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Abstract

Corn silage (CS) is associated with a reduction in milk fat content. The fact that CS is constituted of a grain and a forage fraction could explain this effect. This experiment evaluated the effect of grain fraction of CS on rumen fermentation, production performance and milk composition. Earless CS (ECS) was harvested after manually removing corn ears from the plant. Whole CS (WCS) was harvested from the same field on the same day. Eight (four ruminally fistulated) multiparous Holstein cows (84 days in milk) were utilized in a double 4 × 4 Latin square with 21-day periods. Treatments were (dry matter (DM) basis) (1) 23.0% WCS; (2) 12.4% ECS plus 10.6% high moisture corn (HMC) to obtain reconstituted CS (RCS); (3) 23.0% ECS; and (4) 23.0% timothy silage (TS). Diets were formulated to be isonitrogenous and were fed as total mixed ration once a day. DM intake (DMI), milk yield, 4.0% fat-corrected milk (FCM), as well as protein concentration and yield were higher for WCS than ECS. Compared with WCS, cows tended to eat less with RCS, and produced less milk and milk protein. However, yield of FCM was similar between WCS and RCS. Milk fat concentration and yield, as well as the specific ratio of t11 18:1 to t10 18:1 in milk fat did not differ among diets. Milk urea-N tended to be higher for ECS than WCS and TS, whereas ruminal NH3-N was higher with ECS than TS. Rumen pH decreased linearly with time after feeding but was not different between treatments. Higher acetate and lower propionate concentration resulted in greater acetate to propionate ratio with ECS compared with WCS. In conclusion, removing grain fraction from CS decreased milk production and modified rumen fermentation without affecting milk fat concentration and yield. Moreover, despite some differences in DMI and total ruminal volatile fatty acid concentration between WCS and RCS, the restoration of FCM yield, using HMC in RCS diets, to a level of production similar to WCS highlights the importance of energy and nutrients supplied by the grain fraction of CS to support milk yield.

Implications

Corn silage (CS) has been previously associated with milk fat reduction when offered as the major forage in dairy diets. In this trial, CS was harvested with or without ears to evaluate the effect of grain fraction of this forage on milk production and composition. Milk yield was higher for whole CS than earless CS. However, feeding CS with or without ears did not affect milk fat content and yield. Those results do not support the hypothesis that grain fraction of CS alters rumen fermentation and reduces milk fat synthesis in dairy cows.

Introduction

Corn silage (CS) is considered by nutritionists as an excellent source of energy, and represents a good complement to high-protein forage such as alfalfa (Dhiman and Satter, 1997). For this reason, CS is largely utilized in dairy cow diets especially in early lactation. However, previous studies have shown that increasing CS : alfalfa silage ratio in the diet is associated with linear decreases of milk fat concentration (Onetti et al., 2002; Brito and Broderick, 2006). Moreover, the decrease in milk fat concentration was of a higher magnitude when tallow was added in high CS rations instead of alfalfa (Ruppert et al., 2003; Onetti et al., 2004). Those results make CS often associated with milk fat depression when offered as the major or unique forage in dairy diets (Staples and Cullens, 2005).

Some factors have been identified as influencing milk fat synthesis: forage-to-concentrate ratio (F : C; Macleod et al., 1983; Maekawa et al., 2002; Lechartier and Peyraud, 2010), forage particle size (Grant et al., 1990; Mertens, 1997) and dietary unsaturated fatty acids (UFA; Griinari et al., 1998). Each of those factors are known to influence rumen environment, and then to affect ruminal biohydrogenation process leading to the production of specific fatty acid (FA) isomers (e.g. t10c12 18:2), which are known to inhibit milk fat synthesis (Bauman and Griinari, 2001).

In contrast with other frequently used forages in dairy rations, CS is characterized as being a blend of grain and forage (Broderick, 1985; De Boever et al., 1993). More specifically, the particle size distribution of this latter fraction appears to affect its fiber physical effectiveness, and therefore, chewing activity of cows consuming this forage (Clark and Armentano, 1999).

Finally, CS is also a source of UFA that essentially comes from the grain fraction. In this regard, biohydrogenation of constituent UFA such as c9c12 18:2 can lead to the production of specific conjugated linoleic acid isomers (e.g. t10c12 18:2), which, as mentioned above, may negatively affect milk fat synthesis in the mammary gland (Bauman and Griinari, 2001). Therefore, we hypothesized that non-fiber carbohydrates and lipids in grain fraction of CS alter rumen fermentation and reduce milk fat concentration and yield in dairy cows. The objective of this experiment was to evaluate the effect of grain fraction of CS on rumen fermentation, milk production and composition.

