Predicting Feed Intake of Food-Producing Animals (1987)

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5 Dally Cattle INTRODUCTION Variability in feed intake among dairy cattle intro- ctuces an assortment of complex biological problems. Many of these have not been effectively delineated for dairy cattle; nevertheless, insights and measurements made in the past two decades have resulted in several functional predictive equations, with specificity for the physiological function mimicked through models (Mer- tens and Fly, 1979~. In the presence of enough feed, energy for lactating effects the largest change in feed intake above that for maintenance (Bull et al., 1976; Murdock and Hodgson, 1979; Baile and Della-Fera, 1981; Moe, 1981~. Other changes arise from weight gain or loss, gestation, activ- ity, and climate (Moe, 1981~. Feed intake can be pre- dicted as a function of these independent variables within a lactation period if the energetic efficiency is known and the period of time under consideration is longer than 30 days so that short-term rhythmic changes in intake are averaged (Conrad, 1966; Bines, 1976; Bull et al., 1976; Moe, 1981~. However, knowl- edge of the bounds of the homeorhetic status of cattle and physical constraints on feed intake are significant requirements for developing a prediction equation of feed intake (Conrad et al., 1964; Campling, 1970; Baile and Della-Fera, 1981~. Cows receiving diets with sufficient protein eat 2 to 3.8 times the maintenance requirement for minimal ac- tivity (confinement housing) (Ekern, 1972; Tyrrell and Moe, 1975~. If the proportion of grain fed is large, dry matter consumption may exceed 4 times maintenance partly because of increased milk production and partly because of the suppression in digestibility (Moe et al., 1965~. The suppression in digestibility occurs mostly in the rumen (Tyrrell and Moe, 1975) and may be cor- rected in part by providing additional crude protein in the diet either as protein (Tyrrell and Moe, 1980) or urea (Huber, 1975; Poos et al., 1979~. The reduction in digest- ibility is about 4 percent per multiple of maintenance increase in intake (National Research Council [NRC], 1978~. LACTATION RESPONSE Feed intake response of cows to increased milk pro- duction in early lactation is slow but normally reaches a maximum between 10 and 14 weeks postpartum as less body stores of fat (Ronning and Laben, 1966; Flatt et al., 1969) and amino acids in muscle (Trig" et al., 1981) are secreted in milk. The rise in feed intake during early lactation may be enhanced additionally by providing a high level of protein (Egan and Moir, 1965; Conrad et al., 1977; Roffler and Thacker, 1983~. The feed intake curves of cows eating mixed rations averaged biweekly for the lactation period are readily fitted to quadratic equations for generalization (Stone et al., 1960; Odwongo and Conrad, 1983~; however, the results with individual cows produced spurious devia- tions from the quadratic curves, presumably represent- ing animal-diet interactions in the feed or environmental constraints on the cows. The change in feed intake for the time course of a lactation period is shown for 14 Holstein cows producing 8,100 kg of milk in 305 days  (Figure 5-1~. The equation for those data was: i= i~.2i + To.- 0.0003d2, (I) where Iis kilograms of dry matter per day, and d is days after calving. The "rules of the thumb" commonly used to estimate feed intake of mature cattle are as follows: (l) nonlactat- ing cattle gaining about 800 g daily eat dry matter equiv- alent to 2 percent of their body weight, and (2) lactating cows eat dry matter equivalent to 3.2 percent of their body weight. The difference in the caloric density of the 48

