Voluntary Feed Consumption Research Articles

Abomasal Nitrogen Flow Affects the Relationship between Dietary Nitrogen and Insulin-Like Growth Factor-I in Growing Lambs

Twelve abomasally cannulated wether lambs were fed isocaloric diets containing 9, 12 or 15% crude protein to determine insulin-like growth factor-I (IGF-I) responses to altered abomasal nitrogen flow and nitrogen status. Lambs were offered 1100 g/d of their respective diets. Voluntary feed consumption was not affected by nitrogen intake. Ruminal and total tract digestibilities of dry matter, organic matter and nitrogen increased linearly (P < 0.05) with increased dietary nitrogen. Abomasal flows of total, bacterial and rumen escape nitrogen increased (linear, P < 0.01), whereas dry matter and organic matter flows decreased (linear, P < 0.01). Total amino acid flow was greater (linear, P < 0.01) in lambs fed additional nitrogen due to increased (linear, P < 0.01) flows of essential and nonessential amino acids. Nitrogen retention and blood urea nitrogen increased linearly (P < 0.01). Serum IGF-I concentrations and relative hybridization intensity of hepatic IGF-I mRNA increased (linear, P < 0.05) as lambs consumed more nitrogen. Serum IGF-I and hepatic IGF-I mRNA were correlated positively (P < 0.05) with nitrogen intake and abomasal flows of nitrogen and various amino acids. These data provide evidence of a relationship between abomasal amino acid flow, as influenced by nitrogen intake, and hepatic gene expression and serum concentrations of IGF-I in growing lambs.

THE EFFECT OF SODIUM HYDROXIDE SPRAY TREATMENTS OF MAIZE COBS ON CONSUMPTION AND DIGESTIBILITY BY THE WEST AFRICAN DWARF SHEEP

A Sodium Hydroxide spray treatment of ground maize cobs was evaluated in a feeding trial with West African Dwarf rams. NaOH solution of 6% concentration was sprayed on ground maize cobs at the rate of 50 litres per 100 kg, representing a 3% NaOH treatment level. The treated maize cobs were air dried before being incorporated into the experimental diets. The apparent digestibility coefficients for dry matter (DM), crude protein (CP), crude fibre (CF) and gross energy (GE) were significantly higher in the diet containing NaOH — treated maize cobs (diet B) than the diet containing the untreated maize cobs (diet A). Addition of sucrose (purified cane sugar) at 37 level to the NaOH-treated maize cobs (diet C) produced a slight increase in voluntary feed consumption but did not have any significant effect on nutrient digestibility when compared with diet B. Relative intake (RI), nutritive value index (NVI) and digestible energy (DE) intake values were significantly higher in diets B and C. Average daily weight gains were also significantly higher in the diets containing NaOH-treated maize cobs (diets B and C), while the efficiency of feed utilization values were closely related to the daily weight gains.

Feed consumption, nutrient utilization and serum metabolite profile of captive blackbucks (Antelope cervicapra) fed diets varying in crude protein content

A feeding trial was conducted to determine the optimum level of crude protein (CP) in the diet of captive blackbuck (Antelope cervicapra) in which feed consumption and nutrient utilization are maximal. Fifteen blackbucks (BW 25-34 kg) were distributed into three groups of five each in an experiment of 75-days duration including a digestion trial of 5-day collection period. All the animals were offered 200 g of concentrates and fresh maize fodder ad libitum. The overall CP content of the three respective diets was 6.9%, 10.4% and 12.7%. Blood samples were collected on the last day of the experiment. Intake and digestibility of CP increased (p < 0.01) with the increased level of CP in the diet. Feed consumption and nutrient intake were not significantly different among the groups. However, digestibilities of most of the nutrients were higher in the 10.4% CP diet than in the 6.9% CP diet. The endogenous loss of nitrogen was similar among the groups. Based on the endogenous losses, minimum N requirement was calculated to be 776 mg/kg BW(0.75) /day, and to meet this requirement, diet must contain at least 8.27% CP. Serum urea nitrogen concentration increased (p < 0.01) with increased level of dietary CP. Serum level of glutamate oxaloacetate transaminase and glutamate pyruvate transaminase was higher (p < 0.05) in the group fed 6.9% CP diet. Animals in the group fed low protein diet also lost body mass during the experimental period. It was concluded that a diet containing 10.4% CP was optimum for maximizing nutrient utilization without any adverse effect on voluntary feed consumption and serum metabolite profile of blackbucks.

