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Dietary Forage Concentration Affects the Feed Sorting Behavior of Lactating Dairy Cows

T. J. DeVries^*,,1,2, K. A. Beauchemin^* and M. A. G. von Keyserlingk

^* Agriculture and Agri-Food Canada, PO Box 3000, Research Centre, Lethbridge, Alberta, T1J 4B1, Canada
Animal Welfare Program, The University of British Columbia, 2357 Main Mall, Vancouver, British Columbia, V6T 1Z4, Canada

¹ Corresponding author: tdevries{at}kemptvillec.uoguelph.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The objective of this study was to determine whether the amount of forage in a total mixed ration influences feed sorting by cows and whether the extent of this sorting changes as they adapt to a new diet. Six lactating Holstein cows, individually fed once per day, were provided each of 2 diets in a crossover design (dry matter basis): 1) a higher forage diet (HF; 62.3% forage), and 2) a lower forage diet (LF; 50.7% forage). Dry matter intake, feeding behavior, and sorting activity were monitored for each cow on each diet for 7 d. Fresh feed and orts were sampled daily for each cow and subjected to neutral detergent fiber (NDF) and particle size analysis. The particle size separator contained 2 screens (18 and 9 mm) and a bottom pan, resulting in 3 fractions (long, medium, and short). Sorting activity [for each fraction, NDF and physically effective NDF (peNDF)] was calculated as the actual intake expressed as a percentage of the predicted intake. Overall, sorting activity was greatest on the LF diet, with cows sorting for short particles but against long particles, medium particles, NDF, and peNDF. On the HF diet, cows sorted against long particles, NDF, and peNDF and sorted for short particles. Treatment x day interactions occurred for sorting for short particles and against peNDF, indicating that it took cows 1 d to adjust their sorting behavior to the LF diet. Cows on the LF diet consumed more dry matter but spent less time feeding, which resulted in a greater intake rate compared with cows on the HF diet. These results indicate that cows rapidly adjust their sorting behavior when subjected to a dietary change, and they exhibit more sorting for short particles and against long particles, NDF, and peNDF when fed an LF diet.

Key Words: sorting • forage concentration • particle size

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Providing lactating cows with rations in the form of a TMR, rather than separate feeding of concentrate and forage, is common practice on most commercial dairies. Nocek and Braund (1985) suggested that feeding a TMR is the optimal way to provide the balance of nutrients (including protein and structural and nonstructural carbohydrates) that ruminants need to maintain a stable and efficient microbial population. One concern with TMR feeding is that cows are able to selectively consume (sort) various feed components from their ration. Cows have been shown to preferentially sort for the grain component of a TMR and discriminate against the longer forage components (Leonardi and Armentano, 2003). Moreover, sorting of the diet can lead to the cows consuming an inconsistent ration (Stone, 2004). Cows engaged in sorting behavior are believed to consume higher amounts of grain and lower amounts of fiber, thereby increasing their risk for subacute ruminal acidosis (Cook et al., 2004; Stone, 2004). Alternatively, sorting of the TMR can reduce the nutritive value of the TMR remaining in the feed bunk, particularly in the later hours past the time of feed delivery (DeVries et al., 2005). As a result, cows without access to feed at the time when it is delivered may not be able to maintain the adequate nutrient intake needed to maintain high levels of milk production.

To date, the majority of research on feed sorting in dairy cattle has focused on changing the forage components of the TMR. In research by Leonardi and Armentano (2003), the effects on feed sorting of different quantities, qualities, and lengths of alfalfa hay were tested, without changing the proportion of concentrate in the diet. These researchers found that cows increased their sorting (discrimination against long particles) with more hay and with longer hay, whereas the quality of hay had no effect. Similar effects of particle size have been demonstrated by other researchers (e.g., Kononoff and Heinrichs, 2003; Kononoff et al., 2003; Onetti et al., 2004; Leonardi et al., 2005b). Despite this interest, one of the more basic questions that has received no attention is whether the composition (i.e., forage-to-concentrate ratio) of TMR influences the degree of feed sorting that occurs. Furthermore, no information is available on whether cows adjust their sorting behavior in response to a dietary change. This information is important because dairy producers typically sequentially offer several different rations (with varied forage-to-concentrate ratios) over the course of the lactation cycle.

