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The purpose of this study was to compare the effect of residual feed intake (RFI), residual average daily gain (RADG), feed to gain (F:G) on productivity in Brahman heifers (n=242). Following a 56-d post-weaning period for six calf crops, Brahman heifers were individually fed for periods of 60-d at 2.8% of their body weight in a Calan gate system. Rations were adjusted at weekly intervals. For each contemporary group, the mean and standard deviation of RFI, RADG, and F:G was calculated. Based on the mean plus or minus ½ standard deviation for each group, heifers with the lowest RFI and F:G were classified as efficient (E), while higher RFI and F:G heifers were classified as moderately efficient (ME) or inefficient (I). For RADG, heifers with the highest RADG were classified E, while lower RADG heifers were classified as ME or I. Cow records for age at first calving (AFC), interval from the first to second calf (CI) and calf records for birth weight (BW) and 180-d adjusted weaning weight (AWW) were recorded. Data were analyzed using mixed linear models. The additive genetic effects were modeled as random in all analyses. In analyses of calf traits, maternal additive genetic effects were also random effects. For all traits, cow year of birth was investigated as a fixed effect. For calf traits, calving interval, the calf birth year was a fixed effect. Calf breed, season of birth, and sex were fixed effects in analyses of calf traits. The effects of interest were 3-level classification variables created using the upper and lower quartiles and the two middle quartiles combined of RFI, RADG, and FG. These were modeled in distinct analyses. In analyses of RADG, the high (inefficient) and low (efficient) RFI groups of heifers differed (0.03 ± 0.018 and -0.04 ± 0.02, respectively; P=0.03). In analyses of F:G all RADG groups differed (P<0.01) as expected; that is E (8.0 ± 0.25) < ME (9.5 ± 0.24) < I (12.6 ± 0.25; P< 0.01). Heifer AFC and CI were not significantly influenced by efficiency classifications for RFI, RADG, or F:G (P>0.10). As expected, male calves had greater BW (P<0.05). Classification for efficiency group did not influence either BW or AWW. RFI, RADG, or F:G did not affect reproductive or calf performance in young Brahman cows. Selection for RFI will result in selecting against RADG or F:G. [ABSTRACT FROM AUTHOR]

The forage-animal interface is a vital component of agricultural systems that is often overlooked by both agronomy and animal science programs. As such, current grazing models do not always provide a satisfactory understanding of how forage species and/or cultivars respond to livestock interaction in both production and physiology, how forage can influence the animal, and how the nexus of these two entities may affect the environment. The objective of this effort was to develop a conceptual framework of a dynamic model that combined the forage and ruminant livestock interactions, and to provide the initial parameterization of a model for bermudagrass pastures. The systems dynamic methodology was used. The causal loop diagram (CLD) was first envisioned based on the hypothesis that not only can forage factors affect performance and behavior of the grazing animal, but the grazing animal, through its herbivorous behavior and selective intake, can affect and alter the physiology, growth, and performance of the forage and the pasture. Generation of a preliminary stock-and-flow diagram (SFD) with variable flight simulators was also performed. The main CLD was a composite of four sub-models that represented the agronomic, animal, atmospheric, and soil subsystems. The limitations of this model existed in the lack of meta-analytical publications in which necessary equations have been summarized that could be used in model programming. The parameterization of the model suggested that forage allowance was the driving factor in animal ADG, and forage growth rate was governed by a combination of evapotranspiration, defoliation, soil N, and precipitation. Our simulation of the bermudagrass scenario suggested that under these production conditions, most portions of the model operated in a traditional goal-seeking fashion. The initial results provided the documentation of the interface concept for the benefit of forage agronomy and animal nutrition programs as well as the foundation for advancements of the model to eventually serve as potential decision aid in production-economic strategies. [ABSTRACT FROM AUTHOR]