Hormonal growth promotants: growth and carcase responses
HGPs are a reliable product for increasing liveweight gain to target market weight specifications, provided diet quality enables weight gain. HGPs also impact on other carcase traits such as fatness, so it is important to understand and manage implantation programs to ensure penalties do not occur when cattle are marketed.
A review of hormonal growth promotants was recently undertaken by Bob Hunter, CSIRO Livestock Industries for Meat & Livestock Australia. A priority was given to Australian reports and scientific data, particularly usage in the northern grass fed and feedlot sectors. The following is a summary of HGPs responses in various traits, as outlined in the review.
Dependant on energy intake, oestradiol implants (e.g. Compudose) give growth responses of 0.05–0.1kg/day. When cattle experience both a wet season and a dry season when implanted with 400 day oestradiol, weight responses are usually in the range of 0.06kg/day at a base growth rate of 0.3kg/day, and 0.09kg/day at a base growth rate of 0.6kg/day.
On good quality pasture combination implants which contain oestradiol and trenbolone acetate give growth responses as high as 0.2kg/day. On high energy diets implants containing an androgen and oestrogen give a greater response than implants containing only oestrogen, for the first 100 days after implantation.
Feed intake and feed conversion
A USA review of 85 feedlot experiments found an average increase in feed conversion of 8%. This means feed costs per unit liveweight gain are decreased when HGPs are used.
This review also found an increase in feed intake of six percent. This increase in feed intake is because the cattle are heavier not because the HGP increases feed intake.
Fat composition of the carcase
At any given body weight HGP treated cattle are leaner by five to eight percent. This is because protein deposition occurs at the expense of fat deposition. The degree to which this happens depends on the type of implant, the duration of the implant program, the stage of growth and maturity type of the cattle.
Implants containing androgens: testosterone or trenbolone acetate, produce leaner carcases when comparisons are made. Repeat treatment with a HGP containing an androgen will increase the likelihood of reduced carcase fatness.
Implants which contain oestradiol, whether implanted once or several times, impact much less than the androgens on carcase fatness. Androgens have a direct effect on reducing carcase fat content whereas oestrogens have an indirect effect (cattle reach target weights earlier when animals are less mature and therefore less fat).
Implantation of young cattle in the early growth stage is likely to delay the onset of fat deposition. This is especially so in late maturing genotypes of higher mature weight. When aiming to reduce age of turnoff and market to fat specifications it is important to manage HGP programs to insure downgrading does not occur for inadequate fat cover.
The implants most likely to reduce marbling score are those which contain trenbolone acetate combined with an oestrogen. If implanted during the early stages of growth this likelihood is increased.
The HGP effect on tenderness and eating quality is negative as assessed by taste panels. The negative effect is increased if there is a larger cumulative dose of hormone from repeat implantation over the animal’s life. This is especially the case when combination implants are used.
The individual effects of trenbolone acetate and oestradiol in combination implants appear to be additive in increasing meat toughness.
The review points out that the effect on meat tenderness from implant programs containing only oestradiol has not been adequately addressed. However is it known that when oestradiol 17β was used in repeated 100 day intervals this lead to a reduction in meat tenderness.
Treatment with both oestrogen and androgen HGPs results in higher ossification scores, and the effects are more dramatic when HGPs are applied at a younger age. Higher ossification scores indicate advanced skeletal maturity and reduced eating quality. The number of implants used in a program also has an impact on skeletal maturity.
Dark cutting meat
The incidence of dark cutting is potentially increased when steers are treated with androgens, particularly with trenbolone acetate. This is more the case if steers are slaughtered during the pay-out period. The hormone may result in increased physical exertion which depletes glycogen stores. If this happens before slaughter it may result in dark cutting meat.
Meat Standards Australia (MSA) and HGPs
MSA use a computer model to predict meat eating quality. The model uses a number of measured parameters including weight, fat cover, ossification score, hump height, marbling, meat colour and pH, when calculating a grade for different muscles.
A negative score or penalty for HGP treatment is applied by the MSA model and is specific for different muscles and different aging periods. The model does not differentiate between different HGP implants or implant programs. HGP cattle are not excluded from grading MSA but the negative score for HGP treatment makes grading more difficult. For HGP treated pasture fed cattle to grade Boning Group 10 or better, good scores are needed for weight, ossification, hump height, marbling, and fat cover, to counteract the negative scoring for HGPs.
The MSA model also applies a penalty for Bos indicus content as measured by hump height. This penalty is significantly greater than the penalty applied for HGPs. The majority of cattle from feedlots will have been HGP treated and will grade MSA, as good scores for weight, ossification, hump height, marbling and fat cover will compensate for the HGP penalty.
The effect of HGPs on improving weight gain and enabling producers to meet market weight specifications at a younger age is well proven. When comparisons are made HGP treated cattle will have increased frame size and mature weight, and lower body fat composition.
It is important to review carcase feedback data to determine if inadequate fat cover is resulting in downgrading of carcases (or carcases missing out on eligibility to grade MSA). This problem is more likely to occur with later maturing genotypes or their crosses. However the problem can also arise if weight for age has been improved through breeding and cattle are now slaughtered heavier at a younger age and have yet to reach maturity. While attention should be paid to improvement in genetics and nutrition, it is also important to manage implant programs. If the current HGP program involves repeat implantation, particularly if commencing at an early stage of growth, the program should be reviewed. If the last implant in the program contains both oestradiol and an androgen, then the likelihood of the HGP contributing to reduced fat content of the carcase is increased.
Beef producers consigning cattle MSA may wish to review their HGP program if it involves multiple implants and use of a terminal combination implant. The continued use of HGPs in a less aggressive program is possibly an economical option. While MSA does not directly distinguish between single or multiple implant strategies, or type of implant, a less aggressive program may result in improved ossification, fat cover and marbling scores. However in northern Queensland pasture systems this issue is still to be thoroughly researched and investigated.
The free Meat & Livestock Australia booklet ‘Hormone growth promotants and beef production: a best practice guide’ provides clear and unbiased information on best practice use of HGPs. This includes information on designing implant strategies to suit different market end-points, finishing regimes, management systems and other factors. Hardcopies can be obtained from Meat & Livestock Australia – email email@example.com or phone 1800 023 100.
Felicity Hamlyn-Hill, formerly Queensland Government.