Effects of Feed Composition and Stage of Lactation ... - Science Direct

Effects of Feed Composition and Stage of Lactation on the Short-term Feeding Behavior of Dairy Cows N. C. FRIGGENS,1 B. L. NIELSEN,1 I. KYRIAZAKIS, B. J. TOLKAMP, and G. C. EMMANS Animal Biology Division, Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG, United Kingdom

ABSTRACT Twenty Holstein-Friesian cows were assigned to one of four feeding groups throughout lactation in a full change-over experiment using two total mixed diets. The low concentrate total mixed diet contained 100 g of concentrate/kg of fresh matter, and the high concentrate total mixed diet contained 300 g of concentrate/kg of fresh matter. The remainder of the total mixed diet was grass silage. The two changeover groups switched total mixed diets at 153 d of lactation; the other two treatment groups remained on their assigned diets throughout lactation. For analysis of short-term feeding behavior, four periods of 3 wk each were identified. The midpoints of these periods were –102, –18, 18, and 102 d from the changeover. The concentrate content of the total mixed diet significantly affected dry matter intake and all short-term feeding behavior variables. Cows that consumed the high concentrate total mixed diet had fewer but longer visits to the feeders and ate more feed per visit than did cows consuming the low concentrate total mixed diet. With one exception, no significant effect of stage of lactation was detected for any of the short-term feeding behavior variables. Despite a highly significant decline in dry matter intake as lactation progressed for cows consuming the high concentrate total mixed diet, there were no interactions between total mixed diet and stage of lactation for any of the short-term feeding behavior variables. Large differences in feeding behavior were detected between cows consuming the same total mixed diet. These last two findings suggest that the use of short-term feeding behavior variables to predict daily intake is unlikely to be successful. ( Key words: feeding behavior, feed composition, stage of lactation, dairy cows)

Received March 11, 1998. Accepted August 17, 1998. 1Present address: Danish Institute of Agricultural Sciences, Department of Animal Health and Welfare, Research Center Foulum, PO Box 50, DK-8830 Tjele, Denmark. 1998 J Dairy Sci 81:2368–3277

Abbreviation key: HTMD = high concentrate TMD, LTMD = low concentrate TMD, STFB = short-term feeding behavior, TMD = total mixed diet. INTRODUCTION In diet formulation, the concentration of nutrients to be included in the diet is based on the predicted intake of that diet. Consequently, considerable importance has been attached to the prediction of feed intake by domestic animals. The time unit generally used in describing intake is the day, which has the advantage of being in synchrony with any diurnal patterns in feeding behavior ( 3 0 ) and with husbandry procedures, which usually show a 24-h cycle. However, day may not be an appropriate time unit when the goal is to understand feeding behavior. Both longer (17, 19) and shorter (8, 20) time units have been proposed as more biologically relevant. Most animals, and all domestic species, do not eat continuously throughout the day but eat in bouts. The study of short-term feeding behavior ( STFB) has largely arisen from work that aimed to elucidate, in various species, the physiological mechanisms and controls that initiate and terminate eating bouts (20, 22, 29). Until recently, progress in this field has been limited by the difficulty of manually collecting such data, and, as a consequence, few data sets have been available. However, the development of computerized systems for recording feed intake for a number of farm species, including cattle, has made it possible to increase significantly the amount of feeding behavior information that is recorded automatically, which has led to various suggestions for the analyses and application of such data (25). The equipment used in this study allowed the direct measurement of the number of visits to the feed bin per day, the duration of the visit, and the feed intake per visit. From these primary measures, the values of other variables (daily feed intake, feeding rate, and time spent feeding) can be calculated. In Figure 1, the arithmetic relationships between primary and derived variables are shown by the connecting lines; these lines do not imply any causality.

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SHORT-TERM FEEDING BEHAVIOR OF DAIRY COWS

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A number of basic questions related to the STFB of cows remain unanswered. Data collected in this study provided the opportunity to evaluate the STFB of lactating dairy cows. In this study, the differences in STFB that resulted from two different feeds and whether these differences were affected by stage of lactation were examined. The effect on STFB of changing from one feed to another was also examined. Production aspects of this study have been reported by Friggens et al. (13). MATERIALS AND METHODS Experimental Design

Figure 1. A representation of the arithmetical relationship between components of short-term feeding behavior for data recorded on a visit basis [adapted from Nielsen (25)]. Primary measures are shown at the apices of the triangle. Derived measures, located at the midpoints of the triangle sides, can be calculated from the two primary measures, which are connected by that side.

