Variation In Feed Intake Research Articles

Effect of pelleting and enzyme supplementation on variation in feed value of wheat-based diets fed to broiler chicks

The effect of wheat cultivar, pelleting, and enzyme supplementation was measured with a broiler chick bioassay comparing differences in feed intake, growth, FCR and metabolisable e nergy (ME) level. Samples of two wheat cultivars from each of six classes were ground and were either unpelleted or pelleted and ground before including (80%) in mash diets with or without enzyme (Avizyme 1302). Two common control diets were fed in the two bioassay series required to evaluate the 48 diets. Each diet was fed to four cages of six male broilers from 4 to 17 d of age. Apparent metabolisable energy (AME) was determined on excreta collected at 16 d of age. There was more than a 25% difference in feed intake among the 48 diets; the largest contribution of the variation was due to wheat cultivar. Feed intake was significantly increased by enzyme addition and decreased by pelleting. Variation in feed intake explained more than 70% of the variation in body weight and had only a minor association with determined AME. The AME of all wheat samples, except the two Durum wheats, was increased by both enzyme supplementation and processing. The increase in AME with processing may represent an anomaly of using different bioassay series to measure unpelleted and pelleted wheat samples or it may represent changes in AME of all wheat samples, except Durum wheats, with storage after harvesting. The relative AME and FCR measurements for the 12 wheat samples were similar whether the wheats were pelleted or not. However, there was no relationship between measurements of feed intake when the different cultivars of unpelleted and pelleted wheat were compared. The marked variation in feed intake and growth rate of broilers fed different cultivars of wheat continues to be a concern as it suggests that other factors may limit intake and prevent the broiler from obtaining the required levels of nutrients from the diet. Key words: Wheat, feed processing, enzymes, feed value, broilers

Controlling variation in feed intake through bunk management1

Controlling variation in feed intake through bunk management1

Zufütterung von Getreide an männliche Kälber aus der Mutterkuhhaltung während der Weideperiode

Abstract. Title of the paper: Supplementation of grain to suckling beef calves during the grazing period A study on 65 beef cow-calf pairs was conducted to investigate the intake of a feed supplementation with grain and its interrelations to performance and behaviourial characteristics of suckling calves, as well as the development of body condition and milk yield of the dam. The intake of grain in the group receiving the supplementation (group B) was on average 2.7 kg/ calf/day during the grazing period. A substantial individual variation of feed intake from 1.8 kg to 7.7 kg grain/calf/day was determined. In this period, the daily gains and the weaning weights of the calves in group B were higher (15.4% resp. 7.4%). The advantage of the calves of the group B could be proven only in the second half of the grazing period. The supplementation of grain caused significantly lower suckling activities and grazing periods per day. Effects of a grain supplementation in the feeding of suckling calves on pasture could not be determined on both the body condition and the milk yield of the cows.

Growth performance and sexual maturation in diploid and triploid Atlantic salmon ( Salmo salar L.) in seawater tanks exposed to continuous light or simulated natural photoperiod

Triploid Atlantic salmon are considered to be functionally sterile and the use of these in intensive fish farming would potentially reduce interactions between escapees and wild stocks. However, variable performance under commercial production conditions in seawater has been reported. The experiment described here is part of a larger comprehensive study on triploid Atlantic salmon, and investigates seasonal growth performance in diploid and triploid salmon under simulated natural light, and an accelerated production regime using continuous light during winter and spring. Four full-sib families of diploid and triploid Atlantic salmon postsmolts were held in six 20-m 3 circular indoor tanks in the January–October period after their transfer to seawater in July. Two tanks were given continuous light (LL) from January to June and subsequently put on simulated natural photoperiod (SNP), while four tanks were kept on SNP throughout. Variation in feed intake was monitored by waste feed collection, and fish were sampled monthly for weight and length. A depression in feed consumption was noted in the LL groups during the first 6–8 weeks after onset of light. As expected, the LL groups outperformed their SNP counterparts, but displayed an unexpected equal incidence of sexual maturation. Triploids displayed distinctly enhanced feed intake during their second sea autumn, which resulted in their being heavier and significantly longer than diploid fish in October. This increased feed intake in triploids was advanced in the LL treated fish. Only males showed signs of becoming ‘sexually mature’ (increased GSI) as grilse (after 1.5 years in seawater), and equal proportions of the diploid and triploid fish were characterised as sexually mature. Thus, lack of ‘sexual maturation’ could not explain the improved triploid growth. There were no distinct differences in mortality or occurrence of deformities between ploidy types. Both triploids and diploids responded to the accelerated light regime with an advanced seasonal development of growth and condition factor, long-term enhanced growth and an indicated improved feed conversion. In general, all sibling groups responded to the ploidy and light treatments. Growth differences between sibling groups within treatments were mainly ascribed to different proportions of sexual maturation. This study indicated that triploid Atlantic salmon performed equally, or better, compared to diploid conspecifics reared in indoor seawater tanks.