Material and methods

Silage production

Corn (Elite Fusion, La Coop fédérée, Montréal, Québec, Canada) was planted in a single field at a density of 79 000 seeds/ha. When kernels reached one-third to one-half milkline stage, whole corn plants were picked across the field and dried to obtain dry matter (DM). When corn plant reached a DM concentration between 30% and 35%, corn ears were manually removed from the plants in half of the field. The next day, whole CS (WCS) and earless CS (ECS) were harvested using a forage chopper (model 790; New Holland Inc., New Holland, PA, USA). Both silages were stored in plastic bag silos.

Animals and dietary treatments

All procedures performed in this study were approved by the institutional animal care committee based on the current guidelines of the Canadian Council on Animal Care (1993). Eight multiparous Holstein cows, four with rumen fistulas, averaging 84 (s.d. ± 31) days in milk and 626 (s.d. ± 104) kg of BW at the beginning of the trial were used in a double 4 × 4 Latin square (one square of rumen-fistulated cows). Each experimental period lasted 21 days, with 17 days for adaptation and 4 days for data collection. Cows within each square were randomly assigned to experimental diets. Cows were housed in a tie stall barn, and had free access to water at all time during this experiment.

Treatment diets consisted of (DM basis): (1) 23.0% WCS; (2) 12.4% ECS and 10.6% high moisture corn (HMC) to obtain reconstituted CS (RCS); (3) 23.0% ECS; and (4) 23.0% timothy silage (TS). Each diet contained equal proportions of alfalfa silage and chopped timothy hay to complete the forage fraction. On the basis of initial feed ingredient composition, diets were formulated to be isonitrogenous, and to meet or exceed the National Research Council requirements (NRC, 2001; Table 1). The proportions of ECS and HMC in RCS were established from the grain and the forage fractions previously determined from whole corn plants randomly sampled in the field the day of harvest. Ears (grains, cob and husks) represented 59% of corn plants, of which grains accounted for 80% (46% of whole plant) on a DM basis. In TS diet, part of soybean meal was replaced by soyhulls to achieve CP, ADF and NDF concentrations similar to ECS diet. Corn oil was added at 3% of DM in each treatment to provide an additional supply of UFA available for biohydrogenation in the rumen. Diets were fed as total mixed rations (TMR) at 1000 h daily and the amount offered was adjusted based on previous day's intake to allow for 10% refusals. Silages were sampled every week and dried for 3 days at 70°C to determine DM concentration, and adjust the composition of treatment diets on an as-fed basis.

Table 1 Ingredient composition (% of DM) of experimental diets

DM = dry matter; WCS = whole corn silage; RCS = reconstituted corn silage; ECS = earless corn silage; TS = timothy silage; HMC = high moisture corn.

aPokonobe Industries Inc., Hudson, Qc, Canada. Fatty acid composition was 10.8% 16:0, 1.6% 18:0, 28.6% c9 18:1, 57.1% c9c12 18:2 and 1.0% c9c12c15 18:3.

bContained 20.8% Ca, 3.3% P, 6.9% Mg, 0.9% K, 1.0% S and 2520 mg of Zn, 1650 mg of Mn, 1503 mg of Fe, 714 mg of Cu, 67 mg of I, 12 mg of Co, 21 mg of Se, 352 000 IU of vitamin A, 94 000 IU of vitamin D and 1960 IU of vitamin E/kg of mixture.

Experimental measurements and procedures

BW was measured at 0930 h during 2 consecutive days at the initiation of the trial, and on days 18, 19 and 21 of each period. Samples of TMR and orts were collected from days 18 to 21 of each period. Samples were composited by period, and one fraction of those composites was dried at 70°C for 3 days to determine DM. The second fraction was stored at −20°C for further analyses by wet chemistry (Dairy One Laboratory, Ithaca, NY, USA) of CP, ADF, NDF, ether extract (EE), starch and ash. Intake of individual nutrients was calculated by subtracting their respective weight in orts from their weight in TMR offered. The FA profile of TMR was determined after freeze drying and grinding through a 1-mm screen. FAs were directly transesterified, and FA methyl esters were extracted following the method described by Sukhija and Palmquist (1988) using toluene as solvent.

