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Biomedical subjects

J A Jennings

Publications and source records attributed to J A Jennings.

At least 19 recordsLinked to original sources
PubMed2006

Ruminal in situ disappearance kinetics of nitrogen and neutral detergent insoluble nitrogen from common crabgrass forages sampled on seven dates in northern Arkansas.

Southern crabgrass (Digitaria ciliaris [Retz.] Koel.) is often an undesirable species in field and forage crops, but visual observations suggest that livestock prefer it to many other summer forages. The objectives of this study were to assess the nutritive value of crabgrass sampled weekly between July 11 and August 22, 2001 and then to determine ruminal in situ disappearance kinetics of N and neutral detergent insoluble N (NDIN) for these forages. A secondary objective was to compare these kinetic estimates for crabgrass with those of alfalfa (Medicago sativa L.), bermudagrass (Cynodon dactylon [L.] Pers.), and or-chardgrass (Dactylis glomerata L.) as control hays. All kinetic evaluations were conducted with 5 ruminally cannulated Gelbvieh x Angus x Brangus steers (383 +/- 22.7 kg). Concentrations of N for crabgrass decreased linearly (P < or = 0.002) across sampling dates for leaf, stem, and whole-plant tissues. Conversely, percentages of the total N pool within NDIN and ADIN fractions generally increased over sampling dates in mostly linear patterns. For crabgrass, the immediately soluble portion of the total N pool (fraction A; overall mean = 54.6% of N) was greater (P < 0.001) than for all control hays. Crabgrass exhibited a more rapid N disappearance rate (overall mean = 0.093/h; expressed as a proportion disappearing/h) than that of bermudagrass (0.046/h; P < 0.001), but the disappearance rate for alfalfa N (0.223/h) was considerably faster (P < 0.001) than for crabgrass. The effective ruminal disappearance of N was greater (P < 0.001) for crabgrass (overall mean = 85.4%) than for the alfalfa (83.3%), bermudagrass (72.3%), or orchardgrass (76.0%) control hays. For alfalfa, the ruminal disappearance rate of NDIN (0.150/h) was more rapid (P < 0.001) than for crabgrass (overall mean = 0.110/h); however, the disappearance rate for crabgrass was faster than that for bermudagrass (0.072/h; P < 0.001) or for orchardgrass (0.098/h; P = 0.010). Effective ruminal disappearance of NDIN was greater (P < 0.001) for crabgrass (overall mean = 72.0%) than for the bermudagrass (69.0%) or alfalfa hays (50.5%), but there was no difference (P = 0.865) between crabgrass and orchardgrass (72.1%). Although crabgrass forages exhibited concentrations of total N that were comparable with those of alfalfa and rates of ruminal N disappearance that were < 50% of those for the alfalfa hay control, improvements in N use efficiency relative to alfalfa are questionable because of the excessively large Fraction A for crabgrass.

Animal Feed↗
PubMed2005

Ruminal in situ disappearance kinetics of dry matter and fiber in growing steers for common crabgrass forages sampled on seven dates in northern Arkansas.

Southern crabgrass (Digitaria ciliaris [Retz.] Koel.) is often viewed as an undesirable weed, largely because it encroaches upon field and forage crops, gardens, and lawns. However, visual observations of livestock grazing mixed-species pastures suggest that cattle seem to prefer crabgrass to many other summer forages. The objectives of this study were to assess the nutritive value of crabgrass sampled weekly between July 11, and August 22, 2001, and then to determine ruminal in situ disappearance kinetics of DM and NDF for these crabgrass forages. A secondary objective was to compare these kinetic estimates with those of alfalfa (Medicago sativa L.), bermudagrass (Cynodon dactylon [L.] Pers.), and orchardgrass (Dactylis glomerata L.) control hays. All forages were evaluated in situ using five (383 +/- 22.7 kg) ruminally cannulated crossbred (Gelbvieh x Angus x Brangus) steers. Whole-plant crabgrass exhibited more rapid (P < or = 0.002) ruminal disappearance rates of DM (overall range = 0.069 to 0.084 h(-1)) than did bermudagrass (0.054 h(-1)) and orchardgrass (0.060 h(-1)) hays, but disappearance rates were slower (P < 0.001) for crabgrass than for alfalfa hay (0.143 h(-1)). Effective ruminal disappearance of DM was greater (P < 0.001) for crabgrass (overall range = 69.3 to 75.4%) than for all the control hays. Similarly, disappearance rates of NDF for crabgrass (overall range = 0.069 to 0.086 h(-1)) were more rapid (P < 0.001) than observed for bermudagrass and orchardgrass hays; however, NDF in alfalfa disappeared at a faster (P < 0.001) rate (0.107 h(-1)) than crabgrass. These results indicate that crabgrass offers greater effective ruminal degradability of DM and NDF than orchardgrass or alfalfa of moderate quality. More importantly, it potentially offers faster and more extensive ruminal disappearance than perennial warm-season grasses typically found throughout the southeastern United States, and it should likely support improved performance by ruminant livestock in this region.