Short-term effects of in-feed supplementation of tetracyclines for disease control on feed intake pattern and growth in weaned pigs

Chlortetracycline (CTC) and oxytetracycline (OTC) are often administered in feed to pigs at extra-label doses to increase the efficacy of disease control strategies, but the bitterness of these antibiotics may significantly depress the voluntary feed intake of pigs at risk, compromising growth and the efficacy of the therapeutic regimen. We examined individual feed intake patterns and growth of 5-week-old pigs that received either drug-free feed ( n=36 pigs) or feed medicated with CTC or OTC at 12-h intervals for a period of 5 days. Each tested drug was mixed in feed at concentrations of 550, 1100, 1650 and 2200 mg/kg ( n≥8 pigs/dose/drug). Neither drug nor dose significantly affected the rate of feed intake (excluding interruptions), duration of meal, and the likelihood of leaving feed uneaten at the end of at least one meal. Average daily gain and feed conversion ratio (FCR) showed similar mean values across treatment groups, but variance of FCR varied significantly: CTC<OTC<drug free. Meal duration and feeding rate, and the likelihood of leaving feed uneaten were significantly influenced by the size of pigs, the occurrence of infection or spontaneous feeding interruptions during the meal. Feeding rate increased with experimental days. Results show that CTC and OTC are not feed deterrents in weaned pigs in the range of therapeutic doses currently used, but other sources of variation in feed intake indicate that drug input could vary among pigs.

Using Genetic Variation to Understand Control of Feed Intake in Fish

The overall goal of our research is to develop fish with superior growth, and feed efficiency attributes. Feed intake is integral to these characters. Over the last several years we have been working with channel catfish, Ictalurus punctatus, in two directions tied to feed intake regulation. One direction has been to identify genes functioning as physiological regulators of feed intake, and the other has been to measure and characterize the genetic variation in feed intake within and among distinct catfish populations which reveal a wide range in feeding response. The goals are to uncover the genetic basis of physiological variation, to understand the genes and interactions that produce the phenotypic differences observed and to exploit these differences in a selective breeding program. To investigate the links between genetic variation and physiological variation, differences in voluntary feed consumption were documented in two strains of channel catfish. Treatment with orexigenic compounds affected both strains similarly, though anorexigenic compounds inhibited feeding in one strain more than the other and low temperature treatment affected the strains differently with respect to feed efficiency. Phenotypic variation in feed intake among families within one strain suggested that approximately 40% of the variation in feed intake is due to genetic sources. Future work aimed at utilizing strains and families with divergent phenotypes for identifying the specific genes responsible for this variation is discussed.

L-threonine supplementation increases the amplitude of the feed intake cycle of rats fed a low-protein zinc-deficient diet.

Rats fed a Zn-deficient diet show characteristic variations in feed intake. These variations were followed by means of a personal computer. The specific feed intake patterns in rats fed a zinc-deficient diet before and after supplementation with protein and several essential amino acids were determined. The high-protein diet decreased the amplitude of feed intake under zinc deficiency, probably because of a decrease in sensitivity to the deficiency. Furthermore, the zinc-deficient diet was supplemented with essential amino acids, and of them L-threonine showed the most marked effect on the increased variability of feed intake.

Feed intake in channel catfish: is there a genetic component?