The objective of this study was to determine whether the forage concentration of the diet influences feed sorting by cows and whether the extent of this sorting changes as the cows adapt to a new diet. The hypothesis was that increasing the forage concentration of the diet increases sorting of the ration (i.e., against the forage and for the concentrate) and that sorting within both diets changes over time as cows adjust to a dietary change. This hypothesis was tested in an experiment with cows exposed to 2 different TMR, varying in forage concentration. The extent of sorting of each ration and the time it took cows to adjust their sorting behavior to each ration were measured.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Animals and Housing
Six lactating Holstein cows, 2 primiparous and 4 multiparous (parity = 3.0 ± 1.2; mean ± SD), were used in the study. The animals were 221.5 ± 46.1 DIM at the beginning of the data collection period and had an average milk yield of 22.4 ± 3.9 kg/d over the course of the experiment. The cows were housed together in a free-stall barn located at The University of British Columbia Dairy Education and Research Center (Agassiz, British Columbia, Canada) and were managed according to the guidelines set by the Canadian Council on Animal Care (1993). Cows were housed in a pen with 2 rows of 6 free stalls (mattress covered with sand); 1 row of free stalls was blocked so that each cow had access to 1 stall. Each day, cows were milked between 0730 and 0800 h in the morning and between 1730 and 1800 h in the afternoon. Milk yields were automatically recorded at each milking. Feed (TMR) and water were provided ad libitum by using roughage and water intake control feeders (Insentec B.V., Marknesse, the Netherlands). Each cow was assigned to her own roughage feeder and all cows shared 1 water feeder. Each day, the orts were cleaned out of the feed bins at 0700 h and feed was delivered at 0730 h.

Experimental Design and Diets
The cows were divided into 2 groups of 3, which were balanced according to DIM, milk production, and average parity (2.3 ± 1.4). These groups were created by blocking cows into groups of 2 cows (similar in parity, DIM, and milk production), and then randomly assigning the cows in these blocks to 1 of the 2 experimental groups.

During the week prior to receiving the experimental diets, the cows were housed together to facilitate social adaptation and to train them to access feed from their specific feeder. During this period, the cows remained on their standard high-production TMR consisting of 29.9% corn silage, 10.3% grass silage, 11.2% alfalfa hay, and 48.6% concentrate mash on a DM basis (DM, 51.0%; CP, 17.5% DM; ADF, 20.8% DM; NDF, 35.5% DM; and NE_L, 1.61 Mcal/kg). After this time period, the cows were exposed to 2 dietary treatments (Table 1) in a crossover design. Initially, the cows were alternately assigned to 1 of 2 dietary treatments (assigned to alternate bins). The treatments were 1) a higher forage diet (HF; 62.3% forage, 37.8% concentrate), and 2) a lower forage diet (LF; 50.7% forage, 49.3% concentrate). The HF diet was mixed in a mixer wagon and delivered to each of the 6 feed bins. The LF diet was made by first adding a set amount of concentrate to the TMR in the bins designated to deliver the LF treatment. The feed was then dumped onto the cement floor in front of each bin and thoroughly mixed for 10 min by hand. After mixing was complete, the feed was placed back into its original bin. The amount of feed offered to each cow was adjusted daily to ensure a similar percentage of orts among animals and between treatments. Cows were kept on their respective treatments for 7 d and then switched to the other treatment and fed for an additional 7 d.

Meal Analysis
Visits to the feed intake system were separated into meals by using an individual meal criterion (minimum time interval that a cow was away from the feeder to be considered a meal) for each cow. These meal criteria were calculated by using a mixed distribution model, as outlined by DeVries et al. (2003). The meal criterion averaged 26.0 ± 10.0 (mean ± SD) min. Meal frequency, duration, and size were calculated by using these criteria.

Feed Sampling and Analysis
Representative samples of the TMR were taken daily in duplicate (one for chemical analysis, one for particle size separation) for each cow at the time of feed delivery (offered feed) and from the orts. Samples for particle size separation were immediately sieved by using the 2-screen (19- and 8-mm) Penn State Particle Separator (PSPS; Lammers et al., 1996). This separated the particles into 3 particle size fractions: long particles (>19 mm), medium particles (<19 mm, >8 mm), and short particles (<8 mm). The separated fractions were then pooled by fraction, across animals, within day, and within treatment to create a composite for DM analysis.