Dairy Cattle 49 30 co - ~ 20 LL 10 o Mil king 1 1 1 1 1 1 1 1 1 1 1 1 Dry 0 1 2 3 4 5 6 7 8 Tl M E AFTE R CALV I NG (mo) food accounts for part of the narrow difference between the two. Monteiro (1972) developed a dynamic model for appetite control during lactation. He used a closed- loop scheme, thereby eliminating consideration of phys- ical limits on feed intake. He reasoned that these limits were of relatively short duration and cows would adjust quickly to a new physiological state in which metabolic controls determined the limits of feed intake. The model is dependent on the input of three independent vari- ables, maintenance requirement, milk production, and gain, but does not account for differences in chemical energy and physical structure of feeds. It is character- ized by delay functions for feed intake while cows are transferring body stores of fat and muscle to milk. The equation was: Ft = d W~+aMt+y .~(B' i+l-Bt-i) 1 = 1 t (1-CL'1)-P ~ (M' i+l-Mt i)8i, i = 1 (2) where We and its coefficients are weight-based mainte- nance functions; Mf is milk at any time I; B is the weight change; fly is the conversion factor of weight into milk; ~ is the delay factor for gain; p is the delay parameter for milk; and ~ is the coefficient for evaluating the food deficit not attributable to weight change. Monteiro (1972) noted that the delay parameters were relatively constant and could be set at 0.65 and 0.80, respectively, for milk and gain. In this computation the delayed lactation represents the amount of milk the cow would produce if there was an instantaneous linear rela- tionship between milk and feed intake. The difference between observed and delayed lactation curves mea- sures successively the deficit or excess in feed intake over milk production and/or gain in weight. Appropriate  constants have not been developed for breeds and diets used in the United States. Although it is difficult to predict feed intake measured 9 lo 1, ,2 FIGURE 5-1 Feed intake of Holstein cows during and after lactation. as dry matter or digestible energy, the approaches to this problem are worthy of comparison before a method of computing estimates is selected. LIMITING FORAGE INTAKE The voluntary intake of different foods and the amount of digesta in the reticulorumen are interdepen- dent (sampling, 1970~. When sheep were offered sev- eral roughages such as hay and dried grass, there was evidence that they ate to a constant fill (Blaxter et al., 1961~. The importance of plant cell wall as the primary restrictive determinant of intake has been demon- strated by Mertens (1973~. Presently, the concentration of neutral detergent fiber (NDF) for optimum milk fat production is under investigation in several laborato- r~es. It is generally assumed that intake and digestibility of forages are directly related, as in equations 5, 6, and 7 cited below. While they are somewhat interrelated, in- take and digestibility of forages are separate measures of quality. Intake is dependent upon the structural vol- ume measured by the cell wall content, while digestibil- ity is dependent on the chemical content and its availability (VanSoest, 1982~. This is particularly noted if one compares the differences in intake of grasses and legumes. To date no one has devised a successful mathe- matical model for evaluating on a chemical basis the differences in intake among all forage species, except by using a standard animal for comparing fill. Neverthe- less, the mathematical model is an important consider- ation. Using alfalfa, timothy hay, and orchard grass, Conrad et al. (1964) found that the variance-attributable to forage differences might be as high as 43 percent of the total dry matter intake. A major constraint to feed intake in ruminant foods is