Effect of physical form of a lucerne hay on digestion and rate of passage in sheep

Six 2-year-old Corriedale wethers fitted with rumen and abomasal cannulas were given a lucerne hay to appetite then at restricted intakes, first as chopped hay (CH) then as ground and pelleted hay (GP). Voluntary feed consumption (VFC) was 43% greater on GP hay. Water intake increased as feed intake increased and was greater on GP. Organic matter digestibility (OMD) decreased as intake increased (P &lt; 0.001) but was greater on GP hay at restricted intakes (P &lt; 0.001). Potential OMD was marginally greater for GP hay. The digestibility of fibre constituents decreased as intake increased but was not significantly affected by grinding and pelleting. Rumen OM content and OM flow to the duodenum increased as intake increased on CH hay (P &lt; 0.001), but only rumen OM content increased with grinding and pelleting (P &lt; 0.001). Consequently, the mean retention times (MRT) of solutes, particles, and microbial protein decreased as intake increased (P &lt; 0.001) and increased with grinding and pelleting (P &lt; 0.001). However, at VFC, the MRT of each component was not affected by grinding and pelleting. Nitrogen digestibility decreased as intake increased but was greater on GP hay at restricted intakes. Non-ammonia nitrogen flow to and digestion in the intestines increased with intake but was not affected by grinding and pelleting. Ammonia synthesis in the rumen increased as feed intake increased (P &lt; 0.001) but its 26–32% contribution to non-ammonia nitrogen flowing to the duodenum, which reflects microbial synthesis, was not affected by intake or grinding and pelleting. It was concluded that rate of passage cannot be accurately predicted because it is a derived parameter, determined by the differential responses of rumen digesta content and digesta outflow to the dietary, environmental, and physiological factors affecting the animal. These differential responses are involved in determining the effects of intake and grinding of hay on its digestibility.

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.

Evidence of a role for neuropeptide Y and monoamines in mediating the appetite-suppressive effect of GH.

Among the many responses to GH administration is suppression of voluntary feed intake (FI) in some species, attributed to improvement in the efficiency of nutrient utilization and, therefore, reduced need for ingested substrates. Commercial broiler chickens have been genetically selected for generations for rapid growth, realized largely via the major correlated response of increased voluntary feed consumption. Neuropeptide Y (NPY) and monoamines play very important roles in the central regulation of feeding. Preliminary studies from our laboratory suggest that the appetite-suppressive effect of GH may be independent of its actions as a repartitioning agent, and may involve alterations in NPY expression at the pre-translational level. The purpose of this investigation was to explore the dose-response nature of the appetite-suppressive effect of GH in juvenile broilers, and the possible involvement of NPY and monoamines in this process. A GH dose-response study was conducted using 8-week-old female broilers infused i.v. with GH in a pulsatile pattern for 7 days at 0, 10, 50, 100 or 200 microgram/kg body weight per day. Hypothalamic NPY and epinephrine (EP) concentrations decreased in a dose-related manner with GH. At the highest dosage, voluntary FI decreased 19% (P<0.05) and hypothalamic NPY mRNA decreased approximately 50% in the infundibular nuclei and midline region (P<0.0001). In contrast, birds pairfed to the high-GH dosage group did not differ from controls, verifying that changes in NPY and monoamines were not secondary to reduced FI. We conclude that hypothalamic NPY and EP are likely candidates to explore further as mediators of the appetite-suppressive effect of GH.