The criterion used to denote a discrete episode of feeding was the visit, which was initiated when a cow was first identified by a feeder and finished when the cow left the feeder. However, the relationships shown in Figure 1 apply equally to meals ( 7 ) or other definitions of bouts (36). Nielsen ( 2 5 ) has discussed the problems of analyzing and interpreting these interrelated measures. For clarity, in this paper, STFB is defined as all of the measures shown in Figure 1 except total daily intake. Because daily feed intake is the product of the number of visits per day and the feed intake per visit (Figure 1), it has been suggested that STFB could be used to predict daily feed intake (11, 28). However, an alternative use of STFB may be to quantify differences between cows in their feeding strategy (1, 25). Different cows may achieve the same daily feed intake through markedly different combinations of number of visits per day and feed intake per visit (36). Variation both within and between cows in the feeding strategies they exhibit may be useful in quantifying the effects of the social and physical environment (6, 16), and genotype ( 1 0 ) of the cow, which, until recently, were difficult to estimate.

The effect of two total mixed diets ( TMD) on the pattern of intake from the start of lactation through to the start of the subsequent dry period was evaluated using 20 cows in a full 2 × 2 change-over design with control treatments. The two TMD consisted of grass silage and concentrate and varied only in the proportions of these components. The high concentrate TMD ( HTMD) contained 300 g of concentrate/ kg of fresh matter, and the low concentrate TMD ( LTMD) contained 100 g of concentrate/kg of fresh matter. The mean concentrate contents of HTMD and LTMD, expressed on a DM basis, were 59 and 27%, respectively. Each of the four treatment groups was offered a single TMD for ad libitum intake, at any one time. Two groups remained on their allocated TMD throughout the experiment; the other two groups changed from one TMD to the other at 153 d of lactation (SD = 3.0 d). For analysis of STFB, four periods of 3 wk each were chosen as is shown in Figure 2. Periods 2 and 3 were, respectively, immediately prior to and immediately following the changeover, allowing for adaptation to the new TMD. Periods 1 and 4 were, respectively, 3 mo prior to and 3 mo following the changeover (Figure 2). Period 4 was chosen to be as long after the changeover as possible, although care was taken to ensure that none of the cows had started to dry off by this time. Period 1 was chosen to be an equal time before the changeover. Cows, Housing, and Milking The cows were all Holstein-Friesians ranging from the second to the eighth parity with a mean parity of 4.6 (SD = 1.26). The cows calved between September 14, 1995 and March 1, 1996 and were introduced into the experiment as soon after calving as they were considered fit ( X = 11 d of lactation; SD = 5.0 d). Cows were kept in a loose housing system with nonexperimental cows in one group. The floor was concrete, and there were 50 cubicles (bedded on rubber mats and sawdust) and 28 feeding positions. Tolkamp and Journal of Dairy Science Vol. 81, No. 12, 1998

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Figure 2. Representation of the four periods chosen for the analysis of short-term feeding behavior (bars) relative to the changeover (dashed line) from one total mixed diet to the other [high concentrated total mixed diet (300 g of concentrate/kg of fresh matter; HTMD) to low concentrate total mixed diet (100 g of concentrate/kg of fresh matter; LTMD) or LTMD to HTMD] and the observed pattern of DMI by cows fed HTMD and LTMD throughout lactation (solid lines).

