Corn is the main source of energy used to feed confined cattle. In Brazil, flint corn is the predominant type used (Oliveira and Millen, 2014; Pinto and Millen, 2019) and is known to have a lower degradability rate and, therefore, lower energy availability (Philippeau and Michalet-Doreau, 1997; Correa et al., 2002; Stock and Erickson, 2006). Grain processing practices aimed at modifying its physical structure and increasing starch availability are necessary (Ferraretto et al., 2015; González García et al., 2018; Silva et al., 2019).

Thus, processing is critical and can range from coarser grinding to more efficient and optimizing methods such as ensiling grains harvested with high moisture content or rehydrating grain with moisture content reconstitution (Macken et al., 2006; Ferraretto et al., 2014; Silva et al., 2018). Reducing particle size enhances energy availability, increases surface area for microbial colonization, and starch digestibility, consequently improving animal performance (McAllister et al., 2006; Owens and Soderlund, 2006; Zinn et al., 2007; Ferraretto et al., 2015; Arcari et al., 2016). Reducing particle size, however, can reduce chewing and rumination, as well as increase passage rate (Cozannet et al., 2018; González García et al., 2018).

Rehydrated and ensiled corn grain is an alternative to minimize some common problems in production systems that use corn as an energy source for beef cattle. This technology allows reducing and/or eliminating costs with fees and discounts, taxes, transportation, freight, and storage, as well as reducing losses from insect and rodent attacks, which is very common in dry corn storage (Macken et al., 2006; Silva et al., 2018; Paschoaloto et al., 2019). In addition, there are possibilities of buying grains during periods of lower prices, especially in the harvest season, with economic appeal, providing a reduction in cattle production costs (Arcari et al., 2016).

Ensiling rehydrated corn grain is a strategy to improve starch digestibility (Owens et al., 1986; Benton et al., 2005). During the storage period, the action of proteolytic bacteria and kernel proteases break down the protein matrix (Junges et al., 2017), increasing the availability of starch to animal digestion (Zinn et al., 2007; Hoffman et al., 2011; Ferraretto et al., 2015).

In recent meta-analyses about the effect of ensiling on the feeding value of flint corn grain for feedlot beef cattle, Jacovaci et al. (2021) found that the inclusion of ensiled corn in diets increased total tract digestibility of DM by 4.59% and starch by 3.33%, decreased DM intake by 14.1%, and increased feed efficiency by 18.3% but did not affect average daily gain (ADG). However, these benefits of grain ensiling are based on three major factors (Gomes et al., 2020): moisture content (Owens et al., 1997), particle size (Rémond et al., 2004), and length of storage time (Hoffman et al., 2011).

We hypothesized that mean particle size (MPS) and ensiling process of corn grain could increase animal performance by increasing the digestibility of starch, feed efficiency, and average daily gain (ADG) to allow a complete substitution of cracked dry corn grain. The objective was to evaluate the performance of young bulls fed finishing diets composed of rehydrated and ensiled corn grain, finely or coarsely ground, in substitution of dry corn grain, finely or coarsely ground.

All procedures were approved by the Animal Use Ethics Committee (CEUA) under protocol number 029/2018.

In the present study, there were no interaction effects for parameters initial BW, final BW, ADG, feed efficiency, DMI, ADF intake, and ME of diets (Table 3). Statistical differences (P<0.01) were found for feed efficiency and DMI, when comparing diets containing dry ground corn and rehydrated corn grain silage, respectively. Diets with rehydrated and ensiled corn decreased the DMI (10.3%). Compared with diets containing dry corn, diets balanced with ensiled corn increased feed efficiency without affecting ADG.