Cows were milked at 0700 and 1700 h daily, and milk production was measured with calibrated milk meters (Flomaster Pro, DeLaval, Tumba, Sweden) at each milking from days 18 to 20. Daily milk composites were made from evening and morning milk samples proportionally to milk production. A first sub-sample of those composites was preserved in bronopol and stored at 4°C before they were analyzed for fat, protein, lactose, somatic cell count and milk urea-N (MUN) concentrations by infrared analysis with a Foss MilkoScan FT 6000 instrument (Foss Electric, Hillerød, Denmark) at Valacta (Dairy Production Center of Expertise, Quebec and Atlantic Provinces, Ste-Anne-de-Bellevue, Quebec, Canada). A second sub-sample was stored at –20°C until FA analysis. Before lipid extraction, milk samples were thawed in water at 36°C for 30 min. Lipids were extracted and methylated following the procedure described by Chouinard et al. (1997). Milk FA profile was determined with a gas chromatograph (Agilent 7890A; Agilent Technologies, Santa Clara, CA, USA) equipped with a 100-m CP-Sil-88 capillary column (0.25 μm i.d., 0.20 μm film thickness; Agilent Technologies Canada Inc., Mississauga, Canada), and a flame ionization detector. Three oven temperature programs were utilized in this study. To screen milk FA composition, a first temperature program was as follows: at the time of sample injection, the column temperature was 80°C for 1 min, then ramped at 2°C/minute to 215°C and maintained for 21.5 min. Two more isothermal temperature programs (150°C and 175°C) as described by Kramer et al. (2008) were used to separate isomers that were coeluting in the first temperature program.

Rumen fluid was collected from the ventral sac of fistulated cows on day 21 at 0, 1, 2, 4 and 6 h relative to feeding time. Rumen fluid was immediately filtrated through four layers of cheesecloth and pH was measured. Triplicates of 10 ml of rumen fluid were then stabilized in 0.2 ml of sulfuric acid (50%) and frozen at −20°C.

For the analysis of NH3-N and volatile FAs (VFA), samples of ruminal fluid were thawed at room temperature, and centrifuged at 16 060 × g for 15 min at 4°C. The supernatant was analyzed for NH3-N with the indophenol-blue method (Novozamsky et al., 1974) using a Milton Roy Spectronic 1201 spectrophotometer (Milton Roy Company, Miami, FL, USA) at 630 nm. VFA were analyzed using an Agilent HPLC system (Agilent 1200 Series, Agilent Technologies).

Statistical analysis

Data were analyzed with the MIXED procedure of SAS (SAS Institute Inc., Cary, NC, USA) as a duplicated 4 × 4 Latin square design according to the following model:

$$ {{Y}_{ijkl}}\:{\rm{ = }}\:\mu \:{\rm{ + }}\:{{T}_i}\:{\rm{ + }}{{P}_j}\:{\rm{ + }}\:{{C}_k}\:{\rm{ + }}\:{{S}_l}\:{\rm{ + }}\:{{{\epsilon}}_{ijkl}} $$

where Yijkl is the individual observation, μ the overall mean, Ti the fixed effect of treatment (i = 1 to 4), Pj the fixed effect of period (j = 1 to 4), Ck the random effect of cow (k = 1 to 8), Sl the random effect of square (l = 1 and 2), εijkl the residual error term and where the subject of the repeated statement was cow within square. The covariance structure selection between autoregressive 1, compound symmetry and variance components was based on the Akaike's information criterion.

Rumen fluid pH, VFA and NH3-N were analyzed as repeated measures according to the following model:

$${{Y}_{ijkl}}\:{\rm{ = }}\:\mu \:{\rm{ + }}\:{{T}_i}\:{\rm{ + }}\:{{P}_j}\:{\rm{ + }}\:{{C}_k}\:{\rm{ + }}\:{{H}_l}\:{\rm{ + }}\:{{{\rm{(}}T\:\times \:H{\rm{)}}}_{il}}\:{\rm{ + }}\:{{{\epsilon}}_{ijkl}}$$

where Hl is the effect of sampling time (l = 0, 1, 2, 4 and 6 h) and (T × H)il the effect of the interaction between treatment and time of sampling. Cow was considered as a random effect the subject of the repeated measurements was cow (period × treatment), and the spatial covariance structure SP(POW) was used to estimate covariances. Because there were no sampling time × treatment interactions, mean values for rumen fluid were combined for statistical analysis according to the first model previously described.