Kyriazakis ( 3 4 ) have previously described the experimental facility. Cows were milked twice daily, between 0600 and 0800 h in the morning and between 1600 and 1730 h in the afternoon. The experiment was carried out at the Langhill Dairy Research Center, Edinburgh, Scotland (longitude, 3° 10′ W; latitude, 55° 52′ N ) between October 10, 1995 and December 16, 1996. Feeds and Feeding Both feeds were TMD consisting of grass silage and concentrate. The silage was made from a single, second-cutting perennial ryegrass and contained 269 g of DM/kg of fresh matter and 166 g of CP, 501 g of NDF, and 305 g of ADF/kg of DM. The concentrate was the same in both feeds. As fed, the concentrate consisted of 455 g/kg of barley, 100 g/kg of citrus pulp, 75 g/kg of soybean meal, 83 g/kg of protected soybean meal (Soypass; Cargill plc, Cobham, United Kingdom), 150 g/kg of maize gluten feed, 73.3 g/kg of rapeseed meal, 25 g/kg of a 50% fat premix (UFAC Ltd., Newmarket, United Kingdom), 7.6 g/kg of salt, 20.3 g/kg of dicalcium phosphate, 4.6 g/kg of calcined magnesite, 2.7 g/kg of limestone flour, and 3.5 g/kg of dairy minerals and vitamins (Scotmin Dairy/Beef Supplement; Scotmin Nutrition, Ayr, Scotland). The concentrate contained 895 g of DM/kg of fresh matter and 199 g of CP, 233 g of NDF, and 109 g of ADF/kg of DM. The DM of the feeds was measured by drying in a forced-air oven at 80°C for 24 h. Crude protein was measured by the semi-automated Kjeldahl Journal of Dairy Science Vol. 81, No. 12, 1998

method ( 3 ) , and NDF was measured by the method of Robertson and van Soest (26). The method of Goering and van Soest ( 1 5 ) was used to measure ADF. Sampling, methods of analysis, and compositions of the silage, HTMD, and LTMD have been reported in greater detail by Friggens et al. (13). The TMD were mixed fresh each morning to contain either 100 g of concentrate/kg of fresh matter (LTMD) or 300 g of concentrate/kg of fresh matter (HTMD). The LTMD contained 331 g of DM/kg of fresh matter and 175 g of CP, 11.7 MJ of metabolizable energy, and 429 g of NDF/kg of DM. The HTMD contained 457 g of DM/kg of fresh matter and 185 g of CP, 12.8 MJ of metabolizable energy, and 343 g of NDF/kg of DM. Approximately three-quarters of each TMD was then evenly distributed into six automated intake recording feeders (Hoko Farm RIC feeders; Insentec BV, Marknesse, The Netherlands). The cows were denied access to the feeders when they were being replenished in the morning (0800 to 0930 h). The remainder of the TMD was distributed into the feeders during the afternoon milking. The amounts of TMD offered were calculated daily to achieve orts equal to 10% of daily intake. In addition, cows received 0.5 kg of concentrate in the milking parlor at each milking. Throughout the experiment, the ratio of cows to feeders did not exceed 2. Daily intakes and STFB of each cow were recorded by automated intake recording feeders, which recorded the identity of the cow, the amount consumed per visit (to the nearest 0.1 kg), the time of day, and the duration of each visit to the feeders. Tolkamp and Kyriazakis ( 3 4 ) have described these feeders in detail. For 1 wk of the experiment, it was not possible to measure intake because essential maintenance work was carried out in the experimental facility. During that time, cows continued to receive the experimental feeds. Intake records were screened for errors by plotting intake per visit against duration of visit for individual cows over the entire experiment and inspecting all points that yielded a residual that was numerically greater than ±7 standard deviation when intake per visit was regressed on duration of visit. Deviant records that could not be corrected by logical deduction from the preceding and following visit records were coded as missing values. Missing intake values accounted for 0.07% of the records. Missing durations accounted for 0.12% of the records. The number of visits recorded was unaffected by this screening process because no records were deleted from the data.