Dry matter contents of the composite particle fractions and the samples for chemical analysis were determined by oven-drying at 55°C for 48 h. The dried samples for chemical analysis were then ground to pass through a 1-mm screen (Brinkmann Mill, Brinkmann Instruments Co., Westbury, NY). The NDF contents of all the individual TMR samples (fresh feed and orts) were determined by using an Ankom²⁰⁰ Fiber Analyzer (Ankom Technology, Fairport, NY) according to the methodology supplied by the company, which is based on the methods described by Van Soest et al. (1991). Heat-stable -amylase and sodium sulfite were used in this procedure. The physical effectiveness factor (pef) of the TMR and orts were determined as the sum of the proportions of DM retained on 2 sieves (19 and 8 mm) of the PSPS (Lammers et al., 1996). The physically effective NDF (peNDF) was calculated by multiplying the NDF content of the feed by the pef. The offered feed samples were also pooled by treatment, within period, and analyzed in duplicate for standard nutrients by using wet chemistry procedures (n = 8; Cumberland Valley Analytical Services Inc., Maugansville, MD). Grass silage and corn silage samples were taken in duplicate at the beginning of each treatment period and analyzed for standard nutrients by near-infrared spectroscopy (n = 8; Unifeed Ltd., Chilliwack, British Columbia, Canada).

Calculations and Statistical Analysis
Sorting activity (for each fraction of the PSPS and for NDF and peNDF) was calculated as the actual intake of each variable expressed as a percentage of the predicted intake of that variable (Leonardi and Armentano, 2003). The predicted intake of an individual variable was calculated as the product of the DMI of the total diet multiplied by the DM percentage of that variable in the fed TMR. Values equal to 100% indicate no sorting, < 100% indicate selective refusals (sorting against), and > 100% indicate preferential consumption (sorting for).

Data for the DM content, NDF, particle size distribution, pef, and peNDF of the diets as well as the daily DMI, DM percentage of orts, NDF intake, peNDF intake, water intake, and feeding behavior data were summarized for each cow by treatment period and were analyzed by using PROC MIXED of SAS (SAS Institute, 1999). The model included the fixed effects of period, order, and treatment, the random effect of cow within order, and the residual error.

To test whether sorting of the experimental diets occurred, sorting activity for each fraction of the PSPS, NDF, and peNDF was summarized by treatment and tested for a difference from 100 by using t-tests. Further, to test whether sorting activity differed between dietary treatments and over time, the data for sorting activity for each screen were summarized for each cow for each day on the treatment and were analyzed by using PROC MIXED of SAS (SAS Institute, 1999), treating day as a repeated measure. Compound symmetry was selected as the covariance structure on the basis of best fit according to Schwarz’s Bayesian information criterion. The final model included the fixed effects of period, order, day, treatment, and day x treatment interaction, the random effect of cow within order, and the residual error. Day x period and day x order interactions were tested in the initial model and were not significant; therefore, they were dropped from the final analysis.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Dietary Composition
Nutrient composition and particle size distribution of the dietary treatments are reported in Tables 1 and 2, respectively. The HF diet had a lower DM content than the LF diet (SE = 0.2, P < 0.001) but had a greater proportion of NDF (SE = 0.3, P < 0.001). Decreasing the amount of forage in the TMR decreased the DM retained on the 8- and 19-mm sieves and increased the DM found in the pan (<8 mm). The pef and peNDF were also reduced in the LF diet compared with the HF diet.

View larger version (15K): [in this window] [in a new window]   Figure 1. Sorting activity [100 x (screen i intake/screen i predicted intake)] of cows on a higher forage diet (62.3% forage, 37.7% concentrate) and a lower forage diet (50.7% forage, 49.3% concentrate) for A) long particles (>19 mm), B) medium particles (<19 mm, >8 mm), and C) short particles (<8 mm). Data are averaged for 6 cows on each treatment. Sorting values equal to 100% indicate no sorting, <100% indicate selective refusals (sorting against), and >100% indicate preferential consumption (sorting for).

Cows sorted for short particles (<8 mm) when fed the HF diet (P = 0.001) and the LF diet (P < 0.001). The extent of sorting for these short particles was greater on the LF diet compared with the HF diet (P < 0.001; Figure 1C), with a treatment x day interaction (P = 0.05).

Cows sorted against NDF when fed the HF diet (P = 0.009) and the LF diet (P < 0.001). The extent to which cows sorted against NDF was greater on the LF diet compared with the HF diet (P < 0.001; Figure 2A).

View larger version (14K): [in this window] [in a new window]   Figure 2. Sorting activity [100 x (n intake/n predicted intake)] of cows on a higher forage diet (62.3% forage, 37.7% concentrate) and a lower forage diet (50.7% forage, 49.3% concentrate) of A) NDF and B) physically effective NDF (peNDF). Data are averaged for 6 cows on each treatment. Sorting values equal to 100% indicate no sorting, <100% indicate selective refusals (sorting against), and >100% indicate preferential consumption (sorting for).

