This habilitation thesis focuses on the impact of acute and maternal heat stress (HS) on production and functional traits and on metabolism of dairy and dual-purpose cattle and offspring considering the temperature humidity index (THI) as environmental descriptor. In this context, the comprehensive introduction, the discussion and the 10 original research papers address the different “omics” levels “phenomics” (phenotypic association studies), “genomics” (genetic parameters, genome wide association studies, gene annotations), “epigenomics” (DNA methylation profiles), “metabolomics” (metabolite concentrations) and “transcriptomics” (gene expression levels).
The original research paper 1 focused on the impact of acute HS during the early life of dual-purpose calves on their weight development and of acute and time-lagged HS on fertility traits of dams under pasture-based conditions. Across-generation effects of maternal HS during late gestation are also content of this paper. Our results indicated acute and time-lagged HS effects and limited acclimatization processes to heat, especially during the cooler spring and autumn months. Similarly, the original research paper 2 focused on across-generation effects of maternal HS during late gestation on production and functional traits, however, in dairy cattle. We found an unfavorable impact of HS from the dry period on time-lagged performances in offspring on most primary and functional traits, even on longevity.
Metabolic aspects of acute HS during late gestation on German dairy cows and of across-generation effects of maternal HS on female progeny are content of original research paper 3 and 4. The results showed altered concentrations of different plasma metabolites from different biochemical classes between animals suffering from direct or maternal HS or not with potential to serve as biomarkers for HS.
The impact of time-lagged HS during late gestation and postpartum on genetic parameter estimates for birth weight and weight gain in dual-purpose calves is presented in original research paper 5. Additive genetic variances and heritabilities of all traits increased under challenging climatic conditions indicating a higher selection response in beef cattle traits under HS conditions. Genetic correlations substantially smaller than 0.80 indicated genotype by climate interactions, proven by substantial alterations of sire EBVs for production traits in dependency of time-lagged climatic alterations. In a similar context, maternal HS during late gestation was analyzed for its impact on production and functional traits in dairy cattle in the original research paper 6. Birth weights and birth weight genetic parameters of dual-purpose and dairy calves in context of time-lagged effects due to HS during late gestation and genotype by climate interactions for birth weight are also presented in this paper. The results showed detrimental effects of prenatal HS on birth weight in offspring. Direct and maternal heritabilities for calf birth weight were independent from prenatal HS effects. No genotype by climate interactions on calf birth weight were identified for maternal genetic effects. We concluded that the decline in birth weight is more likely caused by maternal permanent environmental effects instead of genetics, why we suggested to avoid HS during the dry period of dams.
The original research paper 7 focused on direct and maternal genetic effects and the annotation of potential candidate genes for weight and meat quality traits of dual-purpose cattle, raised under outdoor climatic conditions. For intramuscular fat content (IMF), we estimated quite high heritabilities and we identified a quite strong maternal genetic component on weight traits as well as on IMF. This is probably due to the intensive calf-cow relationship in the outdoor suckler system. We suggest the application of models with maternal genetic effect in genetic evaluations, and we recommended to develop a breeding goal including IMF for the used dual-purpose cattle breed.
Direct and maternal genetic effects, and maternal genetic sensitivity on prenatal HS for calf diseases and corresponding genomic loci in German Holsteins were analyzed in the original research paper 8. Genotype by climate interactions for calf diseases in dairy calves, suffering from prenatal HS were analyzed. Our results showed that HS, particularly during the last week of gestation, contributed to increased calf disease incidence. Maternal heritabilities for pneumonia increased with increasing heat. We identified 30 suggestive and 2 significant SNPs from the GWAS and 43 genes, annotated as potential candidate genes. Based on these genes, 3 biological processes were inferred, indicating association between the genetics of prenatal HS mechanisms and immune physiology and disease resistance mechanisms. Original research paper 9 addressed the estimation of genotype by time-lagged HS variance components for milk production traits of the offspring generation, and main and interaction SNP-marker effects for maternal HS during late pregnancy. The results indicated quite small effects of HS during late pregnancy on the test day and lactation milk production traits. Genotype by HS interaction variances varied, depending on the week with in-utero HS. Eight out of 31 suggestive SNPs affect cattle functionality, indicating a possible favorable correlation between functional traits and heat tolerance. Two biological processes contributing to immune response mechanisms, were inferred with suggestive interaction effects.
In original research paper 10, we studied HS effects on 3 different “omics features” which were methylations, gene expressions and metabolic pattern from a direct perspective in pregnant cows and from an indirect time-lagged intergenerational perspective in offspring. Differences in DNA methylation between maternally heat stressed and not heat stressed calves were proven in this paper, supporting the hypothesis of an epigenetic percentage, explaining across generation effects of HS. With regard to the intergenerational perspective, transcriptomic and metabolic profiles showed obvious clustering between dams and calves. In the context of HS separation, gene expression and methylation data clearly separated HS and control groups. However, in the multi-omics analysis, methylation information was the least important omics tier to classify dams in terms of direct HS, when compared to the other omics datasets.
Overall, the studies of this habilitation thesis present important findings for future research studies regarding heat stress mitigation strategies in cattle under German climatic conditions and breeding programs in context of heat stress resistance.