Statistical Analyses Four periods that were 3 wk in duration were used for the analyses, two before (periods 1 and 2 ) the changeover period and two after (periods 3 and 4). The midpoints of periods 1, 2, 3, and 4 were –102, –18, 18, and 102 d from the changeover. In 4 out of 80 cases, a period was moved by 1 wk because it coincided with the week when intake and STFB could not be recorded (cows no. 23, 42, and 131 in periods 4, 3, and 4, respectively) or with a bout of ill health (cow no. 115 in period 4). Within the four periods, records for 55 cow-days (3.3% of total cow-days) were discarded because of equipment failure. Analyses were carried out in two parts using Genstat ( 1 4 ) on the individual period means. First, using only the data from periods 2 and 3 (immediately before and after the changeover), ANOVA was used to test for residual effects of feeding treatment during period 2 on STFB during period 3. Cow was fitted as the block structure, and TMD, period, and the interaction between TMD and period, which quantifies possible residual effects of prior TMD were fitted as treatments. The full data set for the four periods was subsequently analyzed using REML (37); cow was the random effect, and TMD, period, and the interaction between TMD and period were the fixed effects. The model used for both the ANOVA and the REML analysis was Yij = m + Fij + pj + ci + Fijpj + eij where Yij = STFB during period j of cow i, m = overall mean, Fij = effect of TMD applied during period j to cowi, pj = effect of period j, ci = effect of cow i, and eij = unaccounted for variation for cow i during period j. Residual maximum likelihood analysis was used because it accommodated the changeover structure within the four-period design (37). Whether the size of the response to a change from HTMD to LTMD was different from the size of the response to a change from LTMD to HTMD was tested using a t test that compared the sum of the response means to 0. Differences were considered to be significant at P < 0.05. RESULTS Residual Effects of Previous TMD Total mixed diet and period effects on total daily DMI during the periods chosen for STFB analysis reflect the overall patterns of intake relative to stage of lactation reported by Friggens et al. (13). As was expected, daily intakes of HTMD steadily declined after an initial peak as lactation progressed, but daily intakes of LTMD did not change after an initial adap-

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tation (Table 1; Figure 2). Because of the different patterns of DMI relative to stage of lactation, there was an interaction between TMD and period on DMI ( P < 0.01). However, no significant interactions between TMD and period were detected for any of the STFB variables (Table 1), which was true for both the two-period analysis for residual effects of prior TMD and the full, four-period analysis. Therefore, effects of TMD and period on STFB could be tested independently of prior feeding treatment. Effect of TMD Composition The composition of the TMD significantly affected the values of all STFB variables (Table 1). The number of visits per day was higher ( P < 0.001) for cows fed LTMD than for cows fed HTMD. Conversely, intake per visit, both fresh and dry, was lower ( P < 0.001) for cows fed LTMD. The DMI per visit was reduced by half compared with that measured for cows fed HTMD. The duration of visits was also shorter ( P < 0.001) for cows fed LTMD, which was reflected in the different rates of feeding observed for cows consuming the two TMD ( P < 0.001; Table 1). The rate of intake of fresh matter per visit (grams per minute) for cows fed LTMD was 0.80 times that for cows fed HTMD (Table 1). Because the two feeds contained the same concentrate and silage expressed on a fresh weight basis in ratios of 1:9 (LTMD) and 3:7 (HTMD), it was possible to calculate, assuming additivity ( 9 ) , the mean feeding rates of silage and concentrate: 226 and 541 g of fresh matter per minute, respectively. From Table 1, using daily DMI as the example, it can be seen that multiplication of the two mean values of the primary STFB measures, mean number of visits per day, and mean DMI per visit always produce a value that is higher than the mean value for daily DMI. The mean of all of the values of the product of number of visits and DMI per visit is only equal to the product of the mean number of visits and mean intake per visit when these two measures are linearly related. In fact, for a given intake, these two measures are inversely related, which causes a positive bias when the means are inappropriately combined. The same is true for all of the STFB measures, although, in principle, feeding rate should be a linear combination of its component measures (Figure 1). Effect of Stage of Lactation No significant differences were found among the four periods of measurement for any of the STFB variables except daily feeding time (minutes per day), which showed a small, but significant, reduction ( P < 0.05) during period 4 compared with the Journal of Dairy Science Vol. 81, No. 12, 1998

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TABLE 1. Mean daily values of daily feed intake and short-term feeding behavior variables produced by REML for the two total mixed diets (TMD) during the four periods.1,2. HTMD3 Period 1