Intakes and Behavior
Cows had greater DMI on the LF diet (Table 3). Despite higher intake, their feeding time was reduced on this diet, resulting in a greater rate of intake compared with when cows were provided the HF diet. The hourly DMI was consistently greater for the LF diet throughout the day (Figure 3A). The difference in hourly feeding times between diets was less pronounced during periods of peak feeding activity (Figure 3B). These differences in hourly DMI and feeding time translated into cows consuming the LF diet at a much greater feeding rate throughout the day, particularly during periods of peak feeding activity (Figure 3C). Despite no difference in meal number, cows tended to have shorter daily meal time and larger meals with the LF diet (Table 3). There was no difference in the percentage of feed refused (orts) by the animals (HF = 16.6%, LF = 18.7%; SE = 1.5, P = 0.2).

View larger version (18K): [in this window] [in a new window]   Figure 3. Hourly averages for A) DMI (kg) B) feeding time (min), and C) feeding rate (kg/min) for cows fed a higher forage diet (62.3% forage, 37.7% concentrate) and a lower forage diet (50.7% forage, 49.3% concentrate) diet. Data are averaged over 7 d for 6 cows on each treatment.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

The results clearly indicate that, regardless of dietary treatment, sorting of the feed did occur. Similar to previous findings (e.g., Leonardi and Armentano, 2003; DeVries et al., 2005), cows were clearly sorting against long particles, and hence NDF and peNDF, and sorting for shorter particles. Considering the ingredients used in these diets, it is apparent that the cows were primarily sorting against the long silage particles and for the concentrate mash, which made up the majority of the short particle fraction.

Even though several researchers have shown that sorting against long particles and NDF can be reduced by changing the characteristics of forages within a TMR (e.g., particle size: Kononoff and Heinrichs, 2003; Kononoff et al., 2003; Leonardi and Armentano, 2003; Onetti et al., 2004; Leonardi et al., 2005b), no one has looked at how the proportion of forage in the diet influences sorting behavior. Our results indicate that cows easily discriminated against long particles, NDF, and peNDF, and selected for short particles when fed an LF diet. This finding contradicts our hypothesis that cows would sort more with increased forage in the diet (i.e., the HF treatment). We assumed that cows would be highly motivated to sort for the concentrate portion of their TMR, especially when availability of concentrate was limited. Thus, we assumed that decreasing the concentrate portion in the TMR would motivate cows to sort for concentrate and against forage. The increased sorting of the LF diet indicates, however, that a ration with higher a proportion of concentrate may be more easily sorted, simply because the concentrate content is more accessible.

The amount of feed offered can influence the degree of feed sorting (Leonardi and Armentano, 2007). The percentage of orts, however, was similar between treatments; therefore, the amount of feed offered would not have influenced the degree of feed sorting in our study.

Leonardi et al. (2005a) showed a reduction in the amount of feed sorting (against long particles) when water was added to a dry diet (80% DM). Thus, the greater moisture content of the HF ration may have contributed to the decreased sorting of the HF diet. It should be noted, however, that the "wet" diet described by Leonardi et al. (2005a) was much drier (64% DM) than both diets used in the present study. It could be speculated that moisture content would have had less of an effect on the sorting of the diets fed in the present study, particularly because these diets were both silage based, with high moisture contents. Further research is needed to assess how moisture content affects the sorting of higher moisture diets.

We also hypothesized that cows would adjust their sorting behavior over time as they adapted to a dietary change. This hypothesis was also refuted because cows quickly adjusted their sorting behavior. For both treatments, cows consistently sorted against long particles and NDF throughout the 7-d treatment period. Treatment x day interactions revealed that cows took 1 d to adjust their sorting for short particles and against peNDF when switched to the LF diet. There was also a treatment x day interaction for sorting of medium particles. However, this finding is not very meaningful because there was minimal sorting of medium particles on either diet. The interaction is most likely the result of day-to-day variation in sorting behavior. Leonardi and Armentano (2003) are the only other group to report changes in sorting behavior after introduction to a new diet. However, these researchers measured sorting only on d 1 and 6, and were unable to describe when the change occurred, unlike in the present study. Our study is the first to show the rapid acclimation in sorting behavior by lactating cows. This information may be of particular relevance to cows in early lactation, which are typically switched from an HF to an LF diet immediately upon calving. If early-lactation cows sort in a similar manner, sorting of an LF diet coupled with rapidly increasing DMI in the days following calving will likely exacerbate the intake of short particles and refusal of long particles, NDF, and peNDF. This may result in increased VFA production and low buffering capacity in the rumen, factors that increase the risk of subacute ruminal acidosis (Cook et al., 2004; Stone, 2004).

