Genome-Wide Association Studies for Methane Production in Dairy Cattle.

Genomic selection has been proposed for the mitigation of methane (CH4) emissions by cattle because there is considerable variability in CH4 emissions between individuals fed on the same diet. The genome-wide association study (GWAS) represents an important tool for the detection of candidate genes, haplotypes or single nucleotide polymorphisms (SNP) markers related to characteristics of economic interest. The present study included information for 280 cows in three dairy production systems in Mexico: 1) Dual Purpose (n = 100), 2) Specialized Tropical Dairy (n = 76), 3) Familiar Production System (n = 104). Concentrations of CH4 in a breath of individual cows at the time of milking (MEIm) were estimated through a system of infrared sensors. After quality control analyses, 21,958 SNPs were included. Associations of markers were made using a linear regression model, corrected with principal component analyses. In total, 46 SNPs were identified as significant for CH4 production. Several SNPs associated with CH4 production were found at regions previously described for quantitative trait loci of composition characteristics of meat, milk fatty acids and characteristics related to feed intake. It was concluded that the SNPs identified could be used in genomic selection programs in developing countries and combined with other datasets for global selection.

Economical and biological efficiencies of beef cattle differing in level of milk production.

Economical and biological efficiencies of beef production to weaning and to slaughter were estimated in three groups, different in milk available (low, medium, and high) to the calves but with the same potential for growth. Data from different breed groups of cows (low [L] = Hereford x Angus, medium [M] = Red Poll x Angus, and high [H] = Milking Shorthorn x Angus) were used. Economical efficiency was the ratio of income to expenses and biological efficiency was the ratio of calf weight to total feed energy required. Income was derived from cull cows and calves at weaning or carcasses of calves fed to slaughter. Feed and non-feed expenses for the cowherd and for calves to weaning or to slaughter were included in economical efficiency. Efficiencies were estimated assuming observed reproductive rates and energy requirements for maintenance, as well as for equal reproductive rates and equal energy requirements for maintenance in the M and H groups. With the observed reproductive rates and maintenance requirements, biological efficiencies to weaning and to slaughter were 28.1, 27.2, and 27.5 g of weaning weight and 22.0, 20.4, and 20.3 g of carcass weight per megacalorie of ME for L, M, and H, respectively; the corresponding values using equal reproduction and equal maintenance in M and H were 28.3, 27.2, and 27.4 g of weaning weight and 22.1, 20.5, and 20.5 g of carcass weight per megacalorie of ME.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy requirements for maintenance of crossbred beef cattle with different genetic potential for milk.

Maintenance energy requirements were estimated in two gestation and one lactation feeding trials for three groups of cows. All cows in the second gestation trial completed the first gestation and lactation trials. The three groups were chosen to represent cattle similar in growth rate and mature size but different in amount of milk provided to their calves. The low (L) group included Hereford x Angus, the medium (M) group included Red Poll x Angus and the high (H) group included Milking Shorthorn x Angus cows. Cows were individually fed to maintain net body weight (minus gravid uterus for gestation) constant. Allowances were made in energy intake for gestation and lactation. Cow weights were adjusted to an average condition score in each trial. Daily maintenance requirements during gestation were 18% lower than those during lactation. The H and M cows required 12% more energy per unit metabolic weight than L cows to maintain body weight during both gestation and lactation. Differences in milk production explained 23% of the variation in maintenance requirements, suggesting that important differences exist beyond those associated with milk production potential. Repeatabilities of maintenance requirement measurements ranged from .44 to .64. Maintenance requirements for calves under feedlot conditions in the postweaning phase were estimated from data collected from 494 calves, half-sibs and offspring of the cows described previously. Energy requirements were 11% higher for the H and M groups than for the L group.

Reproductive performance and variation in body weight during annual cycles for crossbred beef cows with different genetic potential for milk.

Reproductive performance and body weights were studied in 81, 79 and 88 spring-calving beef cows from three groups, reasonably similar in growth rate and mature size but different in level of milk production (low [L] = Hereford x Angus, medium [M] = Red Poll x Angus and high [H] = Milking Shorthorn x Angus). Milk group did not have a significant effect on dates of first postpartum estrus and first and last breeding or in the intervals from calving to first estrus, to first breeding and to last breeding. A significant age-of-cow x milk-group interaction was found for pregnancy rate (PR, P less than .10) and calf-crop percentage (CC, P less than .05). In heifers, PR and CC were lowest for the L (85.6 and 73.6%) and highest for the M group (94.2 and 92.5%); in 2-yr-olds there were no differences, and in older cows, PR was highest for the L (94.8%) and lowest for the H group (91.6%), and there were no differences in CC. As estimated from Brody's growth curves, mature sizes were 530 +/- 5.7, 471 +/- 4.6 and 499 +/- 4.2 kg for the L, M and H groups, respectively. Of weights taken on four occasions during the year, October weight was the highest for all cows. February weight, minus the estimated gravid uterus weight, was the lowest for L and M cows, and May weight was lowest for H cows. Weight fluctuation over an annual cycle was greatest for the L group. Under these experimental conditions, cows in all groups were able to consume sufficient energy and gain or lose body reserves to achieve similar reproductive performance while differing in lactation performance.

Biological efficiency to weaning and to slaughter of crossbred beef cattle with different genetic potential for milk.

Biological efficiency of beef production to weaning and to slaughter was estimated in three groups of cattle reasonably similar in growth and mature size but different in the amount of milk available to the calves (low (L) = Hereford x Angus, medium (M) = Red Poll x Angus and high (H) = Milking Shorthorn x Angus). Efficiency was defined as the ratio of estimated kilograms of calf weight weaned or carcass weight produced by a herd of 100 first-cross cows to estimated metabolizable energy (ME) intake by the cows and preweaning non-milk ME intake by the calves or preweaning non-milk and postweaning ME intake by the calves. Efficiencies were estimated assuming observed and average reproductive rates, and using observed energy requirements for maintenance, as well as the same requirements for maintenance in the M and H groups. With the observed reproductive rates and requirements for maintenance, efficiencies to weaning and to slaughter were 28.1, 27.4 and 27.4 g weaning weight per Mcal ME and 22.0, 20.6 and 20.3 g carcass weight per Mcal ME for the L, M and H groups, respectively. With average requirements for maintenance, efficiencies were 28.2 and 27.5 to weaning and 20.8 and 20.4 to slaughter for the M and H groups, respectively. Under equal reproductive rates and observed maintenance requirements, efficiencies were 28.3, 27.2 and 27.7 to weaning, and 22.1, 20.6 and 20.6 to slaughter for the L, M and H groups, respectively. With average maintenance requirements, efficiencies were 27.5 and 27.4 to weaning and 20.7 and 20.5 to slaughter for the M and H groups, respectively. Across the production output and input assumption scenarios, the L group consistently was the most efficient, especially when evaluated at slaughter of calves.