Period 3

Period 4

Period 1

Period 2

Period 3

P < Period 4

SED

Feed

Period

Interaction

NS4

30 872

31 793

30 880

29 873

42 475

48 399

48 436

49 410

3.1 61.8

*** ***

NS

NS NS

1882 6.0 162 151 323 23.3 49.9

1743 5.8 164 144 307 22.3 48.1

1943 6.0 154 149 328 21.1 46.5

1873 5.9 139 150 323 19.4 42.0

1388 5.4 201 88 258 16.6 49.2

1142 4.7 208 85 246 16.8 48.5

1305 5.0 216 82 252 16.4 51.5

1191 4.6 192 89 271 16.0 48.9

138.3 0.41 10.0 7.5 18.5 0.63 1.80

*** *** *** *** *** *** **

NS NS ** NS NS *** **

NS NS NS NS NS ** *

1For a measure that is the product of two primary measures, A and B, the mean of all of the values of ( A × B ) is equal to the product of the mean of A and the mean of B only when these two measures are linearly related. Time spent feeding, feeding rate, and daily intake are such measures. 2Each of the four periods lasted 3 wk. The midpoints of periods 1, 2, 3, and 4 were –102, –18, 18, and 102 d from the changeover on d 153 of lactation. Two treatment groups of cows remained on their allocated TMD throughout the experiment; the other two groups changed from one TMD to the other. 3HTMD = High concentrate TMD (300 g of concentrate/kg of fresh matter); LTMD = low concentrate TMD (100 g of concentrate/kg of fresh matter). 4P > 0.05. 5Daily intakes do not include the 0.5 kg of concentrate that was provided in the milking parlor at each milking. *P < 0.05. **P < 0.01. ***P < 0.001.

FRIGGENS ET AL.

Visits, no/d Intake per visit, g of DM Intake per visit, g of fresh matter Duration, min per visit Time spent feeding, min/d Feeding rate, g of DM/min Feeding rate, g of fresh/min Daily DMI,5 kg/d Daily fresh intake,5 kg/d

Period 2

LTMD

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results for the other three periods (Table 1). The observed differences between TMD in STFB were not affected by stage of lactation. A steady and significant ( P < 0.001) decline in daily DMI for cows fed HTMD was observed; this decline was not reflected in either of the component STFB measures, number of visits per day, or intake per visit (Table 1). Even allowing for the effects of combining inversely related measures, as described previously, the absence of trends in the component STFB measures appeared to be counterintuitive. However, as the variation of inversely related component STFB measures increased, the discrepancy between the true mean of the derived measure, in this case, daily DMI, and the product of the means of the component measures increased (25). The coefficients of variation for number of visits per day and DMI per visit for those cows fed HTMD throughout were higher during period 4 (36.3 and 40%, respectively) than during period 1 (24.9 and 27.4%, respectively). These differences in variation were sufficient to account for the increasing discrepancy between mean daily DMI and the product of the means of number of visits per day and DMI per visit but were not large enough to violate the homogeneity of the variance requirements of the statistical analyses. Effect of Changeover The change from one TMD to the other resulted in significant changes in the STFB (Table 2). No change in STFB was observed for cows fed the same

TMD throughout the experiment. No significant differences were detected between the absolute changes in STFB, regardless of whether cows changed from HTMD to LTMD or from LTMD to HTMD (Table 2). Variation Between Individuals Figure 3a shows the DMI per visit plotted against the number of visits per day during period 2 for cows consuming the two TMD. The isolines indicate all of the combinations of intake per visit and number of visits per day during period 2 that result in the mean daily DMI of 16.7 and 22.3 kg of DM/d for cows fed LTMD and HTMD, respectively. For each of the TMD, the variability among cows for daily DMI (12.3 and 10.1% CV for cows fed HTMD and LTMD, respectively, in period 2 ) was less than half the variability for the component measures of intake per visit (37.5 and 22.1% CV for cows fed HTMD and LTMD, respectively) and number of visits per day (42.5 and 22.1% for HTMD and LTMD, respectively). The variability in STFB among cows was large; the within-cow variation in STFB, measured over time, was relatively small. For cows fed HTMD, the within-cow coefficients of variation for intake per visit and number of visits per day during period 2 were 21.6 and 21.9%, respectively, which was approximately one-half of the values for the between-cow variation. The within-cow variation for cows fed LTMD was not significantly different from the within-cow variation for cows fed HTMD. The same was found for the other STFB measures.

