Geographical location

The study was done in the city of Mexicali, Baja California, Mexico (32°32’00” N; 115°12’41” W). The region has a dry desert climate with an average temperature of 34.7 °C (-5 °C winter; 50 °C summer), average rainfall of 37 mm, and average relative humidity of 50 %¹⁸.

Study design

The experimental animals were 720 Holstein males of the same origin, 7 to 8 mo’ age on arrival at the growth facility, with a 240 kg average live weight. Two treatments were applied: surgically-castrated (SC) males (T1) and immunocastrated (IC) males (T2). The animals were randomly assigned to the treatments, with 90 animals per pen and four pens per treatment. Those in the T1 treatment were castrated 24 h after birth at a dairy farm.

Handling after reception at growth facility

Twenty-four hours after arriving at the growth facility the animals were vaccinated, wormed, and an implant applied (contains trenbolone acetate, estradiol and tylosin). The animals in T2 were immunocastrated by subcutaneously administering 1 ml Bopriva^® (Laboratorios Zoetis, Salud Animal, Mexico) at four times after arrival: 24 h (d 1), and on d 21, 101 and 181 of the experiment). The SC animals were administered 1 ml saline solution on the same days. Live weight (LW) was recorded for each animal on days 1, 21, 101 and 181 of the experiment and before slaughter. The animals were fed twice a day, following a six-diet program widely used in northern Mexico which consists of wheat hay, Sudan grass, tallow, DDGS (dry distiller’s grains with solubles) and a mineral premix.

Serum testosterone levels

Ten animals were randomly selected from each pen to measure serum testosterone levels; each was identified with an additional earring. Blood samples were taken on the same days as Bopriva application. A final sample was taken at slaughter at the bleeding station in the slaughterhouse production line. Approximately 5 ml blood were extracted from the coccygeal vein. The samples were centrifuged at 3,500 rpm (TRIAC centrifuge, Clay Adams, Model 0200, New Jersey, USA) to obtain the serum and stored at -20 °C until analysis. Testosterone concentration was measured using the ELISA (Bovine) Testosterone Kit (Abnova Corporation, Taipei City, Taiwan), following manufacturer instructions.

Slaughter

The animals were killed at 242 d after attaining a 607.85 ± 12.89 kg average weight. On the day of slaughter the animals were herded by a wrangler on horseback about 1.5 km to the slaughterhouse. Here they were kept in rest pens with access to water for approximately 5 h. They were slaughtered in a Federal Inspection Type (Tipo Inspeccion Federal - TIF) slaughterhouse, following established procedures (NOM-033-SAG/ZOO-2014).

Carcass evaluation

The carcasses from both treatments were stored at 2 °C for 24 h. Sample cuts were made between rib 12 and 13 to collect carcass data. A total of 120 carcasses per treatment (T1 and T2) were made available by the slaughterhouse for carcass trait data collection: hot carcass weight (HCW); cold carcass weight (CCW); back fat thickness (BFT); kidney, pelvic and heart fat (KPH); marbling; ribeye area (REA), pH and color (L*, a*, b*, C* and H*). Back fat deposition was measured in mm using a metric ruler¹⁹. Ribeye area was evaluated using a plastic template following the Iowa State University method. Estimated KPH was determined subjectively and expressed as a percentage of HCW, and marbling was estimated with a seven-level scale (traces, light, small, modest, moderate, slightly abundant and moderately abundant)²⁰.

Meat quality

Meat samples (approx. 1,000 g each) were taken 48 hours after slaughter from the Longissimus dorsi muscle of ten randomly selected carcasses from each pen in both treatments (total samples 80). These were vacuum-packed, refrigerated and sent to the Animal Quality Products Laboratory of the Veterinary Science Research Institute of the Autonomous University of Baja California. The pH was measured with a potentiometer (Hannah Instruments, Inc., pH 101). Color values (L*, a*, b*, C*, H*) were measured on the surface of the Longissimus dorsi muscle cut using a spectrophotometer (Minolta CM-2002, Minolta Camera, Co., Ltd., Japan). The specular component included (SCI) mode was used with illuminant D₆₅, and a 10° observer; L* is the brightness index, a* is red intensity and b* is yellow intensity. Shear force (SF) was quantified using previously cooked pieces of meat extracted perpendicular to the muscle fibers with a 1 cm diameter punch and placed in a texturometer (Lloyd Instruments, England) equipped with Warner-Bratzler blades. All measurements were done in triplicate.

Statistical analyses

Total variation was analyzed with a linear model:

Where:

Y_ij are meat pH, color and SF values as response variables;

µ is the general mean, τ_i is the fixed effect of treatment (surgically-castrated vs. immunocastrated);

ξ_ij is the residual random effect [ξ_ij ~ NI(0,σe2)].

When the treatments were a source of significant variation (P≤0.05), a Tukey was applied to compare the treatments’ mean values; this was done with the GLM procedure in the SAS statistics package (SAS Inst. Inc., Cary, NC)²¹. Mean serum testosterone levels were compared between treatments and over time (days 1, 21, 101 and 181, and at bleeding) with a mixed linear model:

Where:

Y_ijk is testosterone concentration of the k-th animal taken at j-th time and belonging to the i-th treatment, as a response variable;

µ is the general mean, τ_i is the fixed effect of treatment;

A_k(i) is the random effect of animal within treatment [A_k(i) ~ NI 0, σ a 2 ],

D_j is the fixed effect of time, in days, (τD)_{i j} is the effect of the treatment × time interaction;

ξ_ijk is the residual random effect [ξ _ij ~ NI(0,σe2)].