Table 3

Item	Particle size		Source		SEM	P-value
	FG	CG	DC	RC		PART (P)	Source (S)	P×S
Animal performance
Initial BW (kg)	374.2	374.3	374.5	374.0	3.14	0.98	0.92	0.75
Final BW (kg)	503.9	504.9	505.0	503.7	1.01	0.99	0.46	0.75
ADG (kg)	1.52	1.54	1.53	1.53	0.05	0.80	0.92	0.17
Feed efficiency	0.14	0.14	0.13	0.15	0.41	0.65	<0.01	0.12
Nutrient intake
DM (kg/day)	10.8	11.2	11.6	10.4	0.14	0.28	<0.01	0.97
ADF (kg/day)	0.59	0.70	0.75	0.54	<0.01	0.04	<0.01	0.62
ME (Mcal/day)	34.3	31.6	34.0	32.0	1.61	0.41	0.48	0.81

BW - body weight; ADG - average daily gain; DM - dry matter; ADF - acid detergent fiber; ME - metabolizable energy.

The NEm value of the diets was similar for rehydrated and dry corn treatments. However, the NEg for RCC increased values of the diet by 1.4% (RCF), 4.1%(DCF), and 10.9% (DCC).

For ADF intake, differences were found for the influence of MPS (P = 0.04) and grain source (P<0.01). There was a higher intake of ADF for animals fed dry corn and corn grain coarsely ground when compared with finely ground and rehydrated corn. In contrast, no significant differences (P = 0.48) were detected in ME intake when comparing grain source and particle size.

The ensiling process provided a 10.2% reduction in protein intake when compared with the use of ground corn, regardless of the particle size used. There was also an interaction effect of grain source on this trait showing higher protein intake in diets with dry corn over treatments with rehydrated corn, with no particle size effect.

Interaction effects between particle size and grain source were found for the variables daily intake of NEm, NEg, CP, NDF, and starch and for fecal starch and TTSD (Table 4). For variables CP and NDF (kg/d), higher intake values were found for animals fed dry corn in the diet.

Table 4

Item	Treatment1				SEM	P-value
	DCF	DCC	RCF	RCC		PART (P)	Source (S)	P×S
Nutrient intake
Dietary NEm (Mcal/kg)	2.06a	1.90b	1.99ab	2.05a	0.16	2.88	0.20	0.08
Dietary NEg (Mcal/kg)	1.40a	1.30b	1.44a	1.46a	0.12	2.92	0.20	0.09
CP (kg/day)	1.38ab	1.46a	1.27b	1.28b	0.01	0.94	0.04	0.05
NDF (kg/day)	2.89ab	3.09a	2.64bc	2.52c	0.07	<0.01	0.79	0.04
Starch (kg/day)	3.21a	3.15ab	3.15ab	3.08b	0.49	0.07	0.13	0.08
Fecal starch (%)	5.70b	10.52a	2.54c	4.01bc	0.08	<0.01	<0.01	<0.01
TTSD	97.1b	94.1c	98.8a	98.1ab	0.14	<0.01	<0.01	<0.01

1 DCF - dry corn grain finely ground; DCC - dry corn grain coarsely ground; RCF - rehydrated corn grain silage finely ground; RCC - rehydrated corn grain silage coarsely ground.

For starch intake (kg/day), differences were found between experimental diets, and there was influence of particle size (PART) and interactions for this parameter. Animals fed diets containing RCC had the lowest daily starch intake, being 4% lower than DCF.

Particle size and grain source effects were observed for fecal starch (P<0.01) and TTSD (P<0.01), with evidence that animals fed rehydrated corn diets showed greater efficiency on utilization of dietary starch. Our results about TTSD indicated that feeding beef cattle with rehydrated and ensiled corn increased total digestibility of corn starch. Animals fed RCF showed lower fecal starch losses of 37, 55, and 75% when compared with treatments RCC, DCF, and DCC, respectively.

The decrease of DMI observed in diets with rehydrated and ensiled corn did not influence ADG. These results provided an increase of 13.3% in feed efficiency of animals fed rehydrated corn in replacement of dry corn. Similar to the results found in this trial, Jacovaci et al. (2021) observed that the replacement of dry corn with rehydrated corn also did not affect the ADG, but there was a reduction in the DMI of 14.1% and an improvement in feed efficiency of 18.3%.

