Effect of Replacing Corn Silage with Canola Silage on Feed Intake, Nutrient Digestibility, Milk Yield, and Thyroid Hormones of Lactating Dairy Cows

50, and 100% of DM. Results: Dietary concentrations of neutral detergent fiber (NDF) and acid detergent fiber increased when corn silage was replaced by canola silage. Replacing corn silage with canola silage caused a significant decrease in DMI. The apparent digestibility of DM and crude protein were not affected by dietary replacement of corn silage with canola silage; however, organic matter and digestibility of NDF decreased significantly. When corn silage was replaced by canola silage, there were no significant changes in milk yield, fat corrected milk, and milk composition in the investigated cattle, but milk protein changed significantly. The results indicated that 50% substitution of canola silage caused higher milk protein. Rumen fluid pH and its ammonia nitrogen concentration increased significantly in experimental animals. The concentrations of T 3 and T 4 decreased, whereas blood urea nitrogen concentration increased by dietary replacement of corn silage with canola silage. Conclusion: Canola silage can be substituted with corn silage, and feeding canola silage can be beneficial in some aspects.


Introduction
Providing good quality forage in dairy cattle diet is vital for the dairy farm industry 1 . In some semitropical countries, such as Iran, low rain in the last decade has led to a decline in forage production 2 . On the other hand, Iran's government planned to increase vegetable oil production 2,3 , which has increased canola cultivation over the past decade. Canola is one of the oilseed plants that belong to the Brassica species 4 . Decrease in environment temperature or acute damage by pests makes canola cultivation difficult 5 . In this situation, most farmers eliminate damaged canola forage by burning or burying it in the ground. In this condition, there is a question about the possibility of utilizing canola forage in ruminant nutrition. Some studies indicated the nutrient composition of canola silage and its positive effects on dairy cattle performance [6][7][8] . Moreover, a study on the effects of adding different levels of rapeseed (canola seed) silage on the diet of beef cattle indicated that tract digestibility and NDF were greater in high glucosinolate rapeseed varieties 9 . Given the above-mentioned, the current study aimed to determine the effects of substituting corn silage with canola silage on feed intake and apparent digestibility of nutrients, milk yield, and milk composition.

Ethical approval
All animals were treated in accordance with the regulations on the guidelines of the Iranian Council of Animal Care (1995), and the experiment was approved by the Iranian Ministry of Agriculture (experimental permission no. 1828).

Experimental design
In the current study, 9 Holstein dairy cows with a mean parity of 2.5 (mean ± SD), mean body weight of 556 kg (mean ± SD), days in milk (DIM) of 89.3 ± 21.19 (mean ± SD), and milk yield of 41.7 ± 4.0 kg/ d (mean ± SD) were allocated in a 3×3 change over Latin square design study. This study contained three periods, each lasting 20 days. The first 15 days of each period were the adaptation period. Every 5 days, a composite diet sample was collected (4 times during each period). Both corn (Agratech 787, Agra Tech Seeds, Inc., Ashburn GA) and Canola (Hyola-308) were plated in the sandy soil on April 6, 2019, and irrigation was provided to supplement natural rainfall. Corn was chopped and stored in a concrete silo on August 11, 2019, and canola was mown on August 4, 2019, and allowed to wilt to approximately 20% DM before being chopped and stored in a concrete stave silo. Diet was offered twice daily as a total mixed rations (TMR) at 1000 hours and 1500 hours. Forage was 33% DM of the diet (Table 1). In the present study, the experimental diet was substituted for 0, 50, and 100% with corn silage DM. Samples of diet (0.5 kg) were collected during the experiment and stored at -20°C until analysis. The DM content of the experimental diet was determined by drying in the oven at 55°C for 48 hours 10 .
After drying, ingredients and TMR were ground to pass through a 1-mm screen (Wiley mill, Arthur H. Thomas, Philadelphia, PA). Samples were analyzed for DM, CP, ash 11 , ADF, and NDF 12 . Cows were milked twice daily, and milk samples (10 ml) were collected 4 times ( twice in the morning and twice in the evening) in the final two days of each period in the milking pallor before milking from all quarters. Milk samples were transferred to the Dairy Laboratory of Mashhad Agricultural Organization, Mashhad, Iran, for nutrient composition analysis using a spectrum analyzer (FT400; Foss North America Inc., Eden Prairie, MN). Blood samples (5 ml) were collected from the jugular vein at the end of the experiment. Tubes were allowed to clot, and serum was centrifuged at 30000 rpm at 4°C for 45 minutes. Blood concentrations of urea nitrogen and glucose were measured using an enzymatic and colorimetric method, respectively, by zist shimi analyzing kits, Iran. Thyroid hormone was measured by radioimmunoassay procedure. Fecal samples were collected directly from the rectum in the last 5 days of each experimental period. Fecal samples were collected and analyzed in terms of DM, ash, CP, ADF, NDF, and acid insoluble ash (AIA). Acid insoluble ash was used as an indicator to determine the apparent digestibility of nutrients 12 . Ruminal fluid samples were taken 3 hours after feeding time on the last day of each experimental period from the ventral sac of the rumen, and pH was determined. Ruminal fluid samples were preserved by adding 10 ml chloric acid to 10 ml of ruminal fluid and stored at -20°C for further analysis. Samples of strained ruminal fluid were thawed and centrifuged at 30,000 g for 20 minutes at 4°C andsupernatants were analyzed for NH3 using the Kjeldahl method 12 .

