MATERIALS AND METHODS

Master Graze (Master’s Choice Inc., Anna, IL) was planted in a 3.89 hectare (ha) field (Southern Illinois University Carbondale Dairy Farm, Carbondale, IL) on May 12, 2010. The MG was planted at a seeding rate of approximately 74,131 plants ha^-1. An addition of 45.36 kg of N was applied on June 2, 2010, three weeks after the MG was planted. The field was also treated with a Princep and Atrazine pesticide. The MG was chopped on July 11, 2010 using a conventional pull-behind chopper (Ford New Holland 790, New Holland, PA) and chopped into particle lengths of 3/4-1 inch long. The chopped forage was then bagged into a 6 foot by 150 foot silage bag and sealed for eight weeks, until it was opened at the start of the study on September 21, 2010.

Animals and diets: Sixteen Holstein cows, in mid lactation (120±19 DIM), were used in a balanced 4x4 Latin square design with 3 week periods. The cows were divided into four groups according to days in milk, milk yield and parity. The cows were then housed in four separate pens (4.57x9.14 m) with four cows per pen. The trial consisted of a 2 week adaptation period followed by a 1 week experimental period. The cows were fed one of four treatment diets once daily as a Total Mixed Ration (TMR) for 4 consecutive periods. The control diet (CON) consisted of a 60:40 forage:concentrate diet (DM basis) with Corn Silage (CS) and Alfalfa Hay (AH) as the forage source (1:1; DM basis). The concentrate mixture consisted of ground corn, dried distillers grain, soybean meal, dried molasses and a mineral-vitamin mix (Table 1).

Table 1:	The ingredients and chemical composition of treatment diets
CON: Control diet, MAS-16: Control diet with 16% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), MAS-33: Control diet with 33% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), MAS-50: Control diet with 50% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), Salt-TM: NaCl: 955-9.8, Zn: 10.0, Mn: 7.5, Fe: 6.0, Mg: 0.5, Cu: 0.32, I: 0.28, Co: 0.11, Values within rows with different superscripts are statistically different at p<0.05

For the three treatment groups, the MG silage was substituted in place of the corn silage and alfalfa hay in the forage mix at 16% (MAS-16), 33% (MAS-33) and 50% (MAS-50) on a DM basis. All diets were formulated to be isonitrogenous and meet the NRC (2001) nutrient requirements for lactating dairy cows. Protocols for this trial were approved by the Southern Illinois University Carbondale Institute of Animal Care and Use Committee. All the sixteen cows completed the study.

The cows were fed the treatment diets once daily (07:00 h) in amounts to provide approximately 10% orts for ad libitum consumption. The diet refusal rate and the total feed intake for each pen were recorded daily before feeding. Cows were milked twice a day at approximately 05:30 and 16:30 h and daily milk yield was recorded. Cows had free access to fresh water at all times except during milking times. Cow’s body weight (BW) and BCS were recorded at the start of the experiment and at the end of each period.

Sampling: Feed ingredient samples were collected on day 7 of each week and DM content was determined by drying the samples in an oven at 55°C for 72 h. The samples were then ground to pass through a 2 mm screen using a Wiley mill (Arthur H. Thomas, Philadelphia, PA). The ingredient samples were then composited per ingredient per period and stored at room temperature until analyzed for DM, OM, starch, CP (LECO FP-528, St. Joseph, MI) and ether extract (ANKOM XT10, Macedon, NY), ADF, NDF (ANKOM 200 Fiber Analyzer, Macedon, NY) and lignin (AOAC, 2000; Van Soest et al., 1991).

Milk samples were collected from each cow during the milking process by an automated milk sampler unit. Milk samples were collected from three consecutive pm and am milkings at the end of each period. The milk samples (am and pm) were composited per cow per day, amounts proportional to milk yield at each time and sent to Prairie Farms (Carlinville, IL) for analysis of fat, protein, lactose, total solids, Milk Urea Nitrogen (MUN) by mid infrared spectrophotometry (AOAC, 2000). Somatic Cell Counts (SCC) were determined using a Fossomatic 90 instrument (Prairie Farms, Carlinville, IL).