TABLE 2. Mean difference between periods 2 and 3 in short-term feeding behavior of cows that changed from one total mixed diet to the other. For all measures, the response1 was not significantly different from 0. Changeover2

Visits, no./d Intake per visit, g of DM Intake per visit, g of fresh matter Duration, min per visit Time spent feeding, min/d Feeding rate, g of DM/min Feeding rate, g of fresh/min

HTMD to LTMD

LTMD to HTMD

Response

SED3

–18.3 351 453 0.99 –47 61 54

16.3 –496 –807 –1.40 50 –60 –72

–2.0 –145 –354 –0.41 3 1 –18

7.01 133.4 256.9 0.675 17.1 15.8 27.3

1The response was calculated as the sum of the mean effect of changing from the high concentrate total mixed diet (HTMD; 300 g of concentrate/kg of fresh matter) to the low concentrate total mixed diet (LTMD; 100 g of concentrate/kg of fresh matter) and the mean effect of changing from LTMD to HTMD. 2The cows were switched from one total mixed diet to the other at d 153 of lactation. The data from cows in the control treatment groups that did not change over are excluded. 3Standard error of the difference from a t test comparing the effect of changing from HTMD to LTMD multiplied by –1 with the effect of changing from LTMD to HTMD.


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The relationships between DMI per visit, number of visits, and daily DMI are shown in Figure 3, b and c. The correlation between DMI per visit and daily DMI was high for cows consuming both TMD, but in opposite directions ( r = –0.59 and 0.69) for cows fed HTMD and LTMD, respectively, (df = 8). The correlation between number of visits per day and daily DMI was significant for cows fed HTMD ( r = 0.82; P < 0.01), but this relationship depended heavily on one cow. Removal of this cow resulted in an R2 value that was one half of the original R2 value. The correlation between number of visits per day and daily DMI for cows fed LTMD was r = 0.17 (NS; P = 0.65). DISCUSSION The objectives of these analyses were to evaluate the effects of TMD composition, stage of lactation, and changeover from one TMD to another on STFB. Within the analysis, it was also possible to examine the variation between and within individual cows fed the same TMD; these aspects are discussed subsequently. Effects of TMD Composition and Changeover

Figure 3. a ) Mean intake per visit (kilograms of DM ± SEM) plotted against mean number of visits per day ( ± SEM) during period 2 for cows offered the high concentrate total mixed diet (300 g of concentrate/kg of fresh matter; HTMD; …) or the low concentrate total mixed diet (100 g of concentrate/kg of fresh matter; LTMD; o) . Periods were 3 wk in duration; period 2 was before the changeover from one total mixed diet to the other. Lines indicate combinations of intake per visit and number of visits per day that result in the same daily DMI. The mean DMI during period 2 was ) and 16.7 kg of 22.3 kg of DM/d for cows fed the HTMD ( DM/d for cows fed the LTMD ( - - -). Also shown are b ) the mean DMI (kilograms per day; ± SEM) plotted against mean intake per visit (kilograms of DM ± SEM) and c ) the mean DMI (kilograms per day; ± SEM) plotted against mean number of visits per day ( ± SEM) during period 2 for cows offered the HTMD or the LTMD. Journal of Dairy Science Vol. 81, No. 12, 1998

The TMD composition had a highly significant influence on all of the STFB variables (Table 1). The HTMD, the TMD that contained the least silage, was eaten at a significantly faster rate than was LTMD. Beauchemin ( 4 ) found similar effects for diets containing alfalfa hay as the forage. Cows offered HTMD had less frequent, but longer, visits to the feeders and consumed more per visit than did cows offered LTMD. In addition, when cows were changed from one TMD to the other, there was a rapid change in STFB, such that it could not be distinguished from the STFB of cows that had been consuming the same TMD throughout the experiment. The composition of the TMD affected not only the rate at which TMD was consumed but also other aspects of feeding behavior (Table 1). Evidence from the literature (4, 5, 7 ) indicates that the observed differences in STFB of cows consuming the two TMD may in large part be related to differences in the physical properties of the two feeds. Different aspects of the physical properties of feeds, such as particle size and physical structure, have been shown (21, 32, 35) to affect STFB, particularly in feeding rate. However, the literature (2, 5, 7, 23, 31, 32) concerning the effects of the physical properties of feeds on feeding rate and other aspects of STFB is not consistent. Dado and Allen ( 7 ) found that feeding rate averaged 76 g of DM/min for cows