50 Predicting Feed Intake the indigestible dry matter (Crampton, 1957; Conrad, 1966; Baumgardt, 1970; Campling, 1970; Mertens, 1985~. The relationship between available nutrients and voluntary food intake in ruminants is biphasic (Forbes, 1977~. There is a positive correlation between the con- tent of available energy and the amount of food eaten with poor- and medium-quality roughages and a nega- tive correlation with high-quality roughages and cereal- based diets (Conrad et al., 1964; Baumgardt, 1970; Bull et al., 1976~. When physical limitations of gut capacity set an upper limit for food intake, predictability is com- plicated by individual differences in the amount of ab- dominal space and competition for space by increasing abdominal fat and fetal growth (sampling, 1970; Baile and Forbes, 1974~. Conrad et al. (1964) found that all variables associated with the rate of passage of fecal dry matter accounted for 42 percent of the variation in feed intake. Forbes (1977) developed a computer analog model for predicting variations in feed intake of lactating and/or pregnant cows using metabolic, physical, and endocrine factors. A more sophisticated dynamic model is under development by Mertens and Ely (19791. Neither of these dynamic models has advanced to a state of art for general usage. Hibbs and Conrad (1975) developed equations to esti- mate dry matter intake, maximum feed intake, and min- imum allowable digestibility. They have been modified here to include newer terms for energy requirements for lactation maintenance and activity and weight gain (Moe, 19811. The requirements for net energy are: NE = 0.080W075 + 0.74M+ 5.~^Wforcowsgaining weight (3) or NE = 0.080W075 + 0.74M- 4.91`Wforcowslosingweight, (3a) where NE is net energy lactation in Meal, W is body weight in kg, Mis 4 percent fat-corrected milk in kg, and Wis weight change in kg/day. NE may be converted to metabolizable energy (ME) by dividing NE by 0.60, which is the proportion of ME converted to milk. ME can be converted to digestible energy (DE) by dividing ME by 0.83, which accounts for losses as methane and in urine from DE. DE can be determined from NE by multiplying by 2.0 (1/0.60 x 1/0.83~. DMIe = 2 (0.08 W0.75 + 0.74M + 5.1^ W)/4.4, (4) where DMIe is dry matter intake needed for energy utili- zation. When physical factors limit feed intake, then the amount consumed is (Conrad, 1966~: DM]f = 500(i - dE) TABLE 5-1 Comparison of Intake Predictions (per- cent of BW/day) for a Cow Weighing 600 kg with Zero BW Changes - 4% Fat Corrected Equation Reference No.a Milk (kg) 1 2 3 10 15 20  25 30 35 40 45 4 2.2 2.53 2.4 2.69 2.7 2.87 3.0 3.03 3.2 3.20 3.4 3.37 3.6 3.53 3.8 3.70 2.12 2.42 2.25 2.72 2.55 3.02 2.84 3.32 3.13 3.56 3.43 3.72 3.94 5 6 2.50 2.42 2.82 2.77 3.09 3.03 3.32 3.22 3.50 3.46 3.64 3.57 3.74 3.74 3.79 3.89 aEquation reference numbers are as follows: 1. National Research Council (1978). 2. Agricultural Research Council (1976). 3. Theoretical equation predictions for average grass (65% NDF); Mertens (1985). 4. Theoretical equation predictions for average legume (50% NDF); Mertens (1985). 5. McCullough (1981). 6. Odwongo and Conrad (1983). where DMIf is dry matter intake in kg/day, W is body weight in kg/day, and dE is the proportion of digestible energy in the dry matter. Maximum feed intake (DMImaX) is: DMImaX = 550W + 2(0.08W~75 + 0.74M + 5.1^W) (6) Minimum allowable digestibility (DMDmin) is: DMDmjn= F. I(0.08W 4+0.74M+5.1^W) (7) Bull et al. (1976) concluded that most cattle eat amounts approximating the maximum feed intake set by ballast constraints. Mertens (1985) has devised a method of predicting feed intake using the amount of NDF. This method appears promising but underesti- mates the feed intake of cows eating legumes at produc- tion levels below 30 kg. A table of comparison abstracted from Mertens' (1985) compilation is pre- sented in Table 5-1. COMPUTING ESTIMATES OF DRY MATTER INTAKE Recently Odwongo and Conrad (1983) introduced a prediction equation which provides for changing rela-  tionships among amount of milk produced, weight gain or loss, amount of feed needed for maintenance, and intake. Thus, DE = 0.453 W0593 M033 e0 ~6 ~w (8)

