7 factors that affect nutrient utilization from a given feedstuff
species age physiological state type of GI tract level of consumption physical form of nutrients infectious diseases/parasites
define growth in both a narrow and broad sense
narrow: the constructive or assimilatory synthesis of one substance at the expense of another (nutrient) that undergoes dissimulation
broad: increase in body weight resulting from assimilation of body tissues of ingested nutrients
why do nutrient requirements for growing animals change throughout the growing period?
change throughout the growing period is in direct response to the changing needs of individual organs & organ systems making up the whole animal
what measurements do growth experiments usually include?
- absolute gain in body weight
- rate of gain = average daily or weekly gain
what type of diets are usually fed to animals in a growth trial?
test diets concurrent with similar animals fed a standard diet of known nutritive quality that allows for normal growth (basically, a default/standard diet that allows normal growth for that species)
why is weight gain not always a precise estimate of diet utilization?
growth is often used interchangeably with weight gain. the 2 terms are different b/c an equal increase in body weight b/n animals does not necessarily indicate equal growth of body tissues (i.e. one animal may deposit more lean muscle mass and its pen mate may deposit more fat, which as a higher energy content).
Thus, the rate of weight gain does not provide a precise estimate of diet utilization.
what is feed efficiency?
- feed required per unit of weight gain or
- weight gain per unit of feed
why is feed efficiency a useful estimate of nutrient adequacy of a test diet?
diets that promote a high rate of gain will usually result in a greater efficiency than diets that do not allow rapid gain
what is the advantage of a paired feeding experiment?
in order to rule out factors such as physical characteristics of diet, nutrient content, or palatability
what are the 4 advantages of growth trials?
- allows accumulation of relatively large amounts of data at reasonable costs
- animals usually can be maintained under conditions that are similar to normal environmental situations
- measurements are easily obtained
- results are a reflection of a fundamental biological response, yet can be applied directly to commercial production systems
in a growth trial, how can we deal with the fact that growth is a variable biological parameter?
by using 12 to 15+ animals per treatment to detect statistical differences between dietary treatments
in order to determine changes in body composition during growth in beef cattle without grinding a whole carcass, what can we do?
9-10-11 rib cut has been shown to give a relatively accurate estimate of the total carcass for fat, protein, water & ash (minerals) content
what is the point of a “weight-suckle-weigh”?
milk production is estimated indirectly by measuring gain of young or weighing them before & after suckling - common practice for lab animals, pigs & beef cattle
what is the point of digestion trials?
used to determine proportion of nutrients in a feed or diet that are absorbed from the GI tract
why must there be a preliminary period in a conventional digestion trial?
to void GI tract of residues of pretest feed & to allow adaptation of the animal to the diet
how long is the collection period in a conventional digestion trial?
4-10 days
how many animals per diet do I need for a digestion trial?
4-6 animals
why do we use the term “apparent digestibility” instead of simply “digestibility”?
it takes into account both unabsorbed feed residues & the components of the feces that are endogenous origin
I fed a mouse 40 g of protein. I collected 30 g of protein its feces for the corresponding time period. what is the apparent digestibility of the protein?
(40−30)/30 (100) = 33%
why use the indicator method instead of a conventional digestion trial?
choice: when it is impossible or inconvenient to measure total feed intake or to collect total feces
what are the problems with using internal indicators to calculate digestibility?
- internal indicators or markers are those such as lignin that are present in the feed but that are digested to a negligible degree if at all
- use of lignin is plagued with problems of incomplete recovery & difficulty in analysis, but even so it has been used extensively with herbivorous species or range animals
list 4 problems with using external markers (chemicals/rare earth elements that are added to feed/given to the animal orally or by the rumen cannula)
- irregular excretion
- incomplete recovery
- mixed in feed
- extensive & tedious processes
why use digestibility by difference?
evaluate the digestibility of a feedstuff when fed in a mixture with one or more other feeds
what is the associative effect in digestibility data?
mixtures of feedstuffs do not always give results that would be predicted from digestibility values of the individual components of the mixture
how does one account for the fact that apparent digestibility of a protein in a feed is influenced by the level of protein in a feed?
this is so b/c amount of endogenous protein tends to be constant, so at a high-protein intake, the endogenous fraction represents a smaller percentage of the total nitrogen excretion
what are the methods used to estimate endogenous excretion of nitrogen?
by subtracting the amount of Nitrogen appearing in the feces of an animal fed a low-protein diet from the amount of Nitrogen appearing in the feces of the animal fed a test diet
what is the usual way of determining true digestibility of nitrogen in ruminants?
- feeding nitrogen free diet & determining amount nitrogen in feces
- feeding several levels of nutrient (nitrogen) & calculating fecal levels by regression analysis to a zero intake of nutrient
- feeding a completely digestible protein
what is the advantage to doing a balance trial rather than just a digestion trial?
- similar to digestion trials, but provide info. on utilization of nutrients after absorption from the GI tract
- intent is to get accurate measure of total intake & total excretion in order to determine whether there is a net retention or loss of nutrient in question
what are the advantages of using purified diets?
derived from experiments in which pure or semi purified sources of nutrients were used to formulate diets - allow one to make quantitative changes in the concentration of a single nutrient without greatly affecting concentrations of other nutrients in the diet
why have purified diets not solved completely the dietary essentiality of all trace minerals?
even highly purified inorganic element sources may contain trace amounts of contaminating elements - for this reason, there is uncertainty about the dietary essentiality of some trace mineral elements
why is it good to have simulation techniques for rumen digestibility?
cattle digestion trials cost a lot of money, whereas in vitro techniques are less
what are 3 reasons of fitting a cow with a rumen cannula?
- allows direct access to the rumen
- animal serves as a donor of rumen fluid to other animals
- aids in determining the digestibility of various feedstuffs
how does feed intake above maintenance affect digestibility in ruminants?
increased food intake above maintenance tends to depress digestiblity
how does stomach capacity affect digestibility?
stomach capacity affects eating rate, amount eaten, & passage rate through GI tract
why do similar animals digest a given diet to different events?
all animals do not digest a given diet to the same extent
what are the 2 advantages of using lab animals as models for farm animals in nutrition studies?
- short life spans = obtain data covering several generations/year
- economically efficient
what are some common lab animals used in nutrition studies?
mice rats guinea pigs mammals birds reptiles fish
why is the pig used as a model for human nutrition?
the pig’s digestive system is similar anatomically & functionally to that of humans