fed a TMD with 51% concentrate and 31% NDF in DM. In comparison, the HTMD used in the present study contained 59% concentrate and 34% NDF in DM and was eaten at a rate (Table 1 ) that averaged twice that in the previous study ( 7 ) . Differences have been found in feeding rates between direct-cut silage (91 g of fresh matter/min; 16 g of DM/min) and wilted, chopped silage [63 g of fresh matter/min; 25 g of DM/min; (32)]. However, Alhadhrami and Huber ( 2 ) found no effect on feeding rate of an increase in the content of ADF in hay, and Beauchemin et al. ( 5 ) found no difference in the DM feeding rate for alfalfa hay or silage, even when the forage was dried and cubed (range, 67 to 75 g of DM/min). Suzuki et al. ( 3 1 ) fed silage to six cows that ate at 473 ± 121 g ( X ± SD) of fresh matter/min. In the present study, the mean feeding rate of silage was estimated to be 226 g of fresh matter/min. Considerable differences between reported values may be attributed in part to two difficulties with the studies cited previously (2, 5, 7, 23, 31, 32). The first difficulty is that all of those studies used small numbers of cows, ranging from two to eight, and, yet, as shown by Suzuki et al. (31), a large individual variation in feeding rate exists. Morita et al. ( 2 3 ) found a similar coefficient of variation between cows. The second difficulty with comparisons across studies is related to differences in methodology for measuring STFB. A third issue, identification of the appropriate feed descriptor relating to chewing or feeding rate that accounts for the effects of both the chemical and physical properties of the feed, is discussed in detail elsewhere (21). Differences in the methods used to measure STFB in different studies can be important because the quantification of feeding rate and other STFB measures is sensitive to the definition of a visit, which is usually a consequence of the type of equipment used. A visit may be defined as the time spent by the cow with her head in one of the feed bins, as was the case in the present study, regardless of how the cow spends that time. Because some of the time included in the visit may be spent on nonfeeding activities, feeding rate can be underestimated using this definition. Others ( 1 2 ) have used feed bins mounted on load cells, which record the weight of the feed bin at a preset time interval, such as 30 s. The start of a visit to the feed bin may then be defined as being when two consecutive weighings differ by a certain amount, and the end of a visit may be defined as being when two (or more) consecutive weighings do not differ. This method can yield substantially more visits per day than the equipment used in the present experi-

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ment (R. Sanderson, 1997, unpublished data) because a cow may spend longer than the preset interval at the feed bin with her head lifted. Feeding rate is overestimated when inactivity, as measured by the load cell, is spent chewing. A further complication can occur when visits are combined into feeding bouts or meals especially when these terms are used interchangeably (7, 28). When the visit data have been clustered into meals, the feeding rate is a within-meal rate that includes the noneating time between the visits during the meals, which may be as much as 58% of the meal duration (36). The use of meals rather than visits is appropriate for certain types of analyses, particularly in relation to the short-term regulation of feed intake (8, 33). In a meal-based analysis, visits that occur within meals are treated as random behavioral events (33), which are then obscured if only meal-based results are considered. If the focus of a study is on behavioral events, using the meal as the basic unit of data may be inappropriate. When cows are housed in groups, a substantial proportion of the behavioral events may be related to the social pressure within the group. Cattle housed in groups form hierarchies (16), and, consequently, different cows may be subject to different degrees of disturbance at the feeder (18), which would be reflected in individual differences in measures based on visits, but not necessarily in measures based on meals. Therefore, in the present study, in which cows were housed in groups and important differences existed between individual cows in STFB, we decided to present the results in terms of visits rather than meals. Effect of Stage of Lactation The two TMD used in this experiment were designed to yield different patterns of DMI relative to stage of lactation (13). The LTMD was formulated to be of a sufficiently poor quality that intake would be constrained by the physical properties of the feed throughout lactation (13). Therefore, we expected that, for cows fed LTMD, there would be no change in DMI with time, once intake was established during early lactation. The HTMD was formulated to be of a sufficiently high quality so that intake would not be constrained at any time in lactation (13). Therefore, it was expected that the intake by cows fed HTMD would rise to a peak and then steadily decline throughout lactation in response to changing energy requirements. These expectations were met (Figure 2). Therefore, this experiment provided the opportunity to examine the effects of stage of lactation on STFB when daily DMI was either constant or changJournal of Dairy Science Vol. 81, No. 12, 1998