Dairy Cattle 5 1 For nonlactating pregnant cows the equation is: DE = 0 453 We 593 en i9 low where e is the base of the natural logarithm for i\ Win kg/ day. DMIf divided by the concentration of DE (Meal/kg) in DM provides feed intake. Validation and efficacy of predicting dry matter intake using Equation 8 is shown in Table 5-2. Weights of cows were taken monthly; thus, predic- tions are extremely useful in the long term, but predic- tions in the short term (per week or per day) may have more error and may not coincide. None of these equations provides for evaluation of olfactory factors, climate, or changing hormonal ef- fects. Because of the complexity of the control of feed intake and its interrelationship with energy balance, prediction equations of feed intake are limited guides when applied to specific situations. The specific limits arise from environmental, managerial, and social fac- tors; previous feeding history; physiological condition and hormonal status; the physical and nutritional quali- ties of feed; and several sensory inputs, all of which can have marked effects on feed intake (Baile and Della- Fera, 1981~. Environmental Temperature Feed intake declines in cattle of European origin, Bos Taurus, above environmental temperatures of 20 to 25°C (Kleiber, 19751. The decline approximates 3.3 per- cent/°C. Continuous heat stress may reduce feed intake until cattle reach negative energy balance, and they may cease eating when climatic temperatures above 40°C are maintained. Thermal stress is more severe in early lactation among cows producing large amounts of milk (McDowell, 1972~. These effects are enhanced directly by relative humidity. Cold temperatures result in in- creased feed intake in direct proportion to energy bal- ance, which is affected by tissue insulation, including fatness and pelage (NRC, 1981~. Thyroid function and maintenance requirements usually increase from nor- mal at 10°C to 1.5 times maintenance at -20°C (NRC, 1981~. Thus the dry matter intake of a cow in weight equilibrium requires 3 additional kg of dry matter at -20°C. In addition to dietary dilution with indigestible mate- rial and hot climate there are numerous other con- straints on feed intake. Ruminants have, in comparison to carnivores, dietary regimens that induce large ther- mogenic effects (Baile and Della-Fera, 19811. Most of this arises from ruminal fermentation as the heat of fer- mentation. Hypothalamic temperature increases with feeding activity whether or not energy is consumed. It is not likely, then, that hypothalamic temperature plays a role in controlling feed intake. Other factors in the ru- men and reticulum influencing feeding behavior are ten- sion receptors (sampling, 1970; Grovum, 1979), osmolarity, pH, and volatile fatty acids (Baile and Forbes, 1974~. Metabolites Large amounts of volatile fatty acids are produced in the rumen (Baile and Forbes, 1974~. Intraruminal injec- tions of acetate or propionate solutions of increasing  incremental concentrations before and during a sched- uled meal depressed feed intake in cattle, sheep, and goats. The surface of the dorsal area appears to be sensi- tive to acetate. With as little as 5 percent of the surface exposed to a 1-M solution of acetate, individual meal quantity and length were depressed. Despite the simi- larity of effect with exposure to acetate and propionate, the receptor sites must be different. Apparently, the receptors for propionate are in the walls of the rumen vein (Baile and Forbes, 1974~. While glucose has long been considered a controlling system for feeding in monogastric animals, it is doubtful whether it has a role in cattle. It is interesting, however, that Stone (1975) was able to completely satiate the need of