Growth and development of mammals.

Influence of Nutrition on Mammary Development in Pre- and Postpubertal Heifers

Effect of nutrient intake during pregnancy on fetal and placental growth and vascular development

The literature on bat growth and survival is extensive. Nevertheless, few studies have made or permit within or among species comparisons of performance under varied combinations of biological and physiological parameters. In particular, surprisingly few studies have taken advantage of the fact that many species occupy wide ranges of environmental conditions. Growth and survival frequently have been treated as species-specific constants. Such treatment neglects many important determinants of success and likely has inhibited meaningful hypothesis testing and further investigation. Despite these limitations, existing studies demonstrate impressive variation. In fact, available literature indicates that few if any details of growth and survival are constant over time, even for individual colonies within a single species.

Growth and Survival of Bats

Seventy-nine Finnish Landrace x Dorset Horn ewes in lamb to Suffolk rams were used in a comparative slaughter experiment to assess the effects of numbers of foetuses on the growth of the products of conception. The mean number of foetuses was 2·7, with a range from 1 to 5. The ewes were individually penned and given a standard diet with metabolizable energy concentration 7·7 MJ/kg and N concentration 21 g/kg, rationed at 2 kg/day during the first month and 1·25 kg/day during the second and third. Allowances beyond that time were on two scales and depended on the number of foetuses being carried as diagnosed by radiography. The ewes were slaughtered between 50 and 145 days of gestation. The gravid uterus was dissected into foetal, placental, foetal fluid and empty uterus components. Equations were fitted to the weights of each component to describe the effects of stage of gestation, litter size and ewe weight. For the mean weight per foetus (Y, kg) the preferred equation isIn (Y)= 2·419–17·574e-0.01976t–00079ft+0–0046w,where tis the time in days from conception, / is the number of foetuses and wis the weight (kg) of the ewe at mating. This is a version of the Gompertz equation, with additional terms to express the effects of / and w.The weights of the placenta and of the empty uterus were similarly fitted by versions of the Gompertz equation but the mean weight of fluids per foetus (Z, kg) or, rather, its natural logarithm was best described by a third degree polynomial, which isIn(Z) = –11·518 + 0–326t;–0·00316t2+0·0000102t3.None of the weights was significantly affected by the level of feeding in late pregnancy.Estimates of mean weights from the equations and of daily rates of gain in weight from the first differentials of the equations are tabulated against stage of gestation and litter size, and the forms of the weight, growth rate and specific growth-rate curves are illustrated graphically.The equation for foetal weight estimates that at the end of pregnancy the mean weight per foetus is reduced by a factor of 0–89 for each additional foetus being carried. The mathematical model implies that the differences originate in early pregnancy, when the factor is very close to unity, and that the mean weights gradually diverge. In the absence of direct evidence this would appear to be the simplest hypothesis, rather than the assumption in most of the earlier literature that the effect is entirely confined to the last 4 or 5 weeks of gestation.Just before parturition the total daily weight gain of quadruplet foetuses was about 250 g and was associated with a similar gain in weight of foetal fluids, the ratio of fluid weight to foetal weight appearing to increase with litter size. The ewes were clearly under considerable physical stress. It is suggested that this aspect must be closely considered when greater prolificacy is sought.

Studies on reproduction in prolific ewes

1. Some criticisms of the classical approach to constructing a selection index are advanced. This approach (the economic model) is contrasted with an alternative one (the biological model) which is derived from the concept of the biological efficiency of production of lean tissues. The selection objective which is proposed from the biological model is the improvement of lean tissue feed conversion. The most important single means of achieving this is increasing the lean tissue growth rate.2. The approaches were compared in three studies: (a) the effects of errors in economic and genetic parameters of the economic index; (b) phenotypic relationships between biological objectives and other measures of performance; (c) a theoretical model based on the utilization of metabolizable energy.3. In a fixed situation the results were similar for both models, but the main advantage of the biological model is that it gives good indications of what happens, and what selection policies should be, in different situations. In addition, lean tissue feed conversion and lean tissue growth rate are closely related to concepts widely applied in biology, and their use as selection objectives should be helpful in enabling workers in many disciplines to make use of each other's knowledge.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0003356100031482

Growth and development of mammals.

Citations

Influence of Nutrition on Mammary Development in Pre- and Postpubertal Heifers

Effect of nutrient intake during pregnancy on fetal and placental growth and vascular development

Growth and Survival of Bats

Studies on reproduction in prolific ewes

Objectives in pig breeding

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