Intuitively, one would predict that sorting behavior would slow the eating rate, but again our results provide evidence that the opposite occurs. Cows consuming the LF diet ate at a greater rate and in a shorter time period. Johnson and Combs (1992) also found that cows consuming a 50% forage diet spent less time eating than those fed a 74% forage diet. In contrast, Voelker et al. (2002) reported similar feeding times for cows fed a 67 or 44% forage diet. These latter authors suggest that such discrepancies in feeding times may be the result of differences in dietary particle size. However, neither of these previous studies reported dietary particle size. Interestingly, Voelker et al. (2002) did report that the eating time per kilogram of DMI was lesser for the cows fed a 44% forage diet, indicating that these cows consumed their DM faster, similar to the greater feeding rates on the LF diet seen in the present study. Allen (2000) stated that dietary factors that increase eating time could decrease the time available for ruminating, thereby increasing the filling effect of the diet. Similarly, in our study, the HF diet took longer to consume, likely because of its high NDF content and longer particle size. These factors may have contributed to an increased filling effect, accounting for the lower DMI on the HF diet. Both Johnson and Combs (1992) and Voelker et al. (2002) found DMI to be lower on their higher forage diets. The effect of lesser DMI as a result of increased forage concentration is well founded, as demonstrated in a meta-analysis by Allen (2000), who showed that as dietary NDF content increases through substitution of grain with forage, DMI generally declines. In our study the intake of NDF was similar between the 2 diets, as a result of greater overall DMI on the LF diet. Voelker et al. (2002) found NDF intake to be greater on their higher forage diet, which resulted from a smaller difference in DMI between diets compared with that seen in the present study. Despite no difference in NDF intake, the intake of peNDF was much lower on the LF diet because of differences in particle size of the diets and the greater degree of sorting against peNDF.

Johnson and Combs (1992) found no effect of forage concentration on meal number, but did report that cows had longer meals when consuming a 74% forage diet compared with a 50% forage diet. Similarly, in the present study, we found no effect of diet on meal number. Although cows tended to have longer total daily meal times when on the HF diet, there was no difference in meal length. This discrepancy may be the result of differences in meal calculation. Unfortunately, we can only speculate that Johnson and Combs (1992) used a very short meal criterion because they failed to report how they calculated meal numbers.

The high consumption rates of the LF diet observed during the time period following feed delivery is of concern, given the higher degree of sorting of this ration. DeVries and von Keyserlingk (2005) demonstrated that the motivation to feed is greatest immediately following the delivery of fresh feed, a time when competition at the feed bunk is also greatest. In situations where cows are consuming an LF diet in a highly competitive environment, dominant cows may be rapidly consuming increased amounts of fermentable carbohydrates and low amounts of physically effective fiber, putting them at risk for subacute ruminal acidosis. Alternatively, subordinate cows that are unable to access the bunk during this time may be forced to access feed at a later time, when the nutrient profile of the remaining feed may not match their specific requirement. Feed bunk management strategies, including additional space or use of feed stalls, make it easier for all cows to access feed at any given time, and will act to reduce the between-cow variation in TMR consumed (DeVries and von Keyserlingk, 2006). The fact that feed was delivered only once per day in the present study must not be overlooked. Previous research on feeding frequency has indicated that delivery of feed at least 2 times per day will improve access to feed, reduce feed sorting, and reduce between-cow variation in TMR consumed (De-Vries et al., 2005).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Cows rapidly adjusted their sorting behavior when subjected to a dietary change and exhibited more sorting for short particles and against long particles, NDF, and peNDF when fed an LF diet. Feeding an LF diet also increased DMI, decreased feeding time, and increased feeding rate, particularly during the time period following feed delivery. These sorting and feeding behavior results suggest that feeding an LF diet may increase the risk of cows’ succumbing to subacute rumen acidosis. These results indicate that measures such as reducing competition at the feed bunk and increasing the frequency of feed delivery must be considered when feeding an LF diet to reduce sorting and the between-cow variation in composition of TMR consumed.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
We thank the staff and students at The University of British Columbia’s Dairy Education and Research Centre. In particular we thank Doug Veira and Audrey Nadalin for their help with the feed sample drying. We also thank Bev Farr and Sheila Frank of the Lethbridge Research Centre for their assistance in performing laboratory analyses. Trevor DeVries was supported by a Natural Sciences and Engineering Research Council of Canada Visiting Fellowship. The project was funded through joint contributions from the Westgen Endowment Fund and Investment Agriculture Foundation of British Columbia.

	FOOTNOTES

 
² Current address: University of Guelph, Kemptville Campus, 830 Prescott Street, PO Box 2003, Kemptville, ON, Canada K0G 1J0.

Received for publication May 16, 2007. Accepted for publication August 19, 2007.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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