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ing. No significant effects of stage of lactation were detected on STFB on either TMD, with the exception of feeding time, which was depressed by 10% in the last period (Table 2). No obvious explanation could be found for this effect on time spent feeding per day. The observed, highly significant change in mean daily DMI for cows consuming HTMD was not reflected in either mean number of visits per day or mean DMI per visit, the two STFB components of daily DMI (Figure 1). This finding demonstrates that the relationships between the STFB measures shown in Figure 1, although arithmetically correct for the individual, do not necessarily apply to group means. Similar findings have been reported by Dado and Allen ( 7 ) , who found no differences in STFB between primiparous and multiparous cows, although the multiparous cows had a higher DMI (24.8 kg of DM/d) than did the primiparous cows (20.2 kg of DM/d; P < 0.05). The observation that a highly significant change in mean DMI can arise from nonsignificant modulations in the component STFB measures suggests that STFB is of little value in the development of general models to predict daily intake. Variation Between Individual Cows The same daily feed intake can be obtained through different combinations of visit frequency and size (Figure 1). In the present study, this was reflected in the poor or inconsistent correlations between number of visits per day, feed intake per visit, and daily DMI (Figure 3 ) and in much greater variation between individual cows in the number of visits per day and feed intake per visit than in daily DMI. Substantial variation in STFB between individual cows housed within the same environment (23, 31) suggests that cows use a range of feeding strategies to achieve their desired daily intake. Further, these strategies may be influenced by the social environment. Krohn and Konggaard ( 1 8 ) found that, when separated from older cows, heifers ate more frequently, spent more time eating, and had higher intakes. If the variability between cows in feeding strategies cannot be accounted for by readily measurable traits of the cow, then prediction of daily DMI from STFB, in general, will have to contend with large error terms. However, this same variability between individuals within groups may be used as a measure of the pressure that the social environment exerts on animals within a given physical environment (25). The variability may even be indicative of underlying differences Journal of Dairy Science Vol. 81, No. 12, 1998

in physiology ( 6 ) . In Figure 3, cows fed HTMD had more between-cow variation in STFB in terms of both feeding frequency and intake per visit, occupying a much greater part of the relevant isoline of constant daily DMI than cows fed the LTMD, which indicates that cows fed a TMD with a relatively high amount of concentrate are able to express a much wider STFB repertoire than cows fed a diet with a low amount of concentrate (27). Although there is considerable variation between cows in their STFB, the variation within cows was smaller for cows fed HTMD. The within-cow variation of intake per visit and number of visits per day (error bars, in Figure 3a) is very low relative to the range between cows. The within-cow coefficients of variation were one-half of the between-cow values. Apparently, in a given environment and on a given feed, individual cows have a persistent feeding strategy (25, 36), which implies that different cows may be affected to different degrees by any given feeding or social environment. We suggest that the individual differences in STFB may be used to quantify the degree to which a given environment impinges on the different individuals within a group. Large, but repeatable, differences between individuals also afford the opportunity for genetic selection for animals that display appropriate social behavior ( 2 4 ) or for feeding strategies that are best suited for efficient feeding management (30). CONCLUSIONS The ratio of forage to concentrate in a TMD significantly affected the STFB of dairy cows. The size and duration of visits were higher, and the number of visits per day was lower, for cows offered HTMD. These effects were independent of stage of lactation, which had no effect on STFB, regardless of whether daily DMI was constant or changing throughout lactation. Measures of STFB varied considerably between cows fed the same TMD, but the STFB of individual cows fed a given TMD varied little with time. Cows used very different short-term feeding strategies to obtain similar amounts of daily intake. Although these differences may prove useful as a means to characterize individual cows, differences in STFB did not correlate with variation in daily intake, and, therefore, appear to be of little value for the prediction of daily feed intake. ACKNOWLEDGMENTS The technical assistance of the staff at the Langhill Dairy Research Center (Edinburgh, Scotland) is


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