Aproximación conceptual

Regular assessment of the lactation performance of beef cattle breeds is not a commonly performed on-farm routine and has only been possible due to the development and improvement of the techniques here reviewed; however, despite the great efforts in minimising sampling errors through complex experimental designs, to date, no unbiased

Machine-milking trained animals or machine milking whilst the calf is suckling may not reflect the behavioural characteristics of beef cattle. Milk data obtained from these procedures tends to be low, mostly from underestimation of milk consumption due to insufficient milk let down. The techniques can be laborious in terms of animal training and constant sampling which may become expensive, hard to replicate and of limited biological use because of the overexposure of animals to extensive handling practices that will further limit the number of animals that can be handled within a certain time frame.

The oxytocin method has the disadvantage that by itself it is only useful to measure the amount of milk produced in the cow. When using this method, estimation of calf consumption cannot be determined. Also it’s questionable that the amount of oxytocin needed to extract milk from animals may not be of biological usefulness other than giving a rough estimate of milk production of a particular animal in a certain population. However, when combined with the WSW method, it may offer the advantage of overcoming the problem of underestimation of calf consumption during early lactation and, therefore, better estimates at this stage can improve later correlation with the calf’s growth.

Newer methods such as those using isotope dilution and/or transfer techniques may offer advantages. Stress caused by over handling animals and separating cows and calves for several hours and the variation in milk yield estimates due to disturbance of behavioural patterns is almost completely removed. Suckling occurs normally without disturbance over long periods of time ranging from 7d to 15d and handling is only required during short periods for blood sampling, weighing and isotope injection and equilibration. This allows an even greater number of animals to be tested over the sampling period and could also increase the number of measurements possible for a whole lactation which may lead to increased accuracy and reliability of estimates. The disadvantage of the isotope dilution technique is that it relies on the assumption that the animals are only ingesting water coming from milk and consequently it may only be possible to use this technique during the first two weeks of lactation. Once corrections for changing pool size, isotope recycling and sources of water other than milk are made, the isotope transfer technique offers great practicality and biological significance to the research worker. However, isotopes are expensive to obtain and in the case of radioactive isotopes, there are also waste handling costs involved. They require

a consequence, the large capital investment required when using these techniques may over shadow their advantages.

It seems that the most suitable and cost-effective method to determine milk intake is the WSW technique, however it has two major disadvantages, firstly, during early lactation when milk production of the cow exceeds calf intake, WSW may result in an underestimation of milk consumption. Measurements of milk consumption prior to 28-30 days postpartum tend only to decrease the accuracy and reliability of the estimates. Secondly, long periods of separation between the calf and its mother may not reflect actual suckling behavioural patterns showed under normal, undisturbed conditions, indeed infrequent suckling stimulation may reduce milk output. It is questionable that measurements taken during a period of stress to the animal may not be of any biological usefulness when extrapolated to the field. However if careful, well-timed and detailed measurements are taken, the reliability of the technique can increase considerably. The WSW method is undemanding for both the researcher and the experimental animals, and offers the greatest practicality for grazing systems where animals are handled relatively few times a year. After a period of acclimatization to the facilities and procedures, animals are comfortable if adequate handling is provided and pose minimal risk to the researcher. Financially, it is the most affordable method since it makes no use of external inputs to the systems and only requires labour and good facilities.

Milk production influences calf liveweight gain, however, the importance of this relationship has varied across studies Some authors have reported no correlation between milk production and liveweight gain, whilst other have reported correlations that ranged from 0.14 to 0.84. Similarly, research has shown that a relationship exists between milk production and calf weaning weight, the hypothesis being that high producing cows would be able to wean heavier calves. The size of this relationship has varied between authors and consequently, reported correlations have ranged from 0.17 to 0.94. The differences between authors are attributed to differences in the breeds studied, methods of estimation of milk yield and particularly the herd management. Correlations tend to be lower for pure breed studies compared to crossbred studies. Estimation of milk yield by the oxytocin technique resulted in more consistent correlations between milk yield and calf liveweight gain and calf weaning weight across studies when compared to the WSW technique, however, reported correlation

Milk consumption is important during the first months of lactation as it supplies the animals with enough water and nutrients for their development. Research has shown that milk supplied about 86% of the nutrients needed to grow in 44 days-old calves. However, at some point during the pre-weaning period, calf growth may be limited by the consumption of a diet consisting solely in milk. Some authors have reported that by week 9 to 13, milk production is inadequate to support calf growth. As calves grow in age, pasture consumption becomes important to maintain a desirable daily gain. Pasture intake is therefore, inversely correlated with milk consumption. Research has demonstrated that calves receiving less milk can compensate by consuming more pasture and therefore gaining similar weight to those receiving more milk. However, consumption of nutrients from sources other than milk not always resulted in increased liveweight gains and some authors have reported that the increased pasture consumption seen in calves consuming low levels of milk may not be sufficient to support high daily gains.

In the study reported in the remainder of this thesis, the WSW technique was used to estimate calf consumption of beef bred calves. The objective was to determine the suitability of a random regression methodology to predict the milk yield of beef cows based on a limited number of test-day records obtained by the WSW technique. A secondary objective of this experiment was to identify the most significant sampling time point throughout lactation to optimise future WSW sampling schemes. Research has demonstrated that dairy breed crossbreeding in beef production systems have resulted in increased milk yield, and consequently a second objective of the first experiment was to determine and quantify the lactational performance of straightbred Angus cows and Angus×Friesian, Angus×Jersey and Angus×Kiwi-Cross cows. In the study reported here, calves were reared by dams of different milk production potential and their lactation curves were estimated in the first experiment; consequently, the objective of the second experiment was to determine to what extend any differences in milk yield, and by assumption calf intake, may affect the growth trajectory of calves up to a year of age. Estimation of the theoretical metabolizable energy requirements and the estimated energy consumption from milk were necessary to determine differences in pasture consumption between beef steers.

Chapter 2.

Lactational performance of straightbred