D. T. Chambers

Feed efficiency measured as a function of gain in body weight and feed consumed was studied for 1,324 bull and heifer calves at three experiment stations. Three measures were computed: (1) feed consumption adjusted for differences in gain; (2) gain adjusted for differences in feed consumption; and (3) the ratio of gain to feed consumed. In all three measures, mid-weight was considered simultaneously in an attempt to remove differences in maintenance requirements. Efficiency expressed as gain adjusted for differences in feed consumption (i.e., ±deviation from the regression of gain on consumption) was considered the most accurate mathematical description of the cause and effect relationships and resulted in the highest heritability of the three measures studied. No trends in the heritabilities calculated for each experiment station were noted. The combined heritabilities were 0.65 for gain on test, 0.64 for feed consumed, 0.62 for gain adjusted for differences in feed consumption, 0.28 for feed consumption adjusted for differences in gain, and 0.36 for the ratio of gain to feed consumed. A path analysis of feed efficiency (gain adjusted for feed consumption), feed consumption and gain was made. The analysis indicated that 38% of the variation in gain could be attributed directly to genetic differences in feed efficiency. Genetic differences in feed consumption accounted for 25% of the variation in gain. The remaining 37% of the variation in gain was accounted for by variations in environmental influences. The genetic correlation between feed efficiency and gain was 0.79, between feed consumption and gain was 0.64, and between feed efficiency and feed consumption was 0.04. These results indicate that selecting for gain should be effective and lead to both increased feed efficiency and increased feed consumption. Selecting for feed efficiency would increase feed efficiency and result in increased daily gain, but feed consumption would not be affected. Selection for feed consumption would increase feed consumption and daily gain, but would lead to no improvement in feed efficiency other than that attributable to a smaller portion of the intake being used for body maintenance.

One hundred and eighty head of yearling and two-year-old steers were used over a period of 15 months to study the variations that occur in the amount of fill contained by steers running on pasture. The weight changes between a base weight taken under uniform conditions and subsequent weights taken under controlled conditions constituted the items of study. Under the conditions of the experiment it was found that the availability of water was a more important source of fill variation than the availability of pasture grass. There was an indication that under some conditions the differences in amount of fill among the animals tend to be lessened by shrinking. Animals in different pastures exhibited different amounts of fill when gathered during the afternoon. Using full weights under such conditions would tend to reduce the repeatability of experiments. Steers weighing from eight hundred to a thousand lb. lost about one lb. every ten minutes during the first three to four hours of shrinking whether they were held in a dry lot or were being quietly driven. Animals on pasture early in the morning appeared to lose fill faster than it was replaced. These findings emphasize the necessity for using standardized procedures in weighing cattle. It is suggested that all lots of cattle should have water equally available at all times. Shrinking cattle for a period of 10 to 15 hours before taking the weights that are to be used in the final evaluation of treatments effects may increase experimental efficiency. If several lots are involved, they should be weighed as rapidly as possible to prevent excessive loss of fill by those being weighed last.

In a 3 × 3 factorial experiment, the effects of three levels of nitrogenous fertilizer (0, 336 and 672 kg/N per ha) and three grazing intensities on live‐weight gains and carcass attributes were measured. The live‐weight gains of the animals (per head) over the season were greater at the low and moderate grazing intensities than at the high; this effect occurred mainly from July to October each year. Gains were low on the no‐nitrogen swards from July 1962 to the end of the grazing season and throughout 1963: this was related to the very high clover content of the swards. In all years, as grazing intensities increased, the carcass weights per animal became lower. Carcass balance, carcass length, depth of longissimus dorsi muscle, fat thickness over the longissimus dorsi, weight of kidney suet and channel fat, and weight of the alimentary tract also reflected the effect of treatment on weight gains. The results are discussed in relation to the botanical composition and consumption of the herbage, presented in Part I.

The reproductive performance of a purebred Hereford herd was studied for the period from 1935 through 1952. The reproductive performance of this herd gradually declined during this period. The average reproductive performance for this period was: number of services per conception, 1.7; interval from calving to conception, 108.2 days; gestation length, 285.7 days; calving interval, 394.2 days; interval from calving to first breeding, 75.5 days; and interval from first breeding to conception, 31.0 days. In general, the reproductive performance was higher in summer and fall than in spring and winter. Cows more than 10 years of age declined rapidly in performance. All correlations and regressions of performance on age of cow were positive and significant. The rapid decline at the older ages caused the regression to be curvilinear. During the period of this study 530 cows were taken from the herd at an average age of 7.8 years. Cows that were sent to market at the end of their productive life had an average age of 9.2 years. Thus they had a productive life of 7.5 years. Heritability estimates of reproductive performance were determined by the paternal half-sib intra-class correlation, the correlation between daughter and dam records, and the regression of daughters' records on dams' records. The repeatability estimates were determined by intra-class correlations. The estimates were generally low. Most heritability estimates were near zero. Repeatability estimates varied from 0.03 to 0.15. Cows bred more than 60 days post partum required fewer services per conception than those bred prior to 60 days post partum.