Increasing catfish growth is a primary objective of our broodstock improvement programme, and growth is at least partly dependent on voluntary feed intake. Two experiments were conducted to determine the genetic component of feed intake, and the relationship between feed intake and growth in sib-groups of channel catfish. In the first experiment, 10 fish from each of 31 full-sib families from the USDA-103 strain of channel catfish, Ictalurus punctatus (Rafinesque), were individually identified with passive integrated transponder (PIT) tags and distributed into two replicate tanks, five fish from each family to each tank. Fish were fed to apparent satiation with feed labelled with an X-ray opaque marker for one meal, radiographed, and feed intake was quantified for each individual. Genetic effects (broad sense heritability) accounted for approximately 41% of the phenotypic variation in feed intake. These fish were then grown for 5 months and the mean change in weight of the family groups was significantly correlated with mean feed intake (r = 0.64, P < 0.001). A subsequent experiment compared the feed intake of 100 families of catfish with their growth rate over the previous 2 months. Each family was grown in a separate 800 L tank. Fish were fed to apparent satiation daily and mean weight was determined monthly. After the 2 month growth period, fish were fed the labelled feed to apparent satiation, 28–30 individuals from each family/tank were radiographed, and individual feed intake was determined. A highly significant correlation (r = 0.54, P < 0.0001) between mean specific growth rate and mean feed intake (% consumption) was demonstrated. Taken together, these results suggest that individual feed intake has a heritable basis, and should be responsive to selection. Selection for increased feed intake could result in a correlated response of increased growth.

The role of cholecystokinin in the expression of seasonal variation in the feed intake and eating pattern of red deer ( Cervus elaphus)

The role of cholecystokinin (CCK) in the expression of seasonal variation in feed intake and feeding activity in ruminants was investigated by active immunisation of castrated male red deer ( Cervus elaphus). In April, animals of two groups (five animals per group) were immunised against either CCK or vehicle solution only and booster injections were administered at 2-month intervals for the following year. Measurements were conducted for a period of more than a year from July. There were no significant effects of immunisation on mean daily food intake. However, there was a significant interaction ( P<.01) between immunisation and month (season), with respect to rate of feed ingestion during meals, with animals immunised against CCK exhibiting higher mean rates of ingestion during October to May but lower mean rates during June to September. It is concluded that systemic CCK has a role in the expression of seasonal variation in the rate of feed ingestion during meals in ruminants and that this, in turn, may affect the pattern of seasonal change in daily feed intake. Since there was no evidence of differences with treatment in profiles of insulin, growth hormone (GH), thyroxine (T 4), triiodothyronine (T 3), insulin-like growth factor-1 (IGF-1) or prolactin, it is unlikely that this effect is expressed through changes in the profiles of these hormones.

Genetic variation in feed intake, growth and feed utilization in Atlantic salmon ( Salmo salar)

Feed intake, growth and feed utilization were studied in 14 full-sib family groups of Atlantic salmon (seven sires, two dams per sire, 30 fish per family), with an average initial body weight of 148 g. Each full-sib family was held in replicate tanks with fresh water for two subsequent 5-week periods. Body weight increased by 12–52%, and the average daily feed intake (DFI), thermal growth coefficient (TGC) and feed efficiency ratio (FER) were 0.33% of BW (CV=21), 1.08×10 −3 (CV=35) and 1.14 g weight gain/g feed intake (CV=16), respectively. The effects of family and growth period on DFI, TGC and FER were significant ( P<0.05), with family explaining 31–32% of the variation. FER was positively correlated with TGC ( r=0.79, P<0.001, n=28). The salmon consumed an average of 450 g protein (CV=16) and 22.6 MJ gross energy (CV=18)/kg gain: 43% of the protein and 31% of the energy were deposited. When calculated per kilogram weight gain, the effects of family were significant for feed consumption and visceral deposition of protein and energy. The results indicate a genetic variation for feed intake, growth and feed utilization in Atlantic salmon.

Locomotor activity, core body temperature, and circadian rhythms in mice selected for high or low heat loss.