Statistical analysis
The resulting data were statistically analyzed using the linear model in the statistical package for the social sciences (SPSS 16, Chicago, USA). Tukey test was chosen to determine the mean significant differences between treatments. The p < 0.05 was considered a significant difference between the experimental treatments.

Results and Discussion
The nutrient content of corn silage and canola silage is presented in Table 2. The DM content of corn silage was higher than canola silage (p < 0.05). Concentrations of CP in corn silage were lower than in canola silage, but NDF and ADF contents of canola silage were higher (p < 0.05). Table 1 demonstrates the composition of the experimental diets. The results showed a decrease in the DM content of the diet by replacing corn silage with canola silage. Moreover, there was an increase in the CP, NDF, and ADF of experimental diets, which might be due to the higher fiber content of canola silage, compared to corn silage [7][8][9][10][11][12][13] . The DMI values between experimental treatments were significantly different ( Table 3). The DMI decreased by increasing the substitution level of corn silage with canola silage (p < 0.05). Brassica forage contains 80-95% water that limits DMI due to the gut fill effect 14,15,16 . The NDF content in the ruminants' diet is a very important chemical predictor of DMI 17 . The DMI has a positive correlation with NDF concentration and limits food intake 10,18 . Therefore, an increase in the NDF content of diets caused by substituting corn silage with canola silage can effectively limit DMI 9,19 . Nutrient apparent digestibility of diets is presented in Table 3. Apparent digestibility of DM and CP were not affrected by the dietry replacement of corn silage with canola silage. However, apparent digestibility of NDF (p < 0.05) and OM (p < 0.05) decreased as a result of canola silage replacement. The findings of a study on feed intake and apparent digestibility of diet supplemented with Brassica hay in Lambs showed that hay plus tyfon (Brassica) diets exhibited negative associations in apparent digestibility of NDF, and ADF 9 . Likewise, the results of the current study indicated that canola similar to tyfon decreased the apparent digestibility of NDF.
The result of the current study showed an increase in rumen pH and NH3-N concentration in dietry replacement of corn silage with canola silage (p < 0.05, Table 3). Chewing activity increases by intake of forage due to its fibers 3 . Chewing time increase with an increase in the proportion of NDF in the diet that stimulates saliva secretion 20 . Saliva contains half the bicarbonate entering the rumen that helps to buffer the acids produced during fermentation 18,19,21 .
It is suggested that an increase in diet NDF content by replacing canola silage with corn silage is associated with an increase in ruminal pH value. The substitution of corn  Replacing corn silage with canola silage did not change milk yield, FCM (4%), and milk chemical composition (Table 4), but protein in the group where 50% of corn silage was replaced with canola silage was higher than in other treatments (p < 0.05).
Blood glucose and BUN concentration were influenced as canola silage was replaced with corn silage ( Table 5). Replacement of corn silage with canola silage results in an increase in diets CP content so it can be effective in enhancing BUN concentration 22 . Increased content, solubility, and degradability of dietary protein can lead to increased ammonia concentrations in the rumen, resulting in high blood BUN concentrations.
It is indicated that increasing rumen propionate concentration as a result of fermentation of tyfon (Brassica) forage carbohydrates enhances blood glucose concentration 23 .
Serum T3 and T4 concentrations were affected by increasing substitution levels of corn silage with canola silage (Table 5). Glucosinolate contains Brassica has a negative impact on thyroid gland function by interfering with iodine uptake 24-26.   A study reported a reduction in T4 concentration and an enhancement in T3 concentration in lambs due to diets containing rape 27 .
Seeds are the major source of glucosinolates in the canola plant 7 . Although in this study the concentrations of glucosinolates did not measure. It seems that the canola pods in canola forage can increase glucosinolates concentration in diets as canola silage was replaced with corn silage. It diminished the levels of T4 and T3 in cows treated with 100% canola silage.

Conclusion
The DM content of corn silage was higher than canola silage. Concentrations of CP in corn silage were lower than canola silage, but NDF and ADF content of canola silage was higher than corn silage. Canola silage can be replaced by corn silage, and feeding canola silage can be beneficial in some aspects. The high fiber content of canola silage can limit DMI since it is important to take more studies about the application level of canola silage in dairy cattle diets.

Competing interests
There is no conflict of interest.

Authors' contribution
The final manuscript draft was reviewed by all authors, who also gave their approval.

Funding
The study was funded by the Ferdowsi University of Mashhad and the Centre of Excellence for Animal Science, Iran.

Ethical considerations
Ethical issues (including plagiarism, consent to publish, misconduct, data fabrication and/or falsification, double publication and/or submission, and redundancy) have been checked by all the authors.