Project Number: 6619-31630-003-00
Project Type: Appropriated

Start Date: Jul 16, 2007
End Date: Jul 15, 2012

Objective:
The long-term objective of this research is to develop an improved understanding of those traits and genes that are responsible for the ability of cattle to adapt to and withstand the harsh conditions of the subtropical region of the United States. Over the next 5 years we will focus on two objectives: Obj. 1) Complete the evaluation of the influence of breed and breed-type on cow performance in the subtropics by 1a) Estimate heterosis and breed effects for efficiency and adaptability in cows developed with Romosinuano (tropical Bos taurus), Angus (temperate Bos taurus), and Brahman (tropical Bos indicus) breeds and 1b) Develop and evaluate a tropically-adapted composite targeted for the subtropics, and Obj. 2) Determine the cascade of physiological responses to chronic stressors responsible for and indicative of reduced animal performance in subtropical environments by 2a) Determine if heat and other stressors elicit a differential immune response in cattle with different tolerance to environmental stressors and 2b) Evaluate QTL markers identified for tolerance to internal parasites in cattle in the subtropics.

Approach:
For Obj. 1a) After the opportunity to calve as two-year-olds (born from 2002 through 2005), purebred Romosinuano (R), Brahman (B), and Angus (A) cows (n = 120) and crossbred R-B, B-R, R-A, A-R, B-A, and A-B cows (n = 240) will be bred through their 8th breeding season to produce purebred, F1, and three-breed calves. This mating design and subsequent analyses is appropriate for estimation of heterosis for cow maternal traits, i.e., weight and other size traits of their calves. Reproductive traits evaluated will include pregnancy, calving, and weaning rate. Feed efficiency will be determined on a subset of steer and heifer calves. Heifer calves will be developed as replacements for a subsequent project. For Obj. 1b) Following their 8th breeding season, beginning in 2011, cows will be used in the development of a composite ½ Angus ¼ Romosinuano ¼ Brahman. This will be accomplished by using: R-A, A-R cows and B-A, A-B bulls, and B-A, A-B cows and R-A, A-R bulls. Similar cow and calf traits will be measured as in Obj. 1a. For Obj. 2a) Two experiments will be conducted, the first in environmentally controlled rooms and the second under ambient conditions at STARS. For the first, laboratory analyses will be completed for samples collected from 9 R and 9 A steers following: acclimation to thermal neutral conditions, after animals were subjected to heat stress conditions, and after recovery from heat stress, in response to an an endotoxin challenge. In a second experiment, A, B, and R steers at STARS will be sampled during winter and summer under ambient conditions. Feed intake, rectal temperature, and respiration rate will be collected. For both experiments concentrations of cytokines, acute phase proteins, IGF-I, and growth hormone will be determined. For Obj. 2b) Blood and fecal samples will be obtained from all purebred and crossbred calves prior to weaning. Samples will be sent to ARS-BARC where DNA will be extracted and stored from all calves, and fecal samples will be analyzed for nematode egg counts. The number of eggs per gram of feces (EPG) for each animal will be determined. Estimation of genetic control, i.e., heritability, will be determined from these data. At the same time, data from the purebred Angus calves will be used to evaluate the relationship among the EPG phenotype with QTL markers developed at ARS-BARC.