The inclusion of rehydrated and ensiled corn in diets for finishing cattle can increase feed efficiency by an average of 14% as a result of an average reduction of 12% in feed intake (Tonroy et al., 1974; Benton et al., 2005; Caetano et al., 2015; Caetano et al., 2019; Paschoaloto et al., 2019; Salvo et al., 2020). However, these benefits and results of using grain ensiling are based on major factors: moisture and nutritional content, particle size, quality of the ensiling process, and length of storage time (Owens et al., 1997; Rémond et al., 2004; Hoffman et al., 2011; Silva et al., 2018; Gomes et al., 2020; Salvo et al., 2020).

The action of proteolytic bacteria and kernel proteases break down the protein matrix during the storage period (Junges et al., 2017). Thus, there is an increase in the availability and digestibility of starch in both the rumen and the small intestine (Owens et al., 1986; Owens et al., 1997; Hoffman et al., 2011).

The greater availability and fermentability of starch are associated with hypophagia (Oba and Allen, 2003). The explanation may be related to higher energy availability (NEm and NEg) and the “Hepatic Oxidation Theory”, defended by Allen et al. (2009). According to the authors, with higher starch fermentability, there is an increase in the production of short-chain fatty acids (SCFA) per unit of rumen-fermented organic matter, resulting in changes in the ruminal fermentation pattern, which leads to an increase in the molar proportion of propionate, which has a DMI suppression effect.

A potential explanation for increase in the production of SCFA is that the starch in corn experiences different degrees of exposure to enzymatic attack in the rumen (Beauchemin et al., 1994; Huntington, 1997), which indicates that rehydrated and ensiled corn is a more efficient processing method than grinding corn for beef cattle in feedlot.

The averages of NEg values of corn grain silage were markedly higher, 7% higher than dry flint corn (finely or coarsely ground). Ensiled corn in finishing diets increased starch and DM digestibility and had higher energy contents than diets based on dry corn. Previous studies have suggested that NEg of dry ground flint corn is lower than tabular values in nutritional models (e.g., NRC, 1996; NASEM, 2016) and the ensiling corn grain may increase the NEg to 1.72 Mcal/kg DM (Zinn et al., 2011).

The percentage of fecal starch was influenced by the treatments, in which the highest starch content in the feces was verified when the animals were fed dry corn ground to larger particles, demonstrating that when the corn was rehydrated and ensiled, it provided better utilization of this nutrient. Similar to the present study, Cozannet et al. (2018) showed an increase in the energy of corn-based diets and a reduction in fecal starch content of bulls fed rehydrated and ensiled corn grain (Salvo et al., 2020).

In the same way, Ferraretto et al. (2015) stated that the rehydration of ground corn increased starch digestibility, especially when ensiled, suggesting that these procedures may be viable alternatives under favorable climatic conditions for harvesting and storage. The increase in grain starch digestion was expected because according to Watson (1987), the breakage of corneous endosperm occurs along the cell walls as a result of the strength of the protein matrix.

Another important point to highlight is that the finely ground dry corn grain also improved digestibility by decreasing fecal starch and increasing TTSD when compared with dry coarsely ground corn grain in the diet of young bulls finished in feedlot.

Starch digestibility is inversely proportional to the MPS for dry (Rémond et al., 2004) and rehydrated and high-moisture corn (Ferraretto et al., 2014). In the present trial, a higher percentage of fecal starch was verified for the treatments that contained MPS of 3.53 mm. On the other hand, Gomes et al. (2020) reported that flint corn presents divergent data when compared with dent corn.

The increase of TTSD observed in this trial may have occurred as a result of greater degradation of some zein protein in the starch-protein matrix of rehydrated and ensiled corn, improving greater solubilization of protein matrix and consequently, increasing starch granule surface area for bacterial attachment in the rumen (Huntington et al., 2006; Hoffman et al., 2011; Ferraretto et al., 2015).

Diets containing rehydrated and ensiled finely ground corn grain had higher NEg level accompanied by lower fecal starch loss. This may be an indication that the use in diets for finishing beef cattle requires supply adjustments, because although there may be a decrease in DMI, there was no influence on ADG.

Our results suggested that ensiled rehydrated corn grain improves feed efficiency, and the finely and coarsely ground rehydrated and ensiled corn grains increase the digestibility of starch in substitution of dry corn grain. The fine grinding of dry grain can also be a valid strategy in diet of finishing of young bulls in feedlot.