Digestibility: At the end of each period, the same two cows from each pen were used to determine nutrients digestibility coefficients. A fecal grab sample was obtained and acid-insoluble ash was used as an internal marker according to Van Keulen and Young (1977) for determining nutrients digestibility. The samples were taken at 0700, 1100 and 1900 h on day 20 and at 0300, 1500 and 2300 h on day 21 of each period to represent every 4 h in a theoretical 24 h time clock. Samples were dried in an oven at 55°C for 72 h. Samples were then ground to pass through a 2 mm screen using a Wiley mill (Arthur H. Thomas, Philadelphia, PA), composited for each cow and stored until analyzed for DM, OM, CP and NDF as described previously.

Statistical analysis: Data for milk composition, milk yield, BW and BCS were analyzed as a 4x4 Latin square design using the PROC MIXED of SAS (SAS/STAT, 2010) using the following model:

where, μ is the overall mean, D is the diet effect, P is the period effect, C is the cow effect and e is the residual error. The diets and periods are the fixed effects and the cow is the random effect. Results are expressed as least square means with standard error of the means.

Data for feed intake and digestibility were also analyzed as a 4x4 Latin square design using the PROC MIXED SAS (SAS/STAT, 2010) using same model except pen was the random effect. Significance was declared at p<0.05.

RESULTS

The ingredients and the chemical composition of treatment diets are presented in Table 1. The chemical composition for forages and concentrate mix are presented in Table 2. The DM content of treatment diets was highest in the CON diet and decreased (p<0.05) as the level of MG silage in diets increased. The concentrations of NDF and ADF in treatment diets increased while the concentration of starch decreased as the level of MG silage in the diet increased. Although, diets were balanced to be isonitrogenous, after chemical analysis dietary CP content was highest with the CON diet and decreased (p<0.05) as the level of MG silage in diets increased.

The effects of substituting CS and AH with MG silage on dry mater intake (DMI), milk yield and milk composition are presented in Table 3. The inclusion of MG silage in treatment diets as a substitute for CS and AH had no effects (p>0.05) on feed intake and milk.

Table 2:	Chemical composition of dietary ingredients
CS: Corn silage, MG: Master graze, AH: Alfalfa hay, CM: Concentrate mix

Table 3:	Effects of substituting corn silage and alfalfa hay with Master Graze silage at different levels on feed intake, milk yield and milk composition
CON: Control diet, MAS-16: Control diet with 16% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), MAS-33: Control diet with 33% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), MAS-50: Control diet with 50% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), Values within rows with different superscripts are statistically different at p<0.05

Milk fat percentages, milk fat yield, protein percentages and milk protein yield were also not affected (p>0.05) by MG silage inclusions in treatment diets. Relative to the CON diet, milk lactose content was higher (p<0.05) in cows fed the MAS-33 and MAS-50 diets. Milk total solids, MUN, SCC and cows BW and BCS were also not affected (p>0.05) by MG silage inclusions in treatment diets (Table 3).

The effects of substituting CS and AH with MG silage on nutrient digestibility is presented in Table 4. Relative to the CON diet, the apparent total tract digestibility of DM was higher (p<0.05) in cows fed the MAS-50 diet and tended to increase (p<0.08) in cows fed the MAS-16 and MAS-33 diets.

Table 4:	Effects of substituting corn silage and alfalfa hay with master graze silage on apparent total tract digestibility of nutrients in treatment diets
CON: Control diet, MAS-16: Control diet with 16% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), MAS-33: Control diet with 33% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), MAS-50: Control diet with 50% master graze silage as a replacement of corn silage and of alfalfa hay (DM basis), Values within rows with different superscripts are statistically different at p<0.05

Similarly, the apparent total tract digestibility of OM was higher (p<0.05) in cows fed the MAS-50 diet and tended to increase (p<0.11) in cows fed the MAS-33, in comparison to the CON diet. The apparent digestibility of NDF was higher (p<0.01) in cows fed the MAS-50 diet than in cows fed the MAS-16 or MAS-33 diet and all were higher (p<0.05) than the cows fed the CON diet. Lastly, the apparent total tract digestibility of CP was higher (p<0.05) in cows fed the MAS-50 diet in comparison to cows fed the CON diet.

