Sheep farming in the tropics is mainly focused on meat production. Hair breeds are widely used due to their adaptability to high temperatures, resistance to parasites and high prolificacy¹. Extensive systems are the most common production system in the tropics. Lamb production is the highest value product of these systems and represents the main income source for producers².

Milk production in hair breeds is fundamental to lamb survival, growth and weaning weight^3,4, with any surplus used in human foods. Despite its importance, information on milk production and quality in hair sheep during lactation is limited to a few studies in breeds such as Santa Cruz, Blackbelly, and Katahdin^5-7, as well as some West African breeds⁸. Sheep milk nutritional composition has only been characterized in Blackbelly x Katahdin sheep^9,10. This lack of research is puzzling since sheep milk is popular among human consumers in tropical North and Central America^11,12.

Daily milk intake is the most important factor influencing lamb growth rates. Offspring survival, growth potential and weaning weight therefore depend on milk production during the mother’s lactation^3,4. Genotypes vary widely between tropical sheep breeds. This means that estimation of milk production during lactation, as well as variation in milk composition, are essential data for establishing sheep and lamb management strategies for specific breeds. The present study objective was to characterize milk production and quality in Santa Cruz ewes, from parturition to weaning.

The study was carried out at the Faculty of Agricultural Sciences, Autonomous University of the State of Morelos (Universidad Autónoma del Estado de Morelos). The experimental field is located at 18°56' N and 99°13' W, at an altitude of 2,160 m asl. Average annual temperature in the region is 20 °C, and average annual rainfall is 1,243 mm.

The experimental animals were eighteen multiparous Santa Cruz ewes with a 2.0 years’ average age and 51.22 ± 2.36 kg weight. All had lambed within four 4 d of the beginning of the experiment. Of the ewes included in the study, 12 gave birth to twins, 2 had triple births and 4 single births. During gestation, the ewes were kept in a single group, grazing from 0800 to 1400 h in a pasture with African star grass (Cynodon nlemfluensis). During the remaining hours they were penned and provided a supplement of 300 g commercial concentrate. This concentrate contributed 16 % crude protein (CP) and 2.79 Mcal/kg metabolizable energy (ME), while the forage (alfalfa + sorghum) provided 7.6 % CP and 1.78 Mcal/kg ME. The diet consisted of 50 % concentrate and 50 % forage, meaning it covered the nutritional requirements of a 60 kg sheep, with two young, producing between 0.79 and 1.48 kg milk per day¹³.

During lactation, the ewes were given free access to a forage mixture (30 % alfalfa and 70 % sorghum straw), 1 kg of the same commercial concentrate feed, and water. For the first 5 days postpartum (dpp) each ewe was housed with her offspring in individual 2 x 2 m roofed pens with a cement floor, individual feeder and drinker. From 6 dpp to weaning (60 dpp) all ewes and their lambs were kept in a single group inside a roofed pen (4 m²/ewe), with concrete floor, a trough and a shared drinker. Beginning at 14 dpp, the pen was equipped with a creep feeder to provide the lambs free access to an 18 % CP commercial concentrate.

Starting at 6 dpp, ewe milk production was measured every three days using the “oxytocin method”. This consists of separating the lambs from the ewes, intravenously administering 5 IU synthetic oxytocin (Oxitopisa; Pisa; Hidalgo, México), and completely milking the udder manually. Four hours after the first hormone application, a second dose is applied, the udder milked again, and milk volume recorded. This value is multiplied by six to estimate daily production¹⁴.

Milk quality was measured once a week using a 50 mL milk sample collected from the daily milk production. Milk protein, fat, lactose, and total solids contents were quantified using an infrared spectrophotometry method¹⁵ applied by an outside laboratory (Alimenlab; Jalisco, Mexico).

Statistical analysis of milk production and quality was done by applying a polynomial regression to produce the equation, the R² value and its probability. All analyses were run using the PROC REG procedure¹⁶.