Animals↗
PubMed2001

Associated phoria and the measuring and correcting methodology after H.-J. Haase (MKH).

The test charts included in the Polatest, designed by H.-J. Haase and manufactured by Zeiss, are used in Germany, Switzerland and Scandinavia for prism correction of 'associated phoria.' From clinical experience with the Polatest Haase developed a motor and sensory theory of the different stages of decompensation of 'associated phoria ' and a strategy for its prismatic correction - the MKH (Measuring and Correcting Methodology after H.-J. Haase). The theory challenges many accepted ideas about the plasticity of the visual system and the use of prisms in the treatment of sensory abnormalities. This article, the first full description in English, describes and critically discusses the MKH.

Accommodation, Ocular↗
PubMed1999

The influence of heterophoria measurements on subsequent associated phoria measurement in a refractive routine.

The influence of measuring heterophoria on a subsequent associated phoria measurement was investigated in 50 subjects. After refraction, heterophoria was measured by either von Graefe's technique or Maddox rod and wing. The effect of these procedures was assessed by associated phoria measurements taken before and after the dissociated measures. It is shown that subjects with a history of unstable binocular vision are likely to have an associate phoria induced or changed by the previous dissociated phoria measurement, particularly if von Graefe's technique is employed. It is recommended that, especially for near, associated phoria should be assessed before the dissociated phoria in subjects who are regarded as having a history of unstable binocular vision.

Adult↗
PubMed1997

Analytic approximation of the off-axis modulation transfer function of the eye.

Published experimental measurements of the ocular modulation transfer function (MTF) in the peripheral field are approximated by the expression T(f) = exp[-(f/fc)n], where T(f), f, fc, and n are modulation transfer, spatial frequency, spatial frequency constant and MTF index, respectively. It is shown that n (which describes the shape of the MTF) remains relatively constant at about 0.9 for field angles out to 40 deg but fc (which defines the spatial frequency scaling) declines steeply over this range, depending upon the pupil diameter and conditions of focus. The oblique astigmatism of the eye plays a major role in the off-axis changes in fc at field angles > or = 20 deg. The approximation may be useful in allowing the form of the degraded optical stimulus in studies of the peripheral retinal function to be evaluated.

Astigmatism↗
PubMed1995

Video-photorefraction: the relationship between blur circle diameter and refractive error.

When using a videorefractor (VPR-1) the margins of the defocused blur circles are sometimes poorly defined and difficult to locate. To investigate the effect this may have on the apparent refractive error, the relationship between blur circle diameter and refractive error was found by videoing circles of known size and inputting these data as pupil and blur circle diameters. The results indicate that for accuracy of 0.50 D blur circles need to be defined to approximately equal to 1 mm.

Humans↗
PubMed1985

Anomalous retinal correspondence--a review.

The theories and characteristics of anomalous retinal correspondence (ARC) are critically reviewed and a classification of ARC is proposed which attempts to reconcile various aspects of the different theories. It is suggested that ARC in microtropia, and perhaps some small-angle squints, is a cortical adaptive mechanism, whereas squints with a larger and more variable angle have ARC with different characteristics, which are better explained by the motor theory.

Adaptation, Ocular↗
PubMed1978

Optical image quality in the peripheral retina.

A double-pass, photoelectric, ophthalmoscopic method was used for objective study of the variation in image quality across the retina in terms of the reflected image of a fine line, the line-spread function. Radial and tangential line-spread functions have been studied with active and paralyzed accommodation at eccentricities up to 45 deg. With large pupils, the retinal image quality remained relatively constant over the central 20 deg of field and deteriorated at larger field angles. The results are discusssed with respect to field curvature, peripheral acuity, and peripheral refraction.

Adult↗
PubMed1978

The refraction of the eye in the relation to spherical aberration and pupil size.

Available theoretical and experimental data on the dependence of the subjective refraction of the eye on spherical aberration and pupil size are reviewed and additional experimental results are presented. Under photopic conditions, refraction varies little (similar to 0.3 DS) with pupil diameter, even when substantial amounts of spherical aberration are present (similar to 1.0 DS). It is argued that this result may be attributed to the insensitivity of optimal focus to spherical aberration at the spatial frequencies involved in photopic refraction, to the Stiles-Crawford effect and to the influence of aberrations other than primary spherical aberration in the outer zones of the pupil. The signficance of these findings is indicated in relation to possible discrepancies between the results of objective and subjective refractions when large pupils are in use.

Humans↗