Heritability of mastitis score in pacific northwest dairy herds

Unraveling the Genomic Association for Milk Production Traits and Signatures of Selection of Cattle in a Harsh Tropical Environment

A study was designed to identify the genomic regions associated with milk production traits in a dairy cattle population reared by smallholder farmers in the harsh and challenging tropical savanna climate of Bengaluru, India. This study is a first-of-its-kind attempt to identify the selection sweeps for the dairy cattle breeds reared in such an environment. Two hundred forty lactating dairy cows reared by 68 farmers across the rural-urban transiting regions of Bengaluru were selected for this study. A genome-wide association study (GWAS) was performed to identify candidate genes for test-day milk yield, solids-not-fat (SNF), milk lactose, milk density and clinical mastitis. Furthermore, the cross-population extended haplotype homozygosity (XP-EHH) methodology was adopted to scan the dairy cattle breeds (Holstein Friesian, Jersey and Crossbred) in Bengaluru. Two SNPs, rs109340659 and rs41571523, were observed to be significantly associated with test-day milk yield. No significant SNPs were observed for the remaining production traits. The GWAS for milk lactose revealed one SNP (rs41634101) that was very close to the threshold limit, though not significant. The potential candidate genes fibrosin-like 1 (FBRSL) and calcium voltage-gated channel auxiliary subunit gamma 3 (CACN) were identified to be in close proximity to the SNP identified for test-day milk yield. These genes were observed to be associated with milk production traits based on previous reports. Furthermore, the selection signature analysis revealed a number of regions under selection for the breed-group comparisons (Crossbred-HF, Crossbred-J and HF-J). Functional analysis of these annotated genes under selection indicated pathways and mechanisms involving ubiquitination, cell signaling and immune response. These findings point towards the probable selection of dairy cows in Bengaluru for thermotolerance.

Enhanced clinical mastitis resistance in Holsteins with a FEZL p.Gly105(12_13) polymorphism

Mastitis is a common infectious disease of the mammary gland and a major problem in the dairy industry. We previously reported that forebrain embryonic zinc finger-like (FEZL) encoding a stretch of 12 glycines (p.Gly105[12]) instead of 13 glycines (p.Gly105[13]) is associated with a lower somatic cell score (SCS) in a family derived from Walkway Chief Mark. Here we report that the p.Gly105[12] allele is associated with a significantly decreased incidence of clinical mastitis in a large Holstein population. We genotyped the FEZL polymorphism in 918 randomly collected Holstein sires, and investigated the effect of the polymorphism on the estimated breeding value (EBV) for SCS and milk, fat, solids-not-fat, and protein yield, and on the number of cattle with clinical mastitis among daughters derived from these sires. The average EBV for SCS among sires carrying the heterozygous p.Gly105[12] was significantly lower than that among sires carrying the homozygous p.Gly105[13], whereas we found no unfavorable effects of this polymorphism on EBV for milk, fat, solids-not-fat, and protein yield. The proportion of cows with clinical mastitis derived from sires carrying heterozygous p.Gly105[12] was significantly lower than that of daughters derived from sires carrying the homozygous p.Gly105[13]. Thus, selection of sires carrying p.Gly105[12] could be beneficial in the dairy industry by reducing the incidence of mastitis.

Genetics of health traits in Holstein cattle

Incidences of 22 health related traits were analyzed for 11,008 lactation records provided by 21st Century Genetics. Traits were analyzed both individually and grouped into one of five health categories. Significant sources of variation included herd-year, lactation, cow, and sire effects. Genetic parameters were estimated with multiple-trait REML. Repeatability and heritability estimates were low to moderate. Estimates ranged from .09 to .57 for repeatability and from .02 to .21 for heritability when individual traits were pooled by health category. Reproductive and respiratory traits were the least heritable (less than .10); mammary and locomotive traits were slightly higher (less than .20); and digestive category was most heritable (.21). Genetic correlations of mature equivalent milk with number of inseminations and other reproductive problems were positive (antagonistic), but those with incidences of other health problems were negative. Phenotypic correlations among mature equivalent milk and fat with individual health traits and categories of health traits were small and mostly near zero. Genetic correlations among health traits were positive except for reproduction with mammary and respiratory traits. Phenotypic correlations among health traits were near zero. Results suggest that selection for reduction of health problems is possible. If selection is practiced for improved health, these health traits should be properly weighted relative to production and other traits by the appropriate economic value. Evaluation of animals for health problems should be done on a multiple-trait basis.