sheep for hay by driving the plasma glucose level to about 130 mg/dl. Likewise there is no specific evidence to date that free fatty acids or amino acids provide satiety signals to cat- tle consuming balanced diets. It is quite clear, however, that feeding of higher levels of protein at peak lactation stimulates feed intake (Roffler and Thacker, 1983~. Feed intake may be summarily reduced during meta- bolic diseases such as milk fever, ketosis, D-lactic acido- sis, or bloat (Baile and Della-Fera, 1981~. Infectious mastitis, metritis, and diarrhea either abruptly or chron- ically reduce feed intake and cause major losses in milk production (Natzke, 19811. In cattle estrogen (17-`B-estradiol) administered at periparturient levels depresses feed intake; this is coun- teracted by progesterone (Muir et al., 1972~. Insulin lev- els change during feeding (Chase et al., 1977), but its importance or functional response remains undeter- mined. However, most hormones affect feed intake when given in large amounts, such as the increases in feed intake when surfeit amounts of growth hormone are given (Baile and Della-Fera, 1981~. Meals and Meal Patterns The microstructure of eating behavior in cattle as measured by periodicity, amount, and rate is an impor

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Dairy Cattle 53 tent feature of eating behavior in cattle. These mea- sures are essential to understanding the interaction of a specific feed with an experimental animal. The time required for satiation is highly variable and dependent on the diet. After a hiatus in eating, cows may eat a meal of palatable concentrate within 15 min. whereas to eat a total mixed ration with silage containing fermentation products, 150 min may be re- quired before the animal ceases with nibbling bouts (Conrad et al., 1977; Heinrichs, 1982~. In general, the rate of eating during the first bout of newly adminis- tered feeds resulted in successively increasing meal times for wetted concentrate, silage, and hay (Conrad, 1966~. In cattle, as in other species, there is a postpran- dial correlation, that is, a significant correlation be- tween the size of a meal and the interval that follows it. Typically, in cattle fed once or twice daily there are two large meals that may occur as a normal meal when fresh food is given and immediately after milking. The average time for spontaneous meals is about 5 min. dur- ing which cows eat about 600 g. Normally, cows con- sume 10 to 20 spontaneous meals during 24 h. The initial meals are closely related to the physical form of the diet due to differences in degree of mastication and deglutition that are required (Kertz et al., 1981~. This may also be important in spontaneous meals. Gustatory, olfactory, or tactile stimuli can also influence the amount of total feed intake and feeding patterns in cattle. Three classes of compounds have been found to shorten meal length and meal weight. They are organic acids, amines, and ammoniacal nitrogen or its precur- sors (Clancy et al., 1977~. Because of widespread use of silage which contains all of these compounds, knowl- edge of their quantitative effects has become of interest. The reduction in meal length which coincided with am- monium load in the rumen as a result of injection of ammonium salts or precursors of ammonia showed that ruminants were perturbed quickly, possibly from intra- cellular acidification and intoxication by the increasing ammonia concentration in the rumen (Conrad et al., 1977~. When this was done with urea, the resultant ef- fect was malaise and learned avoidance (Chalupa et al., 1979; Kertz et al., 1982~. Carotid blood ammonia con- centration increased so rapidly after dosing with urea that it was apparent that the ammonia absorbed from the rumen leaked past the liver (Bartley et al., 19811. Thus an alternative site for the action