Daily locomotor activity, core body temperature, and their circadian rhythms were measured in lines of mice selected for high (MH) or low (ML) heat loss and unselected controls (MC). Lines were created by selecting for 16 generations in each of three replicates. Collection of locomotor activity and core temperature data spanned Generations 20 and 21 for a total of 352 mice. Physical activity and core body temperature data were accumulated using implanted transmitters and continuous automated collection. Measurement for each animal was for 3 d. Activity was recorded for each half hour and then averaged for the day; temperature was averaged daily; circadian rhythm was expressed in 12-h (light vs dark) or 6-h periods as well as by fitting cyclic models. Activity means were transformed to log base 2 to lessen heterogeneity of variance within lines. Heat loss for a 15-h period beginning at 1630 and feed intake for 7 d were measured on 74 additional mice in order to estimate the relationship between locomotor activity and heat loss or feed intake. Selection lines were different (P < 0.01) for both locomotor activity and core body temperature. Differences were due to selection (MH-ML, P < 0.01), and there was no evidence of asymmetry of response (P > 0.38). Retransformed from log base 2 to the scale of measurement, mean activity counts were 308, 210, and 150 for MH, MC, and ML, respectively. Mean core temperatures were 37.2, 36.9, and 36.7 degrees C for MH, MC, and ML (P < 0.01), respectively. Females had greater physical activity (P < 0.01) and body temperature (P < 0.01) than males. There was no evidence of a sex x selection criterion interaction for either activity or temperature (P > 0.20). Overall phenotypic correlation between body temperature and log base 2 activity was 0.43 (P < 0.01). Periods during the day were different for both 12- and 6-h analyses (P < 0.01), but there were no period x selection criterion interactions (P > 0.1) for physical activity or body temperature. More sensitive cyclic models revealed significant (P < 0.01) 24-, 12-, 8-, and 6-h cycles that differed (P < 0.01) among lines. Estimated differences between MH and ML mice in feed intake and heat loss due to locomotor activity were 36 and 11.5%, respectively. Variation in activity thus contributed to variation in feed intake.

Genetic variation in residual feed intake and its association with other production traits in British Hereford cattle

Variation in residual feed intake, that is, variation in feed intake in relation to liveweight (LW) and growth rate, was investigated using data from 540 progeny of 154 British Hereford sires, collected over ten 200-day postweaning performance tests conducted between 1979 and 1988. Residual feed intake (RFI Reg) was calculated for each test as the difference between actual feed and expected feed intake predicted from a multiple regression of feed intake on metabolic mid-test LW and average daily gain (ADG). RFI Reg was heritable (0.16, S.E. 0.08) and phenotypically and genetically independent of size and growth rate. RFI Reg had favourable phenotypic and genetic correlations with feed conversion ratio (FCR) and estimated maintenance energy expenditure. It was negatively correlated with estimated lean content of the carcase (LEAN) and appeared to be genetically independent of mature cow LW (COWWT). RFI Reg over the performance test was not affected by differences in pre-test rearing treatments, in contrast to start-of-test LW and end-of-test LW, and in some years, ADG and FCR. Selection against RFI has the potential to increase the efficiency of beef production by reducing feed intake without changing the growth rate of the young animal, or increasing mature cow size.

Influence of Feed Intake Fluctuation, Feeding Frequency, Time of Feeding, and Rate of Gain on Performance by Limit-Fed Steers

Abstract Four experiments were conducted to determine the effects of feed intake fluctuation, feeding frequency, time of feeding, and rate of gain on performance by limit-fed steers. Mean initial BW for steers in Experiments 1, 2, 3, and 4 were 378 ± 43, 225 ± 19, 227 ± 20, and 249 ± 17 kg, respectively. All experiments were complete random designs, and pen was the experimental unit. In Experiment 1, 10% daily variation in feed intake resulted in decreased (P 0.10) ADG or G/F. Average daily gain and G/F by steers programed to gain 1.25 kg/d were not affected (P>0.10) by 10% fluctuation in feed intake or twice daily feeding in Experiment 3. Treatments used in Experiment 4 consisted of 1) steers fed to gain 0.9 kg/d, 2) steers fed to gain 0.9 kg/d with 10% daily feed intake fluctuation, 3) steers fed to gain 1.25 kg/d, and 4) steers fed to gain 1.25 kg/d with 10% daily feed intake fluctuation. Fluctuation in feed intake did not affect (P>0.10) ADG at either rate of gain. Results suggest that feed intake fluctuation in limit-fed cattle might decrease performance early in the feeding period; however, cattle seem to adapt to fluctuating feed intake as the feeding period progresses. Time and frequency of feeding did not affect performance by limit-fed steers.