Genetics statistics and relationships of teat and udder traits, somatic cell counts, and milk production

Teat lengths and diameters, udder clearance, cleft depth, and distances between teats were measured from 1967 to 1982 on Holstein cows. Numbers of observations per trait ranged from 5934 to 11,449. Cows were from six herds owned by the North Carolina Department of Agriculture and two research herds of North Carolina State University. Repeatabilities between adjacent lactations for physical traits of udder were mostly between .60 and .80. Repeatabilities between first and second lactations for average somatic cell count and mean log somatic cell count were .11 +/- .04 and .21 +/- .04. Heritabilities in first lactation were .63 for front teat length, .44 for teat diameter, .56 for rear udder clearance, .10 for cleft depth, .32 for mature equivalent milk, and .18 for somatic cell counts. Heritabilities of distances between teats ranged from .33 to .48. Cows with udder clearance higher from the ground, deeper clefts, smaller distances between teats, and teats of smaller diameter had lower somatic cell counts. Multiple regression analysis showed that dams' somatic cell counts and rear udder clearances were not significant predictors of daughters' somatic cell counts. Lactation average somatic cell counts of daughters were predicted with greater accuracy from averages of somatic cell counts of multiple lactations of dams than from single lactations of dams. Milk production and somatic cell counts seemed positively genetically correlated, more so in first than later lactations.

Udder and teat morphology related to mastitis resistance: a review

Annual losses by mastitis are estimated to be in excess of two billion dollars in the United States. Udder and teat morphologies have been associated with incidences of mastitis without clear conclusion on the closeness. Streak canal diameter was correlated with udder health, but it is difficult to measure. Udder depth and teat-end shape have been associated with udder health. Selection to reduce frequencies of cows with deep udders and flat, disk, or inverted teat ends may reduce mastitis incidence. Heritabilities of udder morphology are moderate to high, and a single score during the lifetime of a cow may be adequate for selection. There is no concensus in the literature on relationships between mastitis and teat size, general teat shape, teat-end lesions, teat pigmentation, or milk flow rate. Different breeds, milking procedures, measures of mastitis, and statistical procedures may account for different conclusions. Genetic approaches to improve mastitis resistance of dairy cattle seem warranted. This paper attempts to summarize evidence dealing with possible physical bases for genetic variation associated with mastitis in dairy cattle.

Genetic and phenotypic relationships between lactation cell counts and milk yield and composition of Holstein cows

Monthly somatic cell count data were collected between February 1977 and February 1982 for Holstein cows in 928 herds enrolled on the Quebec Dairy Herd Analysis Service. The geometric mean of the log monthly cell counts was calculated for each lactation. Official lactation records for 305-day milk, fat, and protein yields, and fat and protein percents were obtained for same cows. There were 18,189 cows in first lactation representing 257 sires, 13,225 in second lactation representing 206 sires, and 8,683 in third lactation representing 151 sires. Heritabilities of yield traits and protein percent increased across three lactations. Heritability of fat percent was similar in first and third lactations but decreased slightly in second lactations. Heritability of lactation cell count was small, being least in second lactations. Genetic correlations between lactation cell count and yield traits were positive in first lactations, small and negative in second lactations, and small and positive in third lactations. Genetic correlations between lactation cell count and fat and protein contents were small in the three lactations. Phenotypic correlations between lactation cell count and production traits were small in each of the three lactations. Genetic correlations between yield traits in early lactation and lactation cell count in a subsequent lactation were positive. The genetic correlation between protein percent in an early lactation and cell count in a later lactation was large between first and second lactations, decreased between second and third lactation, and small between first and third lactations. Genetic correlations were small and negative for fat percent.

Considerations in multiple trait evaluation

Traits of dairy cattle known to contribute to genetic merit are discussed briefly. Focus is on alternative measures and research to reflect more closely genetic pathways of performance because knowledge is expanding about cellular, organ, and animal function. Improved measurement of physiological traits of economic characters portends increased genetic control of animal productivity by considering more alternative traits. Research should clarify multiple trait selection programs to maximize profit. The null hypothesis to test is that selection for milk is optimal index selection for milk income and costs of production.