## 2 The Challenge with the Male Progeny of Dairy Cows  
Reproductive performance is integral to the overall efficiency of any livestock farming system because animals removed from the system need to be replaced. In dairy production, this need for efficiency is even more pronounced, because cows that have not produced a calf, do not start lactating. This has led to the development of different technologies for improving dairy cow reproductive performance. The desire to be able to control the gender of livestock progeny has been expressed for many years, and in the dairy industry, this is mainly due to gender-based differences in the value of the progeny. Female dairy calves are typically worth more than male calves, as females can become herd replacements and be milked themselves. Accordingly,'surplus' dairy calves are nearly all the male calves, plus any female calves not needed as replacements for the milking herd. The fate of the surplus calves varies by country. In Australia and New Zealand, they are often sold as \"bobby\" calves and slaughtered in the first few days of life, whereas elsewhere they can be sold and reared for longer to produce veal or 'dairy' beef. Focussing on New Zealand, it is seen by many as a clean and green country that produces dairy and meat products with a low environmental footprint. For example, a systematic review of the literature, removing inconsistencies in life cycle analyses, illustrated that New Zealand had the lowest GWP100 carbon footprint for milk production out of 19 countries [1]. However, the New Zealand dairy industry sends many four-day-old calves to slaughter every year. This occurs in a short space of time given the system is predominantly seasonal and outdoors, with a compressed calving period in the early spring such that lactating cows can have access to high-quality spring pastures. This creates a welfare dilemma, and while the male calves can be humanely managed and slaughtered, potential customers do not like the thought of killing four-day-old animals, regardless of whether valuable products can be obtained from those calves, or not. The calves slaughtered are also not necessarily all male, because while few female calves are killed immediately, they are nevertheless culled from the production system if they are under-performing for key early traits like growth, or if they have other faults. This challenge is not just peculiar to New Zealand. In Canada and the United States of America, it is also a major issue, and surplus dairy calves commonly experience poor welfare as evidenced by high levels of mortality and morbidity, and negative affective states resulting from limited opportunities to express their natural behaviours [2]. These authors also detail how calves can be transported for more than 24 consecutive hours, and how most calves are sold through auction markets or assembly yards, which increases disease exposure. There were 4.67 million dairy cows in New Zealand in the 2022-2023 season [3], and approximately the same number of calves were born between July and September, but only one-quarter of these were kept as replacements [4], with the remainder either sold to be raised as beef cattle, killed on the farm, or sold as bobby calves and sent to slaughter. Is New Zealand different to other countries? With hundreds of millions of dairy cattle globally, then one might assume (in the absence of the widespread use of sex-selected semen) that there will also be large numbers of male progeny born globally that may be of limited value to meat production. Countries that do not farm outdoors on pasture, and that are less constrained by seasonal feed availability, may be in a better position to spread out and manage their calf production, but the point remains that calves sourced from dairy genetics have little value, unless female with genetic merit for dairy production. The Use of Sex-Selected Semen to Reduce Male Dairy Cow Numbers  
The use of sex-selected semen technology has emerged as a potential solution to the problem of unwanted male dairy calves.