The following contrasts were used to test the effects of treatment diets: (1) effects of grain source (WCS v. RCS); (2) effect of grain fraction of CS (WCS v. ECS); and (3) effect of forage fractions (ECS v. TS). Differences between treatments were declared at P ⩽ 0.05, and tendencies from P > 0.05 to P < 0.10.

Results and discussion

CS and diet compositions

The time of harvest of WCS and ECS was based on the average DM of randomly picked whole corn plants to obtain a DM concentration between 30% and 35%. However, DM concentration was higher in grains compared with stalks at this stage. Consequently, DM of ECS was lower than WCS (Table 2). Removing the grain fraction of the corn plant increased ADF and NDF concentrations in ECS diet by 4.3 and 4.4 percentage units, respectively, compared with WCS (Table 3). Moreover, starch, EE and calculated net energy for lactation (NEL) were lower in ECS compared with WCS. Nevertheless, fermentations of WCS and ECS were quite similar based on organic acid profile (Table 2) and pH (below 4.0), suggesting that enough soluble sugars were present in stalk and leaves to support microbial activity.

Table 2 Chemical composition of corn silages

DM = dry matter; EE = ether extract; nd = not detected.

Table 3 Chemical composition of experimental diets

WCS = whole corn silage; RCS = reconstituted corn silage; ECS = earless corn silage; TS = timothy silage; DM = dry matter; EE = ether extract; NFC = non-fibrous carbohydrates; NDICP = neutral detergent insoluble CP; NEL = net energy for lactation.

aNFC: 100 − [(NDF − NDICP) − CP − EE − Ash].

bNEL calculated (NRC, 2001).

BW and nutrient intake

Actual BW and BW change were not different among treatments (Table 4). DM intake (DMI) was 2.3 kg/day higher when cows were fed WCS compared with ECS (P < 0.01). Lower DMIs have also been noted with corn stover silage v. CS when fed in combination with alfalfa (hay or silage) in steers (Mader and Britton, 1986). However, the maturity of ECS in the current study was identical to the forage fraction in WCS as opposed to the study by Mader and Britton (1986) where corn stover was ensiled after dry grain harvesting. In the current study, higher fiber concentration could have decreased the digestibility of ECS diet. Daynard and Hunter (1975) reported an in vitro DM digestibility of 57% for corn stover compared with 70% for whole plant when harvested at 33% of DM. Higher NDF concentration, and to a lesser degree, variation in digestibility of DM could explain the lower DMI observed with ECS.

Table 4 Effect of experimental diets on BW and intake of nutrients

WCS = whole corn silage; RCS = reconstituted corn silage; ECS = earless corn silage; TS = timothy silage; DM = dry matter; EE = ether extract.

Cows tended (P = 0.08) to consume more with WCS than RCS diet, which explains the greater NDF (P < 0.01), CP (P = 0.03) and EE (P < 0.01) intakes. Despite comparable DMI, the higher NDF content in TS diet led to a higher NDF intake (+0.7 kg/day; P = 0.02). The analysis of orts composition also suggests that sorting of feed may have been more important with ECS compared with TS (data not shown), which could have contributed to the variation in NDF intake. Finally, cows had a lower EE intake with ECS than TS, with a similar DMI (P < 0.01; Table 4). A slightly higher EE concentration in TS seems to be related to this difference.

Milk yield and composition

Cows produced significantly more milk when fed WCS than ECS (P = 0.01; Table 5). Data on the comparison of WCS with ECS or corn stover silage (or stalklage) on a kg for kg basis are scarce. Available data were either obtained several years ago (Morrison et al., 1921) or with low productive breeds of cattle (Sahiwal cows; Rahman et al., 2003). In the early work of Morrison et al. (1921), cows fed WCS produced more milk than those offered equal amount of corn stover silage. In the current experiment, the presence of the grain fraction in WCS led to higher starch, non-fibrous carbohydrates and consequently NEL concentration than ECS. Therefore, combined to a greater DMI, WCS provided cows with more energy and nutrients for milk production than ECS. Increasing corn grain in the diet has been previously reported to improve milk production (Valadares Filho et al., 2000; Oba and Allen, 2003).