Supplementary grain and sodium propionate increase the liveweight gain and glucose entry rates of steers given molasses diets

An experiment was conducted to investigate the effects of supplements of sorghum grain (1·1 kg/day), sodium propionate (65 g/day) and intra-abomasal glucose infusion (200 g/day) on the liveweight gain (LWG), glucose metabolism and nitrogen utilization of Hereford steers (four per group) given molasses-based diets ad libitum (molasses 589, urea 18, sunflower meal 194, pangola grass hay 187, minerals 12 g/kg as fed). There was no significant effect of treatment on voluntary feed consumption (97–106 g dry matter (DM)/kg0·75 per day) or DM digestibility (0·564–0·579). The LWG of steers given grain supplements (899 g/day) and sodium propionate (943 g/day) were significantly (P&lt;0·05) higher than those of steers given the basal diet (741 g/day) and basal diet plus glucose infusion (794 g/day).All supplements decreased plasma urea concentrations and urinary N excretion and increased the efficiency of N utilization from 0·19 to 0·36–0·38 g N retained per g apparently digested N intake. The provision of grain (13% DM intake) increased ammonia and volatile fatty acid (VFA) concentrations in rumen fluid, and both grain and sodium propionate increased the proportion of propionic acid in ruminal VFA. Grain supplementation and glucose infusions significantly (P&lt;0·05) increased glucose entry rates (g/day and g/kg0·75 per day) without affecting plasma glucose concentrations or glucose pool size and space. Sodium propionate supplements increased plasma insulin concentrations to a lesser extent than grain supplements and glucose infusion. Increased glucose availability in the present studies was associated with an improved utilization of dietary protein and, in some cases, with improved LWG.

Substitution of sorghum grain for molasses increases the liveweight gain of steers given molasses-based diets

Sorghum grain was included in a basal diet of molasses (molasses 505, urea 21, sunflower meal 191, pangola grass hay 250, minerals 31 g/kg as fed) at rates of 0, 202, 391 and 707 g/kg, generating diets in which grain replaced 0 (diet A), 33 (diet B), 66 (diet C) and 100% (diet D) of the molasses and hay. The four diets were fed to groups of four Hereford steers (293–334 kg liveweight) over a 96-day period. One half of each treatment group was implanted with a growth promotant (zeranol), and all were slaughtered at a commercial abattoir at the end of the trial. The inclusion of 33% grain increased voluntary feed consumption, digestible dry matter (DM) intake (from 57·6 to 82·0 g digestible DM/kg0·75 per day and significantly increased liveweight gain from 592 to 900 g/day. Zeranol implantation also increased liveweight gain but not feed intake. Steers given only grain (diet D) had the highest liveweight gains (1127 g/day). The addition of grain to molasses diets decreased urinary N excretion and increased ammonia and volatile fatty acid (VFA) concentrations in rumen fluid. Molar proportions of propionic acid in total VFA increased from 0·15 to 0·20, and butyric acid decreased from 0·36 to 0·29 when 33% of the molasses was replaced by sorghum grain. The fat content (depth at sacral position) of the carcasses of steers given grain only (diet D) was significantly greater (14 mm) than that of steers given the basal diet of molasses (4 mm), and carcass fat contents were intermediate (10 and 11 mm) for steers given diets B and C respectively. It was concluded that the inclusion of small amounts of grain in molasses-based diets increased cattle growth principally by increasing digestible energy intake without decreasing molasses intake, thus improving the efficiency of utilization of molasses in molasses-based diets.