Milk production at 6 dpp was 1.95 L, increased to a maximum of 2.31 L at 12 dpp, and decreased to 1.01 L at 57 dpp. The lactation curve was divided into five periods of 9 d each. Production during the first two periods (equivalent to 18 dpp) did not differ (P>0.05). However, when compared to the first two periods, production began to gradually decrease (P<0.05) thereafter; 15 % in the 3^rd period (27 dpp), 26 % in the 4^th (36 dpp) and 39 % in the 5^th (45 dpp). Average ewe weight did not differ during lactation (P>0.05): 52.24 ± 1.19 kg at parturition and 52.00 ± 1.63 kg at weaning. The corresponding regression analysis (Figure 1) identified an inversely proportional relationship over time, adjusted to a polynomial trend line: r²= 0.88; y= -0.0003x²-0.0513x+2.1079 (P= 0.0001).

Figure 1 Lactation curve for Santa Cruz ewes during 60 days postpartum (mean ± SE) (P≤0.001). Polynomic regression 

Total solids percentage began at 18 % in week two of lactation and increased to 20.5 % by week eight (Figure 2). The corresponding regression analysis showed a proportional relationship over time: r²=0.92; y=- 0.0993x²+1.1487x+17.276 (P=0.01).

Figure 2 Composition of milk from Santa Cruz breed ewes during first 60 days postpartum (mean ± SE) (P≤0.001) 

Milkfat increased from 8 % in week two of lactation to 9.8 % at the end of lactation (week eight). The regression analysis confirmed this trend: r² = 0.61; y= -0.3929x² + 2.8471x + 5.32 (P =0.23). The protein percentage increased from 4.86 % in week two to 5.18 % at the end of lactation while the lactose percentage increased from 4.68 to 4.75 %. The regression analysis identified minimal fluctuation throughout lactation for both variables: protein, r² = 0.84, y= 0.0029x²+0.0029x+4.672 (P=0.90); lactose, r² =0.60; y= 0.1779x²-1.0201x+5.83 (P= 0.05).

Milk production in the evaluated ewes was highest at 12 dpp, without a clear peak, and then decreased gradually and constantly. Maximum milk secretion in different sheep breeds is generally reported between the second and fourth weeks of lactation¹⁷. The maximum production at 12 d observed in the present study coincides with previous reports for Katahdin sheep⁷, Ile de France sheep¹⁸ and other breeds¹⁹. However, the quantity of milk produced during this same period was higher in Santa Cruz (2.33 L/d) than in the aforementioned meat breeds (Katahdin =1.38, Ile de France = 0.50 and Others = 0.43 L/d). This highlights the relatively higher milk production in hair breeds²⁰ when compared to meat breeds of European origin. The lack of a clear peak in milk production in Santa Cruz ewes has been reported elsewhere⁷. This may be due to high dietary energy content^21-23; in the present study and the previous one⁷, the same feed concentrate was offered, although the forage was different (alfalfa + sorghum straw vs. corn silage), and feed was freely available.

In milk composition, the total solids and fat percentages were higher at the end of lactation. This is generally to be expected since as milk production decreases, total solids and fat become more concentrated. This is reported in other breeds²⁴, including specialized milk production breeds^25-26. Higher total solids and fat contents are associated with lower quality milk²⁷.

Of note in the present results is that the protein and lactose percentages remained constant throughout lactation. Given the total decrease in milk production as lactation progresses, this suggests an increase in these substances over time. This could be an important advantage in the nutritional characteristics of milk from Santa Cruz ewes, but further research is needed to confirm this finding.

Higher protein and lactose concentration in milk may be a response to the fact that tropical sheep live under humid conditions, reducing the need for lambs to receive water via mother’s milk. An inverse situation has been reported in other species inhabiting desert climates, in which milk water content increases²⁷. Tropical breeds are also lighter in weight than European breeds, suggesting that their lower maintenance requirements could allow for greater nutrient allocation and consequently better milk quality¹⁹.

In conclusion, milk production in Santa Cruz sheep reaches a maximum around the second week of lactation and then decreases constantly and gradually. As a result, the percentages of total solids and fat increase over time, while those of protein and lactose remain constant.