Seasonal patterns of feed intake and growth of Hammerfest and Svalbard Arctic charr maturing at different ages

Age at maturity, feed intake and growth were monitored in two strains (Hammerfest, 70°N and Svalbard, 80°N) of Arctic charr, Salvelinus alpinus, held under comparable rearing conditions for 2 years (age 9 to 32 months after hatching). Feed intake of individually tagged fish was measured using X-radiography, and growth rates and timing of maturation were monitored for the same individuals. There were differences in age at first maturity, both between strains and between sexes. In the Hammerfest charr, 56% of the males matured at age 1+, but no male Svalbard charr matured at this age. At age 2+, 94% of the Hammerfest males were mature, compared to 14% of the Svalbard males. No females matured at age 1+, while 71% of the Hammerfest females matured at age 2+. No Svalbard females matured during the study. There were no differences between strains in weight, feed intake and growth before the first reproductive season at age 1+. When fish were held at constant temperature (7°C), seasonal variations in feed intake and growth were observed in fish of both strains and sexes. Svalbard charr were significantly larger than Hammerfest charr from age 19 months onwards, and by the end of the experiment, the Svalbard charr were 25% larger than Hammerfest charr. The differences between the strains were largest in 2+ mature males. Prior to maturation, maturing fish were significantly larger than immature fish, but in the weeks immediately prior to maturation, the maturing fish grew significantly less than the immatures.

The correlation between feed-intake cycle and nutritional zinc-deficient status in rats.

The characteristic cyclic variation in feed intake of rats fed a Zn-deficient diet (Mills et al, Am J Clin Nutr 22: 1240-1249 (1969)) followed a Cosinor curve, as determined by computer analysis (Tamaki et al, Br J Nutr 73: 711-722 (1995)). The values of amplitude for the feed-intake cycle had a positive correlation to their own day-to-day variations and to the correlation value of their own simulated cycles (r2 = 0.764, df = 50, p < 0.001 and r2 = 0.682, df = 50, p < 0.001, respectively). The cyclic variation in feed intake was accompanied by a cyclic variation in body-weight change in rats fed the Zn-deficient diet, and cyclic variation in body-weight change occurred similarly in pair-fed control rats. There were no differences in the mesors of body-weight change cycles of Zn-deficient rats and pair-fed control rats (Zn-deficient rats: 2.5 +/- 1.0 g/d, pair-fed rats: 2.8 +/- 1.0 g/d, mean +/- SD, df = 18, t = -0.674, ND). Rats fed the Zn-deficient diet were given different amounts of Zn supplementation by daily subcutaneous injection. The amplitude of the feed-intake cycle was decreased with increasing Zn supplementation (r2 = 0.919, df = 5, p < 0.001). The concentration of Zn for the appearance of the feed-intake cycle was estimated to be 71.6 +/- 6.6 micrograms/d per rat. The Zn level in the serum showed a significant decrease in the Zn-deficient diet groups, but the supplement of Zn did not vary in the Zn-deficient rats injected with up to 47.3 micrograms/d per rat. From these results, an analysis of the feed-intake cycle allowed us to estimate the quantitative Zn-deficient status of rats.

Effects of imposed feed intake variation on acidosis and performance of finishing steers.

Four metabolism and two finishing trials were conducted to determine the effects of imposed feed intake variation on acidosis and performance of finishing steers. In Metabolism Trial 1, four ruminally fistulated steers were limit-fed and subjected to either a constant amount of feed per day (C) or low intake variation of .7 kg/d (LV). No treatment differences were found for intake or measures of acidosis. Metabolism Trial 2 was conducted similarly to Metabolism Trial 1 with treatments of C and high intake variation of 1.4 kg/d (HV). Treatment HV increased (P < .05) acidosis, as indicated by the area of ruminal pH below 5.6. In Metabolism Trial 3, four steers were fed at ad libitum levels of intake and subjected to three levels of intake variation: ad libitum intake with no imposed intake variation (AL), LV of .7 kg/d, and HV of 1.4 kg/d. No treatment differences were found. In Metabolism Trial 4, six ruminally fistulated steers were fed at ad libitum levels and subjected to three levels of intake variation: AL, LV of .9 kg/d, and HV of 1.8 kg/d. Average ruminal pH increased (P < .05) and area of ruminal pH below 5.6 decreased (P < .05) as level of intake variation was increased. In Finishing Trial 1, 75 steers were assigned to eight pens and two treatments: AL or HV of 1.8 kg/d. Dry matter intake increased (P < .05) from AL to HV. Daily gain and gain/feed were not affected by treatment. In Finishing Trial 2, 94 steers were assigned to 12 pens and two treatments: AL or HV of 1.8 kg/d. No treatment differences were noted in DMI, daily gain, or gain/ feed. Therefore, results of these trials indicate that intake variation of up to 1.8 kg/d does not increase acidosis or decrease performance of finishing steers fed at ad libitum levels of intake.

