Genetic variability of bovine milk proteins and their associations with health and growth parameters of pre weaned beef and dairy calves
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Beschreibung
Genetic variation in farm animal populations is critical for the adaptation to future changes in climate and consumer demand, as well as for continual genetic improvement of economically important traits. Hence, one aim of this study was to analyze the variability of the caseins, which account for 80% of the total milk proteins, in divergent selection lines of German cattle breeds. Due to their influence on economic important production traits (i.e., milk yield) and moreover on technological properties of milk, milk protein polymorphisms are a valuable tool to detect selection.
In particular, due to their positive impact on milk yield and protein content of milk, milk protein polymorphisms have also gained in importance in beef cattle, as these factors are directly linked to the growth rate of suckling beef calves. Based on this knowledge, a further aim was to analyze the effects of maternal milk protein genotypes on the growth performance of beef calves.
Even more often, milk protein variants are associated as a potential object of functional nutrition, whereby particularly casein-derived bioactive peptides are of special importance. At present, there is an ongoing debate of the health-promoting benefits of the original A2 variant of β-casein (β CN) in comparison to the A1 variant caused by a point mutation during evolution. The focus of this argumentation is on the opioid peptide β-casomorphin-7 (β CM7), which is mainly released by digestion of the A1 variant and may be a serious risk factor in the etiology of human diseases. Against this background, an in vivo study was conducted, in which neonatal calves were fed exclusively with pure A1- or A2 milk. Possible effects of these alternative milk feedings are recorded by several growth and health parameters (i.e., body temperature, fecal score) as well as by plasma levels of β-CM7. These findings will also have an impact on human health by contributing new prospects to the field of functional foods and pharmaceutical products.
On the basis of these investigations, the present study finally highlights the potential of bovine milk protein polymorphisms as a future tool for breeding activities. Especially diverse breeding strategies for the herd conversion into the favorable A2 genotype as well as the challenges of marker assisted selection are given.
As it can be seen here, the present study covers various aspects of milk protein polymorphisms including different data sets. Therefore, the structure of this work is divided into a set of chapters. In order to introduce to the overall topic, chapter 1 serves as a general introduction. Accordingly, the importance of bovine milk in human nutrition but also as the main source for neonatal calves is highlighted. Furthermore, information about the function and properties of especially the four main milk proteins - the caseins - was provided.
Chapter 2 addresses the genetic diversity and population structure in divergent selection lines within the German cattle breeds Holstein Friesian (HF), German Black Pied cattle (DSN) and German Simmental (GS) on the basis of milk protein polymorphisms. Whether the different weighting of traits in individual breeding goals (i.e., pasture ability) already has an impact on allele frequencies of loci under selection, should be analyzed through the example of milk protein polymorphisms. Simultaneous typing of genetic variants of the milk proteins αs1-casein (αs1 CN), β-casein (β CN), αs2 casein (αs2 CN) and κ casein (κ-CN) was carried out on skimmed milk by isoelectric focusing. Allele and haplotype frequency distributions indicate different breeding strategies in production systems with or without grazing. For instance, the pasture-based HF selection lines displayed significant higher allele frequencies for the favorable κ CN B allele in comparison to the HF reference population kept indoors. In grazing systems, the focus of selection has emphasized fertility, fitness and robustness. Thus, the prevalence of the B allele (κ CN) in pasture-based selection lines may be the result of an indirect selection for these traits.
The comparison between present results and historic data reveals noticeable changes in allele frequencies reflecting that casein loci or moreover selected mutations in close proximity to the caseins underlie selective breeding. Increasing frequencies were especially observed for the alleles β-CN A2 and κ-CN B, whereas low-frequent alleles like αs1-CN A and αs2-CN D could not be identified. This loss of alleles goes along with a decreasing diversity at the casein loci and suggests a continual monitoring of the milk protein variability.
Based on the knowledge that milk protein polymorphisms are related to milk composition (e.g., protein content), chapter 3 covers the impacts of maternal casein genotypes on growth traits of their suckling calves. Birth weight, average daily weight gains (ADG) and the age-adjusted weaning weight at 180 days of life (WW180) were recorded for calves of the breeds German Angus (GA) and GS. Linear mixed models were used considering either separate fixed effects of individual casein loci or composite αs1-β-αs2-κ-CN genotypes (order of genes on BTA6) to include interactions among single loci (i.e., epistatic effects). Greater ADG and WW180 were observed for calves from dams with the genotypes κ CN BB and αs1 CN BB, respectively (P < 0.05). For example, WW180 was greater for calves from dams carrying the κ-CN genotype BB (215 kg) compared to calves representing the maternal AB and AA genotypes (both 204 kg). Allowing for epistatic effects, the composite casein genotype BBǀA2A2ǀAAǀAB (αs1-ǀβ-ǀαs2-ǀκ-CN) was associated with greater ADG and WW180 of calves (P < 0.05). The favorable effect of the composite genotype BB|A2A2|AA|AB on growth traits (ADG and WW180) may be caused by other quantitative trait loci (QTL) for body weight, being in linkage disequilibrium with the casein genotype. Such pronounced genetic impact of selectable casein genotypes offers new perspectives for breeding strategies aiming on improvements of pre-weaning calf performance due to the nutritional value of colostrum or milk. Hence, selection on favorably associated casein genotypes might contribute to economic efficiency through a shortened fattening period and reduced feeding costs.
The in vivo study referred to chapter 4 serves to analyze comparative effects of milk containing A1 versus A2 β-CN on health and growth of neonatal dairy calves. This research was based on several studies indicating health impairing effects (i.e., apnea, inflammatory response) of the A1-derived opioid peptide β CM7. In total, forty-seven neonatal calves of the breeds German Holstein (GH), GS and their crossing (GH × GS) were fed either homozygote A1- or homozygote A2-milk during their first three weeks of life. Fecal score (FS), respiratory frequency, and rectal body temperature were recorded daily, whereas body weight was measured at birth and at day 21 to estimate ADG. In consideration of environmental influences (i.e., sex) in linear mixed models, marked differences in daily milk intake and fecal consistency were observed between both feeding groups. Fecal consistency was softer for calves fed A2 milk (P < 0.05). While 44% of A2 calves had diarrhea or revealed a tendency towards it (FS ≥ 3), A1-calves had a prevalence of 21%. Nevertheless, similar ADG and end weights of calves fed A1- or A2-milk (P > 0.05) indicate that A2 milk may compensate higher diarrhea rates and lower daily milk intake due to a higher protein content.
In the course of this feeding study, blood was collected from calves three times during the experimental period. For the first time, the respective plasma samples were analyzed for intact β-CM7 via an enzyme-linked immunosorbent assay (ELISA). Intact β-CM7 was detected in plasma of both feeding groups, while the concentration of β-CM7 in plasma of calves fed A1-milk was 5 times higher than in plasma of calves fed A2-milk. Evidence from this study suggests that, due to the change in the amino acid sequence, A2 milk in animals and probably in humans might be able to prevent or, at least, to minimize the cleavage of β-CM7.
At last, chapter 5 completes the whole thesis with a general discussion on all themes covered and analyzed. Against the background of the current breeding development, an outlook is given to future selection decisions, trends and the potential of the usage of milk protein polymorphisms in cattle breeding.
Autor*in
Lisa Gisela Hohmann