Table 5 Effects of experimental diets on milk yield and composition

WCS = whole corn silage; RCS = reconstituted corn silage; ECS = earless corn silage; TS = timothy silage; FCM = fat-corrected milk; DMI = dry matter intake; MUN = milk urea-N; SCC = somatic cell count.

Milk yield was significantly higher when cows were fed WCS than RCS (P < 0.01). This may be explained by a tendency for a higher DMI (P = 0.08). In the current trial, HMC was selected as the grain source to obtain RCS because it has been fermented, and has not been dried, which more closely mimics grain characteristics in WCS. In addition, the amount of HMC added to ECS in RCS diet was previously determined from the grain fraction of whole corn plants to ensure a grain supply similar to WCS. In this calculation, cobs and husks were considered to have a nutritive value more closely related to the forage fraction of corn plants. Cobs and husks were therefore replaced with ECS in RCS diet. However, this substitution could have brought differences in organic matter digestibility and nutritive value. In this regard, Daynard and Hunter (1975) reported similar in vitro DM digestibilities for corn stover (57%) and corn cobs (55%), but a higher digestibility for corn husks (67%), which represented 10% of total plant DM. Differences of DM concentration in HMC (74.0%) and corn grain in WCS (56.6%) could also have had a significant impact on organic matter and starch digestibility, and therefore on the energy supply for milk production. In feedlot steers, Szasz et al. (2007) reported a linear increase in effective degradability of DM and starch with decreasing levels in DM concentration from 72% to 64% in HMC using the in sacco technique. However, total tract digestibility of starch was not different between treatments (Szasz et al., 2007). This may explain why the 4% fat-corrected milk (FCM) and efficiency of milk production was not different between WCS and RCS in the current experiment (Table 5). Finally, there was a trend to produce more milk with TS than ECS diet (P = 0.07). Although the nutrient supply was adjusted between those two diets, TS may have had a better efficiency of nutrient utilization.

Production of 4% FCM was greater when cows were fed WCS compared with ECS, essentially because they produced more milk (P = 0.03; Table 5). In contrast to our hypothesis, milk fat concentration and yield were not statistically different for these two treatments. Other published studies reported decreases in milk fat percentage and milk fat yield when the dietary proportion of CS was similar (Brito and Broderick, 2006) or higher (Ruppert et al., 2003; Onetti et al., 2004; Brito and Broderick, 2006). According to Sutton (1989), milk fat concentration decreases by 0.17 percentage unit for each percentage unit decrease in dietary ADF concentration. On the basis of this relationship, and using the actual ADF concentration of WCS and ECS diets, one could calculate a potential decrease in milk fat concentration of 0.73 percentage unit. However, consistent with the current trial, Broderick (1985) obtained a higher range in ADF concentration with diets containing 60% alfalfa silage (30% ADF) v. CS (18% ADF) as the sole forage, with no effect on milk fat concentration and yield.

In the current experiment, corn oil has been added at the level of 3% of DM in each diet to supply a sufficient amount of UFA. Indeed, Griinari et al. (1998) previously demonstrated that milk fat depression requires both an altered rumen fermentation, and the presence of UFA in the rumen. It should be noted, however, that average milk fat concentration across periods and treatments was 3.50% (s.d. ± 0.41). This value is lower than mean pre-trial fat concentration of 3.97% for the eight experimental cows. It cannot be excluded that this overall decrease in milk fat percentage may have masked any potential individual treatment effect on milk fat synthesis. No difference was therefore observed either in milk fat concentration or yield when comparing WCS with RCS, or ECS with TS.

Milk protein concentration significantly increased when cows were fed WCS compared with ECS (P < 0.01). Similar increases have also been reported (Batajoo and Shaver, 1994; Valadares Filho et al., 2000) as forage was substituted with grains. In a review by Emery (1978), milk protein concentration was positively correlated (R = 0.42) with energy intake obtained by the substitution of roughage with concentrate. In our study, higher DMI with WCS contributed to increase metabolizable energy supply compared with ECS. Finally, because of a greater milk production, cows had a higher protein yield with WCS than ECS or RCS (P < 0.01). The trend for a higher milk production also resulted in a significantly higher protein yield for TS compared with ECS (P = 0.03).