DISCUSSION

The DM content of the CON diet was higher than the treatment diets due to the DM content of the AH. Additionally, because the DM content for MG (17.5%) was lower than that of CS (27.4%), the DM content decreased as the percentage of MG silage in treatment diets increased. Dann et al. (2008) and Grant et al. (1995) also reported lower DM content in BMR sorghum-Sudan grass silage or sorghum silage than CS. The observed decrease in diets CP content with MG silage substitution is due to lower CP content in MG silage than anticipated (6.0 vs. 8.0%). The increase in the dietary NDF and ADF content with the inclusion of MG silage was expected, as NDF and ADF content for MG silage (69.1 and 43.02%) was higher than that of CS (47.4%, 29.04). The decrease in diet starch content was also expected as starch content of MG (2.11%) was lower than that of CS (24.23%) Other studies (Bernard et al., 2002; Dann et al., 2008) have also reported higher NDF and lower starch contents when CS in diets was substituted with sorghum or ryegrass silages.

The DMI in the current study was not affected by the treatment diets. The effect of replacing CS with other forage silages on DMI has not been consistent. Bernard et al. (2002) and Oliver et al. (2004) reported no differences in DMI when CS was replaced with ryegrass silage or sorghum silage. However, Lusk et al. (1984) reported a higher DMI with CS than BMR-12 sorghum in both dairy heifers and lactating dairy cows. Lance et al. (1964) and Nordquist and Rumery (1967) also observed a higher DMI of the CS diet than the sorghum silage diets. Dann et al. (2008) and Grant et al. (1995) also reported higher DMI with cows fed CS compared to BMR sorghum-Sudan grass silage or BMR sorghum silage. Studies that reported higher DMI with the CS diets had also reported higher digestibility with the CS diets. In the current study, the apparent DM and OM digestibility of treatment diets were higher than that of the CON diet and may explain the lack of effects on DMI.

Milk yield in the current study was not affected by the treatment diets in comparison to the CON diet, presumably because cows were in mid-lactation and their energy needs were being met. Other studies (Lusk et al., 1984; Grant et al., 1995; Oliver et al., 2004; Colombini et al., 2010) have also reported no differences in milk yield between cows fed CS or BMR-sorghum silage. However, milk yield was increased when ryegrass silage (Bernard et al., 2002) and BMR-sorghum silage (Dann et al., 2008) replaced CS in cows diet. Oba and Allen (1999) also reported higher milk yield with cows fed the BMR CS in comparison to the cows fed the normal CS. The studies that reported higher milk yield with BMR sorghum or BMR CS had also reported higher DMI. In the current study, DMI was not affected by treatment diets and may also explain the lack of effects on milk yield.

In the current study there was no effect of treatment on the cow’s milk fat content or fat yield. Lusk et al. (1984), Dann et al. (2008) and Colombini et al. (2010) also reported no differences in milk fat content or yield when the CS in cow’s diets was replaced by BMR sorghum silage. In contrast, Grant et al. (1995) and Oliver et al. (2004) observed that milk fat content was highest in cows fed the CS diet and the BMR sorghum diet but lowest in cows fed the normal sorghum diet. Likewise, in a study done by Aydin et al. (1999), milk fat content was higher in cows fed the BMR sorghum silage diet than cows fed the normal sorghum silage diet. Miron et al. (2007) reported higher milk fat content and yield in cows fed normal and BMR sorghum silage diets in comparison to the cows fed the CS diet. The discrepancies among studies could be attributed to the differences in diet composition, forage quality, stage of lactation and feed intake.