Feed intake, digestion, and renal function in Merino sheep selected for higher clean fleece weight

Two-year-old ewes from 2 Merino lines selected for higher clean fleece weight were compared with their control line when given a lucerne hay diet and a lower quality diet of mixed wheaten and lucerne hays. There were no differences between lines in voluntary feed consumption, feed digestibility, urea synthesis and excretion, or renal function. Voluntary feed consumption, digestibility, urea synthesis and excretion, glomerular filtration rate, and urea and creatinine clearances were higher on the higher quality lucerne hay diet; renal tubular function was not affected by diet. Water intake was higher in the selection lines; consequently, these sheep produced more of a less concentrated urine. The renal clearance of endogenous true creatinine was significantly greater than the glomerular filtration rate, indicating that creatinine was secreted in the kidney tubules. The results support the conclusion that differences between selection lines in physiological parameters not directly associated with wool follicle function are unlikely to be consistent enough to be useful predictors of genetic merit for fleece weight.

The response of thick-lipped grey mullet, Chelon labrosus (Risso), to diets of varied protein-to-energy ratio

Six experimental diets were designed containing two lipid levels (approximately 12% and 24%) and three protein levels (approximately 38%, 49% and 59%) with protein-to-energy ratios ranging from 19.72 to 29.83 mg protein kJ-1 gross energy. At both lipid levels, an increase in the protein content of the diet was achieved by decreasing the carbohydrate input. After 84 days of feeding to appetite, the fish (juvenile thick-lipped grey mullet, Chelon labrosus (Risso)) fed the high-oil diets of low and medium protein content were significantly larger than those fed low-lipid diets containing similar amounts of protein. The greatest weight gain and optimum apparent net protein utilization (ANPU) were recorded for the fish fed the diet with a P:E ratio of 19.72 mg protein kJ-1 gross energy. At both lipid levels, increasing dietary protein supplementation led to a decrease in voluntary feed consumption and ANPU. Whole-body lipid appeared to increase in response to a higher dietary oil component. At low levels of dietary lipid, the carcass protein content increased in response to elevated protein supply. This trend was less obvious at the higher level of lipid supplementation. Hepatic glycogen deposition was significantly lower amongst the fish fed the low-carbohydrate diets at both levels of supplemental oil. A significant increase in hepatosomatic index was also recorded which was not directly correlated with either dietary carbohydrate or protein:energy level. It can be concluded from the present experiment that the optimum protein-to-energy ratio for C. labrosus juveniles is in the order of 19.72 mg protein kJ-1 gross energy when fed the present diets containing 37.9% crude protein and 22.8% lipid. Additionally, for this species, lipid was seen as a more effective source of non-protein energy than a corn starch/dextrin mixture.

Intake and digestion in swamp buffaloes and cattle. 4. Particle size and buoyancy in relation to voluntary intake

SUMMARYThe buoyancy and distribution within the rumen of feed particles were measured to assess the extent to which particle buoyancy modulated the movement of particles within and from the reticulorumen. Two experiments conducted in north Queensland in 1988 involved swamp buffaloes (Bubalus bubalis)and crossbred cattle (Bos indicus × B. taurus) fed rice straw with various supplements of energy and protein, lablab (Lablab purpureus), verano (Stylosanthes hamata cv. verano) and two grasses, sorghum (Sorghum bicolor × S. sudanense) and pangola (Digitaria eriantha). Apparent passage rate of particles from the reticulorumen bore an inverse curvilinear relationship to particle size. Passage rate of the smallest particles, retained by a cloth of aperture 50 μm, was faster in buffaloes. Mean particle size of total rumen digesta was smaller in buffaloes than in cattle in Expt 1 with rice straw, but there was no difference between the forage diets offered in Expt 2. The large particle content in digesta (dry matter per kg) in the dorsal rumen was greater than that in the ventral rumen, which in turn was greater than that in the reticulum. The sedimentation rate of particles was measured by a procedure designed to interfere as little as possible with microbial fermentation. A curvilinear relationship was evident between sedimentation rate and particle size; the medium particles sedimented faster than the fine and large particles. Sedimentation rates of some particle fractions were greater in buffaloes than in cattle. Between diets there were marked differences in sedimentation rates of large particles, but these differences decreased with smaller particle size. Large particles from the dorsal sac sedimented faster than those from the ventral sac, and there was no obvious dorsal raft. There was no apparent relationship between particle buoyancy and ease of particle escape from the reticulorumen. The results are discussed in terms of current theories of the physical factors involved in the control of voluntary feed consumption by ruminants.