Higher lactose concentration in milk produced with WCS compared with ECS (P < 0.01) could be explained by the greater energy supply, as discussed previously. Other researchers (Macleod et al., 1983; Valadares Filho et al., 2000) also noticed an elevation of lactose concentration in milk when decreasing dietary F : C. Yield of lactose was also greater for WCS than ECS (P < 0.01). Moreover, as no diet effect was detected in lactose concentration, increased milk yield explained the higher lactose yield with WCS than RCS (P < 0.01), and with TS than ECS (P = 0.05).

The highest numerical value for MUN was observed with ECS (14.1 mg/dl). Tendencies for higher MUN were therefore observed when comparing this treatment with WCS (P = 0.06) or TS (P = 0.07). Valadares Filho et al. (2000) and Weiss et al. (2009) also observed a decrease in MUN as dietary starch supply increased. Although ECS and TS diets had similar starch concentrations, the tendency for a lower MUN with TS suggests a better synchrony of carbohydrate fermentability and N release in the rumen leading to more efficient N utilization. This is in line with the tendency for a higher milk yield observed with TS. Finally, treatments had no effect on somatic cell count.

Rumen fermentation

Ruminal pH decreased linearly with time after meal (P < 0.01; data not shown), and no interaction was observed between dietary treatments and time (P = 0.85). Moreover, treatments did not significantly affect mean ruminal pH for selected contrasts (P > 0.10; Table 6). Reducing dietary NDF concentration while increasing starch supply is known to decrease ruminal pH (Oba and Allen, 2003). Under our experimental conditions, WCS, which contained less NDF and more starch, numerically decreased average ruminal pH. However, the difference did not reach statistical significance compared with ECS (P = 0.14). During data sampling, ruminal parameters were monitored from 0 to 6 h post feeding. Ruminal pH had not reached a nadir at the last time point. Therefore, it cannot be ruled out that differences in ruminal pH could have been observed after 6 h post feeding.

Table 6 Effect of experimental diets on rumen fermentation

WCS = whole corn silage; RCS = reconstituted corn silage; ECS = earless corn silage; TS = timothy silage; VFA = volatile fatty acids.

Concentration of NH3-N in ruminal fluid was higher when cows received ECS compared with TS (P = 0.04; Table 6). This difference is in accordance with the tendency for higher MUN observed with ECS. In ruminants, ammonia is absorbed across the rumen wall into blood stream and reaches liver where it will be converted into urea (Lobley et al., 1995).

Total VFA concentration was higher for cows fed WCS than RCS (P = 0.03; Table 6). More VFA could have been produced as DMI, and thus the supply of fermentable substrates, increased with WCS compared with RCS (Table 4). This observation also offers a support for the potentially higher availability of starch in the rumen with WCS compared with RCS because of a higher moisture concentration, as discussed above.

No interaction of treatment by time was observed for ruminal VFA profile (P > 0.10). No difference was also noted in molar proportions of individual VFA when comparing WCS with RCS, or ECS with TS (P > 0.1; Table 6) at similar supply of dietary fiber and starch. However, feeding ECS decreased the proportions of propionate and valerate, and increased the proportions of acetate and caproate, as well as the acetate to propionate ratio when comparison is made with WCS (P ⩽ 0.05). Introducing WCS in the diet resulted in 13.5% less NDF and 16.1% more starch concentration when compared with ECS. Oba and Allen (2003) observed similar changes in molar proportions of individual ruminal VFA when increasing the starch concentration of experimental diets, which favours propionate-producing bacteria.

Milk FAs

A shift in UFA biohydrogenation pathway could occur when rumen environment is altered (Bauman and Griinari, 2001), leading to the production of t10c12 18:2 and t10 18:1. The t11 18:1 to t10 18:1 ratio would then decrease in milk fat. Because t10c12 18:2 has been identified as an inhibitor of milk fat synthesis in lactating dairy cows (Bauman and Griinari 2001), a concomitant decrease in milk fat concentration and yield is usually observed. In the current study, milk concentrations of t10c12 18:2 and t10 18:1 were not affected by dietary treatments (P ⩾ 0.10; Table 7). This observation is consistent with moderate yet significant changes observed on rumen fermentation (NH3-N concentration and VFA profile; Table 6), and the lack of treatment effect on milk fat concentration and yield (Table 5).