The analysis was run using the MIXED procedure with the REPEATED label in the SAS package. Analysis of repeated records, including correlations between records of the same animal and heterogeneous variances between records in time was done by evaluating the covariance structures: unstructured (US); compound-symmetry (CS); and first-order autoregressive (1AR). This was done based on the Akaike and de Schwartz criteria, choosing that with the lowest values for these two indicators. In this case the selected covariance structure was unstructured (UN). When the treatment x time interaction was a source of variation (P≤0.05) a Tukey-Kramer procedure was applied to compare the least squares means between treatments for each time increment²².

Weight gain

Animal weight was higher in T1 (surgically-castrated) (P<0.05) starting with the second vaccination (21 d), and remained so until slaughter (Table 1).

The trend observed in the present results differs from previous studies. For example, in a study of live weight in beef cattle, at 280 days males immunocastrated with Bopriva were heavier (P<0.05) than those surgically castrated at 91 days growth¹⁰. In a study comparing males immunocastrated with Bopriva to other males surgically castrated between fifteen and seventeen days post-vaccination in the first group, weight did not differ (P>0.05) up to slaughter⁹. One notable difference in the present study is that the animals in T1 were castrated at birth, meaning recovery time due to infection and weight loss had no impact at seven months’ age when the animals were placed in the growth pens.

Serum testosterone concentrations

Testosterone concentrations in both treatments were below 1 ng/ml on each vaccination day until slaughter, confirming the effectiveness of vaccination in suppressing serum testosterone concentrations in cattle^4,9. The present results also support previous reports that seven months is the optimal age for immunization against GnRH and generates maximum antibody production in Bos taurus males²².

Carcass and meat quality

Both HCW and CCW were higher (P<0.05) in the T1 animals (Table 2), which differs from the lack of difference in CCW reported elsewhere¹⁴. Values for BFT and REA differed (P<0.05) between treatments, with the highest values in T1 carcasses. No differences were observed in KPH between treatments (P>0.05). In a previous study using male Nellore breed carcasses no differences (P>0.05) were apparent in BFT and REA between SC and IC animals¹⁴. Results for BFT more in agreement with the present results have been reported in SC and IC Nellore and Nellore x Angus males¹⁵. Previous reports of REA values are lower than those observed in the present study¹⁵: castrated = 81.06 ± 1.78 cm² vs. immunocastrated = 83.61 ± 1.73 cm². These discrepancies may respond to breed since, for example, Holstein cattle are reported to have a larger and longer body structure and a longer growth period, allowing them to develop larger carcasses than beef cattle breeds²³.

Previous studies comparing SC and IC males in Holstein x Cebu²⁴ and Nellore cattle^10,14, found no differences (P>0.05) in BFT and REA values. No differences in CCW, BFT and REA values (P>0.05) were also observed in a study of Holstein males in which the animals were slaughtered at a lower average weight: 477 kg (SC) and 486 kg (IC)⁹. Weight at slaughter was notably higher in the present results: 600+ kg (T1) and 594 kg (T2). The differences between the present carcass quality trait values and those in previous studies^10,14,15 may be due to surgical castration schedules; in previous studies it was done some days before or contemporaneously with Bopriva application while in the present study it was done 24 h after birth, meaning the animals had fully recovered when placed in growth pens at seven months’ age.

Marbling category frequency did not differ (P>0.05) between the treatments, with the largest number of carcasses in both treatments classified in the “slight” and “small” marbling categories (Table 3). These results agree with previous comparisons between carcasses from IC and SC males²⁵. In another study no differences (P>0.05) were observed in intramuscular fat percentage between the carcasses of SC and IC Holstein males⁹.

Meat pH values did not differ (P>0.05) between the treatments (Table 4), with values in a normal range of 5.5 to 5.8²⁶. These results coincide with the 5.57 pH at 24 h reported for Holstein cattle carcasses²⁷.

Most of the color values (a*, b*, C* and H*) differed (P<0.05) between treatments, the L* value being the one exception (P>0.05). No difference in L* values (P>0.05) has also been reported in previous comparisons of meat from SC and IM male cattle^10,15. In addition, a lack of difference for the L*, a* and b* values was observed between the carcasses of SC males (L* = 33.9; a* = 17.1; b * = 2.6) and IC males (L* = 34.0; a * = 16.9; b* = 2.4)⁹. Although meat pH was normal in the present results, the color values observed here are similar to those reported for DFD meat (L* = 34.8; a* = 18.8; b* = 6.7)²⁸, suggesting that the animals had been exposed to stressors prior to slaughter.

Shear force (SF) did not differ (P<0.05) between treatments. Based on established criteria²⁹, meat tenderness was intermediate (tender: 22.26-35.10N; intermediate: 40.01-52.95N; hard: 57.85-70.60N). The same has been reported elsewhere^10,16, with no differences in SF between meat from SC and IC animals. Indeed, another study found no differences in SF due to sexual condition (SC males: 56.60 ± 0.36N; IC males: 53.37 ± 0.35N; whole males: 48.85 ± 0.35N)¹⁵. Even in males slaughtered at eleven months of age SF did not differ between SC (51.9N) and IC males (52.9N)⁹.