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Crossbreeding programs in dairy cattle currently stand as a viable alternative in the dairy industry, because they may alleviate problems related to elevated homozygosis, which has occurred as a result of intensive selection programs using pure breeds. The data utilized of the set of analysis presented hereby, was collected from four commercial dairy farms located in northern Italy between 2007 and 2013. All the farms had a common management system and were using the same 3-way rotational crossbreeding program, including Holstein(HO), Montbeliarde(MO), Swedish Red (SR). The farmers were using also 2-way rotational crossbreeding program, including Holstein(HO) and Brown Swiss (BS). The most common reason for the interest in the use of cross breeding programs by dairy producers is the need to improve cow fertility, health and survival, traits that have progressively deteriorated during the last few decades. A common index used for the evaluation of fertility is the interval between calving and conception, more commonly known as days open. The reproductive performance of crossbred cows has been evaluated by many authors. However, many of these studies reported a value of days open truncated at 250 d. This approach could reduce fertility differences between breed groups. Therefore, the availability of time related variables, such as days open, are influenced by culling time of the cow and by the time of data collection. Data from those cows for which the insemination event, or the conception, have not occurred by the time of culling or data recording, are missed, which leads to some bias on fertility parameters. The objective of the second chapter was to investigate the effect of crossbreeding on fertility traits, studying reproduction time interval traits through survival analysis and success traits and inseminations to conception through logistic regression of heifers and primiparous cows. Results from both cows and heifers suggested that crossbreeding can improve fertility of purebred dairy herds. Crossbred animals showed early insemination after calving and also became pregnant before purebred Holstein cows. Crossbred cows showed a higher probability to conceive at the first insemination, and required less inseminations to achieve conception. The favorable effect in heifers was less pronounced than in primiparous cows. Differences in milk yield between pure HO cows and crossbreds has been well characterized, however the information available on milk lactation curves of crossbred cows, such as peak yield, time at peak, and persistency, is less abundant. The objective of chapter 3 was to evaluate milk curve parameter of primiparous HO and crossbreds cows, using Wilmink function (1987) and modified Wilmink function , in order to include the effect of pregnancy. Milk lactation curves parameters showed different values between pure HO and crossbred cows. Holsteins showed a higher level of production than all crossbred cows. On the contrary, the increase in milk yield towards the peak was not different between pure HO and crossbreeds. Some crossbreeds showed a difference in the persistency of the lactation The effect of pregnancy on milk production was lower for MO × HO and MO ×( SR × HO) crossbreds cows than for pure Holstein, but the gestation started to affect milk production earlier for these two crossbreds. The use of pregnancy on the model showed a general increase of the accuracy of milk production prediction after the milk peak, relative to the classic Wilmink model. The effect of pregnancy is also reported to had an effect as well on milk yield, as on milk component. Furthermore, as reported before, crossbreds cows showed higher reproductive ability then pure HO, and became pregnant before, so that the effect of gestation on the production start earlier in the lactation than for pure Holsteins. The aim of chapter 4 was to compare milk production and milk components of pure HO and crossbred cows, accounting for the effect of pregnancy stage in the model. Pure HO cows showed higher milk yield than all the crossbreds cows. The superiority of HO for yield traits was found also for protein and fat yield, which were higher for HO cows than all crossbreeds, except for BS × HO crossbreds that showed not differences in these traits, when compared with pure HO. However, this inferiority in production was accompanied by a higher milk quality for crossbreds cows. All crossbreds showed higher protein content than pure HO cows and SR × HO crossbred s cows showed also higher fat content than pure HO. Lower values of SCS were reported for SR × HO and BS × HO crossbreds cows, compared with pure HO. Whether the benefit from increased fat and protein content in milk can offset a reduced yield of milk and milk components will depend on specific pay systems. The marketing of cheese has grown in recent years, and cheese plays an important role in the economics of dairy production. So that is important to consider the quality of milk not only in terms of protein and fat content, but also to consider the aptitude of milk to be transformed into cheese. The objective of the last chapter was to evaluate the effect of crossbreeding on milk quality traits, traditional milk coagulation properties and curd firmness model obtained from individual milk samples. Relative to pure HO, the crossbreds used in our study showed more desirable MCP, showed a faster coagulation time and higher curd firmness values. Furthermore, differences between crossbreds for these parameters were also found. Our results suggest that the use of crossbred animals can impact milk yield, milk composition and its MCP. Depending on the intended use of milk, different types of sires can be chosen to direct the efforts of farm crossbreeding programs. |