and source of malaise may be from a direct effect of ammonia on the central nervous system. Amine nitrogen depresses meal length in a similar manner; however, the site of the effect is not known (Clancy et al., 1977; Phillip and Buchanan-Smith, 1982~. Grinding and Pelleting In mature cows principally offered roughage diets containing adequate protein, control of feed intake by physical factors in the rumen usually ceases between 60 and 70 percent digestibility (Conrad et al., 1964; Mont- gomery and Baumgardt,1965~. Grinding the diets alters the threshold of digestibility downward to a point at which physical limitations may cease to exist (Cam- pling, 1970~. However, there does appear to be a total gut limit in lactating cows and dairy heifers which is 22 percent higher when ground diets are fed (Montgomery  and Baumgardt, 1965; Bull et al., 1976~. This approxi- mates what would be expected if the space in the cecum and large bowel was added to the ruminal space for the pool of indigestible material. Thus, the daily turnover rate of fecal dry matter rises from 1.08 percent body weight (Conrad et al.. 1964) to 1.32 percent (Bull et al., 1976~. In rats, it is 1.4 percent body weight (Adolph, 1947~. This is commensurate also with the observation that grinding and pelleting of the diet result in an in- creased fecal mean particle size from 0.30 to 0.36 mm and more rapid evacuation from the gut. Thus, in gen- eral, dairy cattle consume 20 percent more dry matter of ground and pelleted diets if they do not contain chemical inhibitors such as urea, fatty acids in fat, and alkaloids or if the resultant pellet is injurious to the mouth (Mont- gomery and Baumgardt, 1965; Hibbs and Conrad, 1975~. SUMMARY In the metabolic partitioning of feed, cows expend a sizable portion of energy for maintenance of tempera- ture regulation. Periodically, excess heat may accrue from the high calorigenic effect of fibrous feeds, causing reduced feed intake. Lactation, growth, and fattening are all stimuli for increased feed intake. Under condi- tions of normal temperature and humidity, these are quantitative effects that can be related functionally to body size. There is a consensus that at low digestibili- ties, the level of milk production is determined by the cow's capacity for feed, particularly undigested resi- dues, and the rate at which undigested feed can be moved through the alimentary canal. At high levels of digestibility, the physiological state of the cow is the primary determinant of feed intake. The point among a series of increasing digestion coefficients at which phys- ical limitations on eating capacity vanish and the influ- ence of productive energy demands become dominant varies with body size, production, and amount of fecal

54 Predicting Feed Intake TABLE 5-3 Predicted Dry Matter Intake (DMI) in Dairy Cows Milk, 4~o FCM (kg/d) 15 20 30 35 Change in DE in Diet weight (kg/d) (Meal/kg) - 1.0 2.85 1.0 3.10 0.5 2.85 0.5 3.10 1.0 2.85 1.0 3.10 0.5 2.85 0.5 3.10 0.5 2.85 0.5 3.10 0 2.85 0 3.10 0.5 2.85 0.5 3.10 -0.5 2.85 -0.5 3.10 o.o 2.85 0.0 3.10 -0.5 2.85 -0.5 3.10 o.o 2.85 0.0 3.10 -0.5 2.85 -0.5 3.10 o.o 2.85 0.0 3.10 _0.5 2.85 -0.5 3.10 _0.5 2.85 -0.5 3.10 - 1.0 2.85 - 1.0 3.10 Body Weight (kg) 350 400 450 500 * * * 13.4 14.7 15.7 * * 15.9 14.6 * 12.5 13.6 * * * * 13.8 14.9 * *  * * * * 13.6 14.7 * * * * * 16.0 * 18.2 16.8 16.9 15.5 * 18.5 18.6 17.1 * 18.2 * * * 15.8 16.8 * * * * * 18.0 14.5 15.6 16.6 * * * * * * * * * * * * * * * * * 16.4 17.5 * * * * 550 19.3 17.7 17.9 16.4 * 19.5 19.7 18.0 *  20.6 19.1 17.6 * 20.1 20.1 18.5 * * * 18.3 19.4 * * * * * * 21.8 20.1 18.5 * 21.1 21.2 19.5 * 22.2 22.2 20.4 * 20.2 21.2 22.8 21.1 19.4 24.1 22.2 22.2 20.4 * 23.2 23.3 21.4 * 24.1 24.2 22.2 * 23.8 23.9 - 19.9 21.0 22.0 - 600 650 700 750 800 20.3 21.3 22.2 23.2 24.1 18.7 19.6 20.5 21.3 22.1 18.8 19.7 20.6 21.5 22.3 