Feeding, growth and social interactions during smolting and seawater acclimation in Atlantic salmon, Salmo salar L.

In the wild, parr of Atlantic salmon, Salmo salar L., normally show aggressive, territorial behaviour, whereas smolts form non-aggressive schools. In order to study changes in feed acquisition in hatchery reared Atlantic salmon smolts, individual feed intake and hypoosmoregulatory capacity were monitored during smolting and seawater acclimation, using X-radiography and 24 h seawater challenge tests. In both the seawater transferred fish and the control fish remaining in freshwater, weight-specific feed intake and specific growth rates increased during the spring. One week after seawater transfer, there was a temporary decrease in feed intake and an increase in the number of non-feeding salmon. Four weeks after transfer, the fish had compensated for the growth suppression, and there were no differences in weight, feed intake and growth rates. Feed intake was weakly correlated with hypoosmoregulatory capacity, both in freshwater and after seawater transfer. There were no changes in the coefficient of variation in feed intake during smolting and seawater acclimation, and there was no relationship between weight, feed intake or growth in the freshwater phase and the subsequent feed intake in seawater. The study indicated that the decrease in feed intake after seawater transfer may not be explained as a behavioural change in social interactions, but rather as a transient acclimation to seawater.

Feeding time, feed intake and growth of baltic salmon, Salmo salar, and brown trout, Salmo trutta, reared in monoculture and duoculture at constant low temperature

It has previously been reported that the growth of salmonids is improved when they are reared in duoculture, possibly due to reductions in the levels of intraspecific aggression. These claims were examined in more detail by studying the time of feeding, feed intake and growth in Baltic salmon, Salmo salar, and brown trout, Salmo trutta, held at constant low temperature (2.7–3°C) and reared in monoculture or duoculture for 3 months. Rates of feed intake and growth were initially low, but increased during the course of the experiment, this being particularly evident for the salmon. At the same time, interindividual variations in feed intake and growth tended to decrease, and group feed intakes were negatively correlated with interindividual variations in feed intake. Rates of growth of brown trout did not differ significantly between groups reared in monoculture and duoculture. By contrast, rates of feed intake and growth of Baltic salmon were depressed when they were held together with brown trout. Baltic salmon, held together with brown trout, had a significantly greater incidence of fin damage than those held in monoculture, possibly indicating that they were subject to aggression from the trout. Fish of both species tended to feed during the hours of daylight, and there was no evidence of an increase in nocturnal feeding activity amongst fish of either species when held in duoculture.

Selection for Economic Efficiency of Dairy Cattle Using Information on Live Weight and Feed Intake: A Review

The objective of this study was to review some of the latest evidence on genetic variation in feed intake and feed utilization and to determine how this variation might be used. The most important sources of genetic variation in gross efficiency are likely to be the quantities of feed eaten and used for yield or maintenance and the extent to which body tissue is mobilized. Accounting for just one of these components when selection is for improved feed efficiency might result in undesirable genetic changes. For example, in an ad libitum feeding system, the heritability of body condition score is reported to be 0.43 for heifers; genetic correlations of body condition score with milk production and live weight were −0.46 and 0.66, respectively. Also, the genetic correlation between milk yield and live weight depends on lactation stage. For example, over the first 26 wk of lactation, this correlation was reported to be −0.09, but, after genetic adjustment for body condition score, the correlation was 0.29. When economic values are being derived, energy norms or genetic correlations can be used, and double counting of the feed costs needs to be avoided. An index that contained linear type traits, however, gave high accuracy of selection. Hence, although there appears to be great potential to improve economic efficiency by selecting for feed intake and live weight or by possible indicator traits, there is still uncertainty about some of the genetic parameters, especially among traits related to health, reproduction, and energy balance.

Lysine: Amino acid requirements of broiler breeders

Because feed intake is controlled in broiler breeders, amino acid supply is determined by the composition of the feed and the level of feed intake. Controlling amino acid supply during the laying cycle can be facilitated by the use of a model for calculating requirements. A possible model is outlined and the various components discussed. Typical calculations suggest that the model can provide a useful basis for practical feeding decisions. Model elements include: levels of animal performance; utilization of amino acids for egg production, maintenance, and tissue growth; population structure; and the variation of feed intake and the covariance between feed intake and requirements.