Table 7 Effect of experimental diets on milk fat composition (g/100 g)

WCS = whole corn silage; RCS = reconstituted corn silage; ECS = earless corn silage; TS = timothy silage.

aCoelution with t10 16:1.

bCoelution with c10 16:1.

cCoelution with c10 18:1.

dn-6 fatty acids.

en-3 fatty acids.

Milk fat concentration of trans-18:1 and several individual trans-18:1 isomers were higher in milk fat of cows fed WCS compared with RCS (P < 0.01; Table 7). However, the specific ratio of t11 18:1 to t10 18:1 was similar among those two treatments (P = 0.53). A higher proportion of trans-18:1 FA in milk fat with WCS could be explained by a greater supply of dietary UFA for biohydrogenation, as a tendency for an increased DMI (P = 0.08), and a significantly higher EE intake (P < 0.01; Table 4) were recorded for this treatment compared with RCS. A better availability of FA for biohydrogenation may also explain the higher concentration of trans-18:1 as the corn grain in WCS presented higher moisture concentration, and was potentially more degradable than HMC in RCS.

Removing the grain fraction from the corn plant (WCS v. ECS) increased the concentrations of iso 13:0, iso 14:0 and iso 15:0 FA in milk fat (P < 0.01). As reported by Vlaeminck et al., (2006), iso FA are more abundant in cellulolytic bacteria as opposed to amylolytic bacteria in the rumen. Higher proportions of iso 13:0, iso 14:0 and iso 15:0 in milk (Table 7) is therefore consistent with the increased proportions of ADF and NDF in ECS compared with WCS diets (Table 3). Moreover, a significantly lower (P = 0.03) milk iso 15:0 concentration observed with WCS compared with RCS is in line with the potentially higher starch availability with this treatment.

A comparison between WCS and ECS also reveals that removing the grain fraction from the plant before ensiling decreased the proportions of c9c12 18:2, c9c12c15 18:3 and total trans-18:1 in milk fat (P < 0.05). These effects on milk FA profile could again be attributed to a varying supply of dietary polyunsaturated FA (PUFA), as total FA concentration of diet (Table 3), and EE intake of cows (Table 4) were both lower for ECS compared with WCS.

Total FA concentration of diet (Table 3) and EE intake of cows (Table 4) were both higher for TS compared with ECS, which led to a greater supply of PUFA. Consequently, feeding TS as opposed to ECS increased the proportions of c9c12 18:2 (n-6), c9c12c15 18:3 (n-3), as well as other FA from the n-6 (c5c8c11c14 20:4 and c13c16 22:2) and n-3 (c5c8c11c14c17 20:5 and c7c10c13c16c19 22:5) families in milk fat (P < 0.05). Dietary essential FA (c9c12 18:2 and c9c12c15 18:3) are directly transferred to milk, or alternatively desaturated and elongated to other FA from their respective n-family before their incorporation into milk fat. Feeding TS also increased the proportion of intermediates in biohydrogenation of c9c12c15 18:3 (e.g. c9t11c15 18:3 and t11c15 18:2), and decreased the ratio of n-6 to n-3 FA (P < 0.01).

Conclusions

In this experiment, removing the grain fraction from CS decreased milk yield, and modified rumen fermentation with higher acetate and lower propionate concentration. However, feeding CS with or without ears did not affect milk fat concentration and yield, as well as the concentrations of t10 18:1 and t10c12 18:2 in milk fat. Those results do not support our hypothesis that grain fraction of CS alters the pattern of biohydrogenation of UFA in the rumen, and reduces milk fat concentration and yield.

The reconstitution of CS with a mixture of HMC and ECS led to similar ruminal pH and VFA profile, 4.0% FCM yield, efficiency of milk production, as well as milk fat concentration and yield compared with WCS. Despite some differences in DMI and total ruminal VFA concentration between WCS and RCS, the restoration of FCM yield, using HMC in RCS diets, to a level of production similar to WCS highlights the importance of energy and nutrients supplied by the grain fraction of CS to support milk yield.

Acknowledgments

The authors thank administrative and research staff of the Centre de recherche en sciences animales de Deschambault (Québec, Canada) for their help and support in the production of experimental silages (including corn ear removal), and the care provided to cows during the trial. The authors are also grateful to administration of the Département des sciences animales, Université Laval, for supporting this research, as well as Émie Désilets, Micheline Gingras and Nancy Bolduc of this department for their assistance in samplings and laboratory analyses.

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