17.3 18.1 18.9 19.7 20.5 22.3 23.4 24.5 25.5 26.5 20.6 21.6 22.5 23.5 24.4 20.7 21.7 22.7 23.7 24.7 19.0 20.0 20.9 21.8 22.7 22.0 23.1 24.2 25.2 26.2  19.3 20.2 21.3 22.2 23.2 24.1 19.4 20.4 21.4 22.3 23.3 24.2 17.8 18.8 19.7 20.5 21.4 22.2 * * * 26.0 27.0 28.1 23.9 24.9 25.8 22.0 23.0 23.9 20.3 21.2 22.0 25.2 26.3 27.3 23.2 24.1 25.1 23.2 24.2 25.1 21.3 22.2 23.1 * 27.5 28.6 25.3 26.3 25.3 26.3 23.3 24.2 * 29.7 26.3 27.3 26.3 27.3 24.2 25.1 * 29.2 25.9 26.9 26.0 27.1 23.9 24.9 24.3 24.3 22.4 25.2 25.2 23.2 24.9 25.0 23.0 NOTE: These values are functions of daily milk production (M), change in weight (^ W), digestible energy (DE) concentration in the diet, and total BW and were computed from the formula DE = 0.453 W° 593 M0 33 e0 i6/` W(Odwongo and Conrad, 1983). An asterisk means that the amount of feed computed was in excess of the amount cows would be expected to eat and still maintain a daily turnover of undigestible "gut fill" equal to 1.1% of their BW. dry matter excreted daily. Equations 4 through 7 can be used to approximate this point. Changes in estimated dry matter intake (DMI) in cows as functions of daily milk production (M), change in weight (^ W), digestible energy (DE) concentration in the diet, and total body weight were computed from the formula DE = 0.453W° 593 M~ 33 e0 ~6^W, as indicated in Equation 8. The limits of feed intake were determined with Equation 5. The results are summarized in Table 5- 3 for cows of different sizes. REFERENCES Adolph, E. F. 1947. Urges to eat and drink in rats. Am. J. Physiol. 151:110. Agricultural Research Council. 1976. The Nutrient Requirements of Farm Livestock. No.4. Composition of British feedstuffs. Agricul tural Research Council. (Obtainable from Her Majesty's Stationery Office, London.) Baile, C. A., and M. A. Della-Fera.1981. Nature of hunger and satiety control systems in ruminants. J. Dairy Sci.64:1140. Baile, C. A., and M. J. Forbes.1974. Control of feed intake and regula- tion of energy balance in ruminants. Physiol. Rev. 54:160. Bartley, E. E., T. B. Avery, T. G. Nagaraja, B. R. Watt, A. Davidovich, S. Galitzer, and B. Lassman. 1981. Ammonia toxicity.5. Ammonia concentration of lymph and portal, carotid and jugular blood after the ingestion of urea. J. Anim. Sci.53:494. Baumgardt, B. R. 1970. Control of feed intake in the regulation of  energy balance. Pp.235-254 in Physiology of Digestion and Metab- olism in the Ruminant, A. T. Philipson, ed. Newcastle-upon-Tyne, England: Oriel Press. Bines, J. A.1976. Regulation of food intake in dairy cows in relation to milk production. Livestock Prod. Sci.3:115. Blaxter, K. L., F. W. Wainman, and R. S. Wilson.1961. The regulation of food intake in sheep. Anim. Prod.3:51. Bull, L. S., B. R. Baumgardt, and M. Clancy.1976. Influence of caloric density on energy intake by dairy cows. J. Dairy Sci.59:1078.

Dairy Cattle 55 Campling, R. C. 1970. Physical regulation of voluntary intake. Pp. 227-234 in Physiology of Digestion and Metabolism in the Rumi- nant, A. T. Philipson, ed. Newcastle-upon-Tyne, England: Oriel Press. Chalupa, W., C. A. Baile, C. L. McLaughlin, and J. G. Brand. 1979. Effect of introduction of urea on feeding behavior of Holstein heif- ers. J. Dairy Sci. 62:1278. Chase, L. E., P. J. Wangness, J. K. Kavanaugh, L. E. Griel, and J. H. Gahagan. 1977. Changes in portal blood metabolites and insulin with feeding steers twice daily. J. Dairy Sci. 60:403. Clancy, M., P. J. Wangsness, and B. R. Baumgardt. 1977. Effect of silage extract on voluntary intake, rumen fluid constituents and rumen motility. J. Dairy Sci. 60:580. Conrad, H. R. 1966. Symposium on factors influencing the voluntary intake of herbage of ruminants: Physiological and physical factors limiting feed intake. J. Anim. Sci. 25:227. Conrad, H. R., A. D. Pratt, and J. W. Hibbs. 1964. 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