for milk yield (5.5%) and the milk solids (6.4%) in Holstein-Jersey F1 cows was found to be larger than estimates of 3.7% and 5.8% for milk yield and milk solid in Holstein-Friesian F1 cow respectively (Prendiville et al, 2011) in a small controlled study carried out in Iceland. Crossbreeding is a tool used to reverse a decline in reproductive performance occurred due to “Holstenisation” (Buckley et al, 2014). Heterosis obtained by Coffey et al, 2016 was less than heterosis estimates obtained by Sorensen (2008) and Buckley (2014). Coffey et al (2016) observed a short calving interval of Holstein- Jersey which is consistent with the study by Penasa et al (2010).
Prendiville et al (2009) reported that for herbage intakes, the pure Holstein cow had an intake of 16.9kg of Dry Matter per cow per day and the Holstein and Jersey crossbred cow had an intake of 16.2kg of Dry Matter per cow per day. The Holstein Friesian had a lower intake of Dry Matter per 100kg however, 3.39kg of Dry Matter compared to 3.63kg of Dry Matter for the Holstein and Jersey cow (Vance et al., 2012).