Genetics of the interval from weaning to estrus in first-litter sows: distribution of data, direct response of selection, and heritability.

A selection experiment with a selection and a control line maintained for eight generations was set up to study efficacy of selection for a short interval from weaning to estrus after weaning the first litter and to estimate genetic variation in the Dutch Landrace population. Intervals were recorded without truncation (i.e., intervals up to 234 d were observed). A mating scheme with 10 mating groups was used to avoid inbreeding. Distribution of intervals from weaning to estrus was described by a mixture of a normal and an exponential distribution. Between lines, both underlying distributions were similar, but the contribution to the total distribution was different. From generation 4 onward, intervals were significantly shorter in the selection line than in the control line. Variances within lines, generations, and mating groups varied to a large extent. Response per unit selection differential was calculated for original data and three sets of transformed data, both with and without correction for unequal subclass numbers. Standardized responses were more precise and linear, when data were log-transformed, and when corrected for unequal subclass numbers. The assumed best estimate of the realized heritability was .17. The heritability in the foundation population was estimated at .36 +/- .05, using an animal model including all genetic relationships in the data. It was concluded that genetic selection decreases the average interval from weaning to estrus by reducing the number of sows with a record in the exponential part of the distribution.

Thermal requirements of young calves during standing and lying.

A study of 23, 6-d-old Holstein-Friesian male calves was conducted to evaluate the thermal requirements of young calves in relation to their posture (standing or lying). Calves were housed individually and were assigned to one of four ambient temperatures: 5, 9, 13, or 18 degrees C. Heat production of each calf was measured continuously every 9 min by indirect calorimetry for 5 d. The posture during these 9-min periods was derived from the continuous measurement of physical activity. For both standing and lying, heat production was increased at 5 and 9 degrees C compared with 13 and 18 degrees C. This increase in heat production was larger during standing than during lying. Consequently, the energy cost of standing was affected by ambient temperature (173, 189, 144, and 114 kJ.kg-.75 x d-1 at 5, 9, 13, and 18 degrees C, respectively). The time spent standing was not affected by ambient temperature. The thermal requirements of the calves were dependent on the posture. Lower critical temperature was 13.5 degrees C during lying and 17.0 degrees C during standing. The rate of increase in heat production below the lower critical temperature was 7.47 kJ.kg-.75 x d-1 x Co-1 during lying and 11.24 kJ.kg-.75 x d-1 x Co-1 during standing. These results indicate that the increase in heat loss upon standing in the cold is greater than the increase in heat production upon standing at thermoneutrality. Consequently, young calves require increased cold-induced thermogenesis when they are standing.

Metabolic differences between White Leghorns selected for high and low residual food consumption.

1. From each of two populations of White Leghorn laying hens, 6 'efficient' and 6 'inefficient' hens were used in energy metabolism studies involving indirect calorimetry with activity measurement. 2. Hens classified as efficient or inefficient with respect to residual food consumption had comparable body weight and production. 3. Efficient and inefficient hens showed similar energy metabolisability, cloacal temperature, shank dimension and egg composition. 4. However, efficient hens produced less heat; activity-related heat production accounted for 29 to 54% of the difference in total heat production between efficient and inefficient hens. 5. Plumage quality also explained some of the difference in heat production, but to a lesser extent (14 to 19%). 6. Adjustment for heat increment of production changed the heat production difference slightly (-5 to 1%). 7. What was finally left unexplained (100 - 54 - 14 + 5 = 37% to 100 - 29 - 19 - 1 = 51%) was of the same magnitude as activity-related HP and is discussed in relation to basal metabolic rate.

Residual feed consumption in laying hens. 1. Quantification of phenotypic variation and repeatabilities.

A large fraction (on average, 25%) of the variance of daily feed consumption among individual laying hens appears unaccounted for by a model with metabolic body weight, daily egg mass production, and body weight gain as independent variables. This unexplained term is referred to as "residual feed consumption" (RFC), defined operationally as the difference between the observed feed consumption of a hen and its consumption as predicted from the model. The study described here deals with the quantification of the phenotypic variation of RFC of hens of a White Leghorn population during a 44-wk laying period (20 to 64 wk of age) in 11 time segments of 4 wk each, fed either a commercial or a low-energy diet (11.7 and 10.0 MJ ME/kg, respectively, where 1 MJ = .239 Mcal). The RFC showed a standard deviation of 4 to 8 g/day. The repeatability of RFC was estimated as .52 to .58. For each time segment between 32 and 56 wk of age, phenotypic correlations between RFC and RFC accumulated over the whole laying period were estimated to be around .8. It was concluded that RFC shows a considerable amount of systematic and permanent variation, and that experimental RFC measurements can be limited to the period between 32 and 56 wk. It was also shown that variation in RFC was caused mainly by differences among hens in maintenance requirements per unit (kilogram.75) of metabolic body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Residual feed consumption in laying hens. 2. Genetic variation and correlations.

The study described here deals with the quantification of the genetic variation of "residual feed consumption" (RFC) of hens of a White Leghorn population during a 44-wk laying period (20 to 64 wk of age) in 11 time segments of 4 wk each, fed either a commercial or a low-energy diet (11.7 and 10.0 MJ ME/kg, respectively, where 1 MJ = .239 Mcal). The RFC is defined operationally as the difference between the observed feed consumption of a laying hen and its consumption as predicted from a model with metabolic body weight, egg mass production, and body weight gain as independent variables. The RFC was found to be highly heritable in all periods. The heritability of RFC accumulated over the whole laying period (RFC-T) was estimated as .42 to .62. For each time segment between 32 and 56 wk of age, genetic correlations between RFC and RFC-T were estimated to be larger than .91. The genetic sources causing variation in RFC during the first part of lay seem to differ from those causing variation later on, and to be of less importance during the rest of the laying period. It was concluded that RFC shows a considerable systematic and permanent additive genetic variance, and that RFC measurements for selection can be limited to one to three time segments between 32 and 56 wk. Furthermore, less environmental variance and therefore higher heritabilities and genetic correlations seemed to exist for birds fed the low energy diet in comparison with those fed the commercial diet. No clear differences could be found between genetic and phenotypic correlation estimates of RFC with feed consumption, metabolic body weight, egg mass production, and body weight gain.