Energy and protein value of combinations of maize silage and red clover hay for ruminants, using adult sheep as a model

Four combinations of maize silage and artificially dried red clover were fed consecutively to adult wether sheep ad libitum and at a level designed to maintain liveweight. On a dry matter (DM) basis, the diets contained silage and clover at ratios 1 :0, 2: 1, 1:2, and 0:1. A fifth diet contained maize silage plus urea (as 1% of the silage DM). Energy, nitrogen (N), and carbon balances were measured at both feeding levels and while fasting. The maize silage contained 11 g N and 41 1 g cell wall organic matter/kg DM; corresponding values for red clover hay were 37 and 290 g. Energy and protein metabolisms were monitored on the 5 diets and were related to dietary N content. Feed intake and digestibility increased with the proportion of clover in the diet and, hence, with increasing dietary N content. Energy losses from methane and heat production did not differ among diets, whereas urine energy losses were smallest, and therefore, efficiencies of utilisation of metabolisable energy (ME) greatest, on the silage-only and silage + urea diets. Greater urinary N losses on the diets of 1 : 2 silage: clover and clover-only led to lower N balances at the same apparently digested N intake on these 2 diets. There were positive associative effects between the silage and clover for voluntary feed consumption; digestibilities of energy, N, and cell wall organic matter; and energy and N balances. A ratio of ME to digestible energy of 0.81 underestimated the content of ME in silage by 0.5 MJ/kg DM, yet overestimated it in clover by 0.4 MJ/kg DM. The ME value of the silage + urea diet (11.6 MJ/kg DM) was one of the highest reported in sheep for maize silage using indirect calorimetry.

Physiological state of sheep before and during fattening

SUMMARYVarious physiological characteristics of castrate male sheep fedad libitumwere studied between 1986 and 1987 in Australia. The sheep were between 20 and 45 kg live weight (LW) before and during fattening. Sequential data on body composition, estimated from TOH space (starting at 10 kg), established that the weight gain contained 19% fat and 13% protein belowc.26 kg LW and 55% fat and 10% protein above. For a given feed intake, the rate of fat gain was constant but the rate of body protein gain was 45% lower above 26 kg LW. Voluntary feed consumption per unit weight peaked at 26 kg LW and declined thereafter. There was an increase at 45 kg LW in weight of rumen digesta per unit feed intake and in digestibility; metabolizable energy (ME) per kg feed was increased as a result by 6–9%. Efficiency of use of ME, determined by calorimetry, increased from 44% in the lower weight range to 53% in the higher; utilization of nitrogen decreased from 37 to 28%. Partition of ME and energy gain suggested that the energetic efficiency of neither fat nor protein deposition changed between the two weight ranges. Change in the amount of DNA (‘cell number’) rather than g tissue/mg DNA (‘cell size’) accounted for weight change in three muscles and most digestive organs. Exceptions were the rumen, in which cell number and size both contributed to gain and the large intestine, in which cell size diminished during growth. It was concluded that fattening in sheep reflects diminution of the protein content of weight gain due to a decrease in the efficiency of protein utilization at the tissue level.

Influence of partial suckling of crossbred dairy cows on milk offtake and calf growth in the Ethiopian highlands

The effects of implementing a restricted suckling regime with crossbred dairy cows have been examined in the Ethiopian highlands. Calves were allowed to suckle their dams for two minutes before each milking until weaning and this system was compared with the common practice of rearing by bucket feeding. Restricted suckling significantly increased calf growth rate to weaning, from 0.31 to 0.53 kg/d (P less than 0.01). No differences in calf growth post-weaning occurred, so that the 20 kg difference in liveweight that had been achieved by the restricted suckled calves by the time of weaning persisted until the calves were nine months old, when observations ceased. Total milk offtake was not significantly affected by treatment, although that obtained from the partially-suckled animals over their whole lactation exceeded that from the other treatment by 15%. Partial suckling delayed return to oestrus post partum but this was offset to some degree by fewer services per conception, hence calving interval was not significantly increased. Voluntary feed consumption was similarly unaffected and it is concluded that restricted suckling offers tangible advantages for adoption by smallholders using crossbred cows in dairy production systems.