The treatment diets in the current study had also no effect on milk protein content and protein yield. Lusk et al. (1984), Oliver et al. (2004) and Colombini et al. (2010) also reported no differences in milk protein content or yield when cows were fed either CS or BMR sorghum silage. However, in other studies higher milk protein content was reported in cows fed CS diets in comparison to cows fed BMR sorghum silage diets (Grant et al., 1995; Aydin et al., 1999; Miron et al., 2007) or BMR sorghum-Sudan grass silage (Dann et al., 2008). Milk lactose content was higher in the cows fed the MAS-33 and MAS-50 diets, however, the increase in milk lactose content was marginal and most likely does not represent a biological difference. Grant et al. (1995) also reported higher milk lactose content in cows fed the BMR sorghum diet in comparison to the cows fed the normal sorghum diet. However, most studies reported no effects of replacing CS with sorghum silage on milk lactose content (Aydin et al., 1999; Oliver et al., 2004; Miron et al., 2007; Dann et al., 2008).

The apparent digestibility of DM and OM of treatment diets in this study were improved with the feeding of MG silage, particularly when MG silage replaced one-half of the forages in the cow’s diet. Lusk et al. (1984) and Oliver et al. (2004) also reported higher DM digestibility for BMR sorghum silage than normal sorghum. In contrast to the current study, Bernard et al. (2002) and Dann et al. (2008) reported higher DM digestibility for CS diets in comparison to the ryegrass silage or BMR sorghum-Sudan grass silage diets, respectively. Despite the higher starch content for the CS (24%) relative to the MG silage (2%) and the greater NDF content for the MG silage (69%) relative to the AH (57%), the apparent DM digestibility was still higher with the MG silage diets suggesting that the nutrients (mostly NDF) in MG silage are highly digestible. Because of the lack of differences in DMI among treatment diets in this study, the differences in DM and OM digestibility were unlikely caused by alternation in rumen passage rate or pH.

The apparent digestibility of dietary NDF increased with the inclusion of the MG silage and the increase in dietary NDF digestibility was proportional to the MG silage inclusion level. Oba and Allen (1999) reported that the NDF digestibility was higher for the BMR CS diet in comparison to the normal CS diet. The NDF digestibility was also higher for BMR sorghum silage diet than the normal sorghum (Grant et al., 1995; Aydin et al., 1999; Oliver et al., 2004). In contrast, Dann et al. (2008) reported no differences in NDF digestibility between cows fed CS or BMR sorghum-Sudan grass silage. Despite the increase in dietary NDF content with the inclusion of MG silage in the diets, the apparent NDF digestibility was still higher for the MG diets suggesting that the fiber (NDF) in the MG silage is more digestible than that of CS or AH. The higher digestibility of DM, OM and NDF seen with the inclusion of MG silage may be also attributed in part to the higher apparent CP digestibility for the MG silage. Higher dietary CP digestibility may have resulted in more nitrogen being supplied to rumen microbes, cellulolytic bacteria in particular and therefore higher microbial activity and nutrient digestion in the rumen. Although, dietary CP digestibility was higher when ryegrass silage replaced CS in cow’s diet, DM digestibility and NDF digestibility were still higher with the CS diet (Bernard et al., 2002). Other studies, however, have reported either no changes (Oliver et al., 2004) or increases (Lusk et al., 1984; Dann et al., 2008) in dietary CP digestibility with the CS diets in comparison to the BMR sorghum silage or BMR sorghum-Sudan grass silage diets.

CONCLUSION

The inclusion of MG silage in a lactating dairy cow diet did not affect cows feed intake, milk yield or milk composition. On the other hand, the inclusion of MG silage in the treatment diets improved the apparent total tract digestibility of DM, OM, NDF and CP. The MG silage may replace up to 50% of dietary corn silage-alfalfa hay mix in dairy cow rations without any adverse effect on cow performance.