The effects of steam-pressure treatment, pelleting and ammoniation on the feeding value of sugarcane bagasse for cattle

Digestibility, intake and performance trials were conducted with cattle to evaluate the effects of physical and chemical treatments on the nutritive value of sugarcane (Saccharum officinarum) bagasse. The four treatments in the digestibility trial were: 1, untreated raw bagasse; 2, steam-pressure treatment with a continuous flow digester (SP); 3, steam-pressure treatment plus pelleting (SPP); 4, steam-pressure treatment, 3% anhydrous ammonia and pelleting (SPA). Steam-pressure treatment and pelleting, both with and without ammoniation, increased voluntary feed consumption (P &lt; 0.01) by 80% and 36%, respectively. Apparent digestion coefficients for organic matter, crude protein, cellulose, neutral and acid detergent fiber were not affected by steam-pressure treatment, but were reduced (P &lt; 0.05) when this was followed by pelleting. Sixty Brahman crossbred steers (340 kg) were allotted to 12 groups of five animals (three pens per treatment) and fed one of the treatment diets for 100 d. The four treatments were: 1, cottonseed hulls; 2, pelleted raw bagasse; 3, SPP bagasse; 4, SPA bagasse. Average gains (kg d−1) and feed-to-gain ratios for the four diets were as follows: 1, 0.91 and 8.78; 2, 1.10 and 8.39; 3, 1.12 and 7.72; 4, 1.15 and 7.98. Treatment means did not differ (P &gt; 0.05). SPP bagasse was compared with bermudagrass pellets and corn silage in growing diets for cattle. Ninety-six Brahman crossbred steers (167 kg) were allotted to 12 groups of eight animals (four pens per treatment) for 134 d. SPP bagasse pellets depressed gains and feed conversion ratio (P &lt; 0.05). Gains (kg d−1) and feed-to-gain ratio for SPP bagasse pellets, bermudagrass pellets and corn silage diets were 0.42 and 12.90, 1.02 and 7.65, and 0.94 and 5.87, respectively. This investigation demonstrated that feeding pelleted bagasse at 15% of the diet did not reduce performance of finishing cattle fed a high-grain diet. However, when growing cattle were limit-fed a grain-based supplement with free access to bagasse pellets, bermudagrass pellets or corn silage (approximately 70% of the diet), gains and feed efficiency were significantly (P &lt; 0.05) lower on the bagasse diet. Key words: Sugarcane bagasse, steam-pressure treatment, ammonia, digestibility, cattle performance

Voluntary feed intake of reindeer in relation to ambient temperature

Reindeer live in arctic and subarctic regions, where ambient temperature can vary f rom 4 5 ° C i n mid-winter to + 3 5 ° C in mid-summer. In many reviews on food intake it is concluded that food intake increases i n cold and decreases i n hot environments (Baile & Forbes 1974, A r n o l d 1984). A seasonal decrease i n voluntary feed consumption of ad libitum fed reindeer during winter has been reported earlier (Ryg & Jacobsen 1982). However , increased feed intake of reindeer during cold winter periods is reported by reindeer herders feeding reindeer in practice. O u r objective was to determine the effects of ambient temperature on voluntary feed intake of reindeer. Eight adult female reindeer and eight halfyear-old reindeer calves (4 males and 4 females) were fed w i t h pelleted feed concentrates (contained 9 % ash, 10 % crude protein (CP) , 15 % crude fibre (CF) , 7 % ether extract (EE) and 59 % N-free extract ( N F E ) on dry matter basis) f rom the beginning of N o v e m b e r in 1987 unt i l the calving period in M a y 1988. Af ter the calving the reindeer were changed to a high protein feed (contained 10 % ash, 20 % C P , 13 % C F , 7 % E E and 50 % N F E in D M ) . The females and their newly born calves (4 males and 4 females) were fed together unt i l mid-December 1988, w h e n the calves were weaned and excluded f rom the experiment. The yearlings were also fed w i t h the same feed at the same time. In the middle of September 1988 the female and male yearlings were separated to prevent the mating. F r o m the middle of December 1988 the feeding of reindeer continued w i t h the winter feed unt i l next calving period in M a y 1989. The reindeer were fed twice daily ad libitum. The offered feed and the daily feed remains were weighed. The ambient temperature was measured daily w i t h a thermograf. The reindeer were weighed weekly .

Prediction of Responses to Required Nutrients in Dairy Cows

Nutritional science is developing from the calculation of requirements toward prediction of responses to nutrients. Responses are defined as the consequences of the animal's interaction with its feed and other aspects of its environment For dairy cows the important responses to predict are yields of major milk constituents (fat, protein, and lactose), rates of changes of body fat and protein, and voluntary feed consumption. In order to make predictions of cow responses, equal weight needs to be given to quantitative descriptions of cow characteristics, feed characteristics, and relevant aspects of the environment. This paper presents a framework that allows quantitative descriptions of these various attributes to be combined for the purposes of response prediction. The nutrients of importance are those major end products of digestion, which act as substrates for the generation of animal products. Quantitative aspects of genotype include description of potential milk constituent yield, mature body size and composition, and rate of maturing, including a description of “labile” protein mass and animal capacities for ingestion of potentially constraining feed attributes. A variety of approaches is available to simulate nutrient supply and use for purposes of response prediction. The synthesis of such models adds both intellectual richness to the process of developing nutritional science and improved frameworks for testing theories through experiment.

Factors limiting the intake of feed by sheep. 15. Voluntary feed consumption and digestion in lambs fed chopped roughage diets varying in quality

Feed intake and digestion transactions for diets based on wheaten straw and mature ryegrass diets, both supplemented to provide additional essential nutrients, were compared with those for lucerne hay and high quality forage oats diets in lambs weighing c. 24 kg. The data obtained and comparable published data were used to examine relationships between a range of variables. Decrease in roughage quality, as evidenced by decrease in digestible organic matter (OM) intake, was accompanied by increase in (i) times spent eating, ruminating and chewing rumination boluses, (ii) the quantities of digesta or digesta OM in the reticula-rumen and omasum. It was not associated with increase in the large particle content of rumen or omasum digesta. Over the range of diets, close direct relations were demonstrated between (i) ruminating time and the amount of digesta in the reticulo-rumen, (ii) the amounts of digesta in the reticulo-rumen and omasum, (iii) the OM concentrations in rumen and reticulum digesta, (iv) the OM concentrations in reticulum and omasum digesta, (v) the OM concentration of reticulum digesta and of digesta flowing to the omasum, and (vi) the increase in OM concentration from reticulum to omasum digesta and OM concentration of reticulum digesta. Reticulum digesta were of finer texture than rumen digesta, and the bulk density of digesta particle fractions varied between diets, and sometimes between rumen and large intestine digesta. With a straw-based diet, relative to lucerne hay, small particles ( &lt;600 �m sieve) were cleared more slowly from the reticula-rumen, associated with a larger reticulo-rumen particle pool. The data are discussed in relation to (i) the flow of digesta through the alimentary tract and (ii) the regulation of roughage intake. They were considered to be consistent with a concept that energy metabolism and digesta load in the reticulo-rumen interact in the regulation of roughage intake.

Factors limiting the intake of feed by sheep. 14. Comparisons of voluntary feed consumption and various transactions in the alimentary tract between lambs and sheep fed roughage diets

Comparisons were made of the voluntary intake and digestion of diets based on chopped straw, chopped lucerne and ground lucerne by lambs (c. 23 kg body weight) and adult sheep (c. 53 kg), and of feed intake and digestion by larger lambs (c. 35 kg) and adult sheep (c. 53 kg) grazing senescent pasture. With both the chopped and ground diets the lambs (i) had digesta of finer texture, (ii) cleared larger particles (&gt;600 �m sieve) more slowly from the rumen, (iii) cleared organic matter more rapidly from the omasum and caecum + proximal colon, and (iv) performed more rumination bites. With the chopped diets, digestibility and feed intake pcr unit metabolic body weight were similar for lambs and adults but the lambs (i) spent more time eating, (ii) had more digesta in the reticulorumen per unit reticulo-rumen digesta free body weight, (iii) clcarcd organic matter from the abomasum more rapidly, and (iv) had less ash in abomasal digesta (straw-based diet only). With the ground lucerne diet the lambs (i) consumed more feed per unit of metabolic body weight, (ii) regurgitated more rumination boluses and (iii) cleared organic matter more rapidly from the reticulorumen. At pasture, it was estimated that the lambs (i) consumed more feed per unit metabolic body weight, (ii) had more digesta in the reticulo-rumen per unit rcticulo-rumen digesta free body weight, (iii) clearcd organic matter more rapidly from the omasum and abomasum, (iv) tended to have digesta of finer texture and (v) had less ash in abomasal digesta (-64%). The differences between the lambs and adults and their possible implications arc discussed, and it is suggested that with lower-quality roughages the smaller lamb may be placed at a disadvantage, relative to the adult, because of an inability to exhibit a higher feed intake commensurate with its higher maintenance energy need.

Feed intake and digestion responses in sheep to the addition of inorganic sulfur to a herbage diet of low sulfur content

Young sheep were used in feed intake and digestion studies with a wheaten straw diet of low sulfur (S) content (0.71 g S kg-1 organic matter) fed without additional S (low S diet) or containing sodium sulfate (high S diet). With the high S diet, relative to the low S diet, (i) the levels of sulfide in rumen liquor were elevated, (ii) less time was spent in ruminating activities, (iii) rumen liquor volume tended to be lower (-l0%), (iv) indigestible markers were cleared more rapidly from the rumen (approx. +16%), (v) more organic matter (OM) and acid detergent fibre were digested in the stomach and in the alimentary tract as a whole (+8-16%), (vi) more non-ammonia nitrogen (NAN) flowed into the intestines (+13%) and this NAN had a higher content of sulfur amino acids (+ 17%), (vii) a greater quantity of NAN was digested in the intestines per unit of digestible OM intake (+ 12%), (viii) the concentrations of various amino acids in venous blood were substantially lower, and (ix) fungal activity in the rumen was higher. The number of bites performed on each rumination bolus was inversely related to fungal activity in the rumen, but the latter was not related to OM digestibility or to the sulfur amino acid content of the NAN flowing into the intestines. The OM digestibility was inversely related to the rate of clearance of markers from the rumen with the low S diet but not the high S diet. Voluntary feed consumption was similar with both diets, and in the low S diet it was inversely related to OM digestibility and positively related to marker clearance rate from the rumen. Digestible OM intake under conditions of ad libitum feeding was higher with the high S diet (+11%) and positively related to fungal activity in the rumen and to the sulfur amino acid content of the NAN flowing into the intestines. The data indicate that an inadequate amount of S in the low S diet impaired the metabolism of the rumen microbiota which in turn affected variables relating to digestion and metabolism. A quantitatively significant role for the anaerobic fungi in the structural degradation of fibre is suggested and attention is drawn to the need for studies on both feed consumption regulation with low S diets and the relation between herbage S status and the sulfur